DNA methylation involves the addition of a methyl group to a 5' cytosine residue. This usually occurs at cytosines that form part of a cytosine-guanine dinucleotide (CpG sites). Methylation can lead to activation or repression of gene transcription and is mediated through the activity of DNA methyltransferases (DNMTs). DNMT3A and DNMT3B regulate de novo methylation of CpG sites, while DNMT1 maintains established methylation patterns. S-adenosyl methionine acts as the methyl donor.
One current hypothesis for how DNA methylation contributes
to the storage of memories is that dynamic DNA methylation changes occur
temporally to activate transcription of genes that encode for proteins
whose role is to stabilize memory. Another hypothesis is that changes in
DNA methylation that occur even early in life can persist through
adulthood, affecting how genes are able to be activated in response to
different environmental cues.
The first demonstration about the role of epigenetics in
learning in memory was the landmark work of Szyf and Meaney (PMID
15220929) where they showed that licking and grooming by mother rats
(maternal care) prevented methylation of the glucocorticoid receptor
gene. When these pups become adults, they respond better to stressors
than rats who, as pups, were not licked and groomed by their mothers and
instead had a buildup of methylation in the glucocorticoid receptor
gene.
DNMTs and memory
Miller and Sweatt demonstrated that rats trained in a contextual fear conditioning paradigm had elevated levels of mRNA for DNMT3a and DNMT3b in the hippocampus. Fear conditioning is an associative memory task where a context, like a room, is paired with an aversive stimulus,
like a foot shock; animals who have learned the association show higher
levels of freezing behavior when exposed to the context even in the
absence of the aversive stimulation. However, when rats were treated
with the DNMT inhibitorszebularine or 5-aza-2′-deoxycytidine
immediately after fear-conditioning, they demonstrated reduced learning
(freezing behavior). When treated rats were re-trained 24 hours later,
they performed as well as non-treated rats. Furthermore, it was shown
that when these DNMT inhibitors were given 6 hours after training, and
the rats were tested 24 hours later, the rats displayed normal fear
memory, indicating that DNMTs are involved specifically in memory
consolidation. These findings reveal the importance of dynamic changes in methylation status in memory formation.
Feng et al. created double conditional knock out (DKO) mice for the genes DNMT3a and DNMT1. These mice were shown to have significantly weakened long-term potentiation (LTP) and much more easily stimulated long-term depression (LTD) in the hippocampus. When tested in the Morris water navigation task, which is used to study hippocampus-dependent spatial memory,
the DNMT3a/DNMT1 DKO mice took longer to find the platform than control
mice. Single knock-out mice (SKO) for either DNMT3a or DNMT1 performed
normally. DKO mice were also unable to consolidate memory
after fear-conditioning. Since SKO mice did not exhibit the same
learning and memory defects as the DKO mice, it was concluded that
DNMT3a and DNMT1 play redundant roles in regulating learning and memory.
When DNMTs are inhibited in the prefrontal cortex,
recall of existing memories is impaired, but not the formation of new
ones. This indicates that DNA methylation may be circuit-specific when
it comes to regulating the formation and maintenance of memories.
DNA methylation targets
The memory suppressor gene, protein phosphatase 1 (PP1), was shown to have increased CpG island
methylation after contextual-fear conditioning. This corresponded to
decreased levels of PP1 mRNA in the hippocampus of the trained rats.
When DNMTs were inhibited, increased methylation at the PP1 gene was no longer observed. These data suggest that during memory consolidation in associative
learning tasks, CpG methylation is used to inhibit the expression of PP1, a gene that negatively inhibits memory formation.
Demethylation and memory
While DNA methylation is necessary to inhibit genes involved in memory suppression, DNA demethylation
is important in activating genes whose expression is positively
correlated with memory formation. Sweatt and Miller also showed that the
gene reelin,
which is involved in long term potentiation induction, had a reduced
methylation profile and increased reelin mRNA in fear-conditioned versus
control rats. Brain-derived neurotrophic factor (BDNF),
another important gene in neural plasticity, has also been shown to
have reduced methylation and increased transcription in animals that
have undergone learning. While these studies have been linked to the hippocampus, recent evidence has also shown increased demethylation of reelin and BDNF in the medial prefrontal cortex (mPFC), an area involved in cognition and emotion.
The mechanism behind this experience-dependent
demethylation response was previously not fully understood, with some
evidence showing that DNMTs may be involved in demethylation. It was also suggested that members of the DNA damage repair GADD45 family may contribute to this demethylation process. However, more recently, the pathways illustrated in the Figure below,
titled "Demethylation of 5-Methylcytosine (5mC) in neuron DNA,"
especially the TET dependent pathway, have been confirmed as pathways of DNA demethylation. A role for GADD45 has also recently been indicated, since GADD45 physically interacts with thymine-DNA glycosylase (TDG) and GADD45 may promote the activity of TDG in its role(s) during conversion of 5mC to cytosine.
Methyl-binding domain proteins (MBDs)
Mice that have genetic disruptions for CpG binding protein 2 (MeCP2) have been shown to have significant problems in hippocampus#Role in memory-dependent memory and have impaired hippocampal LTP.
Methylation and learning and memory disorders
Changes in expression of genes associated with post-traumatic stress disorder (PTSD), which is characterized by an impaired extinction of traumatic memory, may be mediated by DNA methylation. In people with schizophrenia, it has been shown that reelin is down-regulated through increased DNA methylation at promoter regions in GABAergic interneurons. DNMT1 has also been shown to be upregulated in these cells.
Histone methylation
Methylation of histones
may either increase or decrease gene transcription depending on which
histone is modified, the amino acid that is modified, and the number of
methyl groups added. In the case of lysinemethylation, three types of modifications exist: monomethylated, dimethylated, or trimethylated lysines. The di- or trimethylation of histone H3
at lysine 9 (H3K9) has been associated with transcriptionally silent
regions, while the di- or trimethylation of histone H3 at lysine 4
(H3K4) is associated with transcriptionally active genes.
Histone 3 lysine 4 trimethylation and memory formation
The hippocampus is an important brain region in memory
formation. H3K4 trimethylation is associated with active transcription.
In contextual fear conditioning experiments in rats, it was found that
levels of H3K4 trimethylation increases in the hippocampus after fear conditioning. In these experiments by Gupta et al., a connection was made between
changes in histone methylation and active gene expression during the
consolidation of associative memories. In this same study, it was also found that these histone methylations
were reversible, as the levels of trimethylation of H3K4 returned to
basal levels after a period of 24 hours. This indicated that active
demethylation was occurring following memory consolidation. To further explore the role of methyltransferases in long-term memory formation, this study applied the same fear conditioning tests on rats deficient in Mll,
a H3K4-specific methyltransferase. The rats with a heterozygous mutant
Mll+/− gene showed a significant reduction in their ability to form long-term memories
compared to normal rats with an intact Mll gene. Therefore, H3K4
methyltransferases, such as Mll, must have an essential role in
long-term memory formation in the hippocampus.
The change in methylation state of histones at the location
of specific gene promoters, as opposed to just genome-wide, is also
involved in memory formation. Zif268 and BDNF genes are critical for memory consolidation. H3K4 trimethylation increases around both of the Zif268 and BDNF
promoters following contextual fear conditioning, when these genes are
transcriptionally active. This demonstrates that at the time of memory
consolidation, the transcription of memory formation genes such as
Zif268 and bdnf is regulated by histone methylation.
Histone 3 lysine 9 dimethylation and memory formation
Histone H3 lysine 9 dimethylation is associated with transcriptional silencing. The G9a/G9a-like protein (GLP) complex is a methyltransferase specific for producing this modification. One study examined the role of G9a/GLP-mediated transcriptional silencing in the hippocampus and entorhinal cortex
(EC) during memory consolidation. It was found that the inhibition of
G9a/GLP in the EC, but not in the hippocampus, results in the
enhancement of long-term memory formation. In addition, G9a/GLP inhibition in the entorhinal cortex altered histone H3 lysine 9 dimethylation in the Cornu Ammonis area 1
of the hippocampus, suggesting the importance of this complex in
mediating connectivity between these two brain regions. Therefore, the
G9a/GLP complex plays an important role in histone methylation and
long-term memory formation in the hippocampus and the EC.
Histone methylation and other epigenetic modifications
Histone methylation marks are also correlated with other epigenetic modifications, such as histone deacetylation
and DNA methylation, in the context of learning and memory. Reduced
histone deacetylation is correlated with an increase in H3K9
dimethylation, a modification associated with transcriptional silencing. Therefore, histone deacetylase inhibitors may be applied to increase
histone acetylation and suppress H3K9 dimethylation, thereby increasing
gene transcription. In the case of DNA methylation, it was found that
increases in H3K4 trimethylation correlate with altered DNA methylation
of CpG sites at the promoter of Zif268,
a gene involved in memory formation, after fear conditioning. Gupta et
al. showed that DNA methylation at the Zif268 promoter increased after
fear conditioning, correlating with an increase in Zif268 gene
expression. This finding was surprising, since it was previously thought that DNA methylation resulted in transcriptional silencing.
Histone acetylation
Acetylation involves the replacement of a hydrogen with an acetyl group. In a biological context, acetylation is most often associated with the modification of proteins, specifically histones. The acetylation reaction is most often catalyzed by enzymes that contain histone acetyltransferase (HAT) activity.
Histone acetyltransferases (HATs)
HATs are enzymes responsible for the acetylation of amino acids. HATs acetylate by converting the lysine side group of amino acids with the addition of an acetyl group from an acetyl CoA molecule, creating acetyl lysine.
HAT enzymes are most often associated with histone proteins and work to
regulate the interaction between histones and the DNA that is wrapped
around them. HATs are not only restricted to the acetylation of histone
but can also acetylate many other proteins implicated in the
manipulation of gene expression like that of transcription factors and
receptor proteins.
Chromatin remodeling
Acetylation is one of the main mechanisms implicated in the process of chromatin remodeling. Chromatin remodeling affects the regulation of gene expression by altering the relationship between nucleosomes
and DNA. Acetylation of histones removes positive charge, which reduces
the level of interaction between the formerly positively charged
histone and the negatively charged phosphate groups of the DNA wrapped
around the nucleosome complex. This alteration in charges causes a
relaxation of DNA from the nucleosome, this relaxed section is seen to
have higher levels of gene expression than non acetylated regions.
Acetylation as an epigenetic marker
Patterns of histone acetylation have been useful as a
source of epigenetic information due to their ability to reflect changes
in transcription rates and the maintenance of gene expression patterns.
This acetylation code can then be read and provide generous information
for the study of inheritance patterns of epigenetic changes like that
of learning, memory and disease states.
Acetlylation as a mechanism for learning and memory
The role of epigenetic mechanisms and chromatin remodeling
has been implicated in both synaptic plasticity and neuronal gene
expression. Studies with histone deactylase complex inhibitors like SAHA, toluene, garcinol, trichostatin A and sodium butyrate
have shown that acetylation is important for the synaptic plasticity of
the brain; by inhibiting deactylase complexes total acetylation rates
in the brain increased leading to increased rates of transcription and enhanced memory consolidation. By using various learning assays like the Morris water maze test
and fear conditioning assays in conjunction with acetylation
influencing drugs it was shown that acetylation patterns in the
hippocampus are integral to memory association and learning behavior. Studies with various HDAC inhibitors
and neural development have shown increased learning and memory, as a
result of an increased acetylation state. Conversely studies conducted
with HAT inhibitors yielded impairment of memory consolidation and an
overall decrease in learning.
ERK/MAPK cascade
Studies have shown that the ERK/MAPK cascade is important for the regulation of lysine acetylation in the insular cortex of the brain (A part of the brain implicated in the formation of taste
memories). The activation of the ERK/MAPK cascade was seen in mice
after the introduction of a new taste, the cascade was shown to be
necessary for the memory of the taste to be formed. The proposed
mechanism for how this cascade works is that MAPK regulates histone
acetylation and subsequent chromatin remodeling by means of downstream
effectors, such as the CREB binding protein (which has HAT activity). By observing the rates of acetylation in the insular cortex researchers
were able to determine which patterns of acetylation were due to
deacetylase or acetylase activity and which were a result of lysine
acetyltransferase activity.
Long term potentiation
Long term potentiation (LTP) is the enhancement of signal strength between neurons. LTP is the basis of synaptic plasticity and plays a pivotal role in memory formation. LTP is dependent on the activity of NMDA receptors in the brain and it has been shown that NMDA
activity influences acetylation. When NMDA receptors are activated they
cause an influx of calcium into the cell which in turn activates
various signal pathways that ultimately activate the ERK pathway which then modulates transcription factors like CREB.
CREB then recruits a HAT to help create and stabilize the long term
formation of memory, often through the self-perpetuation of acetylated
histones. Studies done on Acetylation of histone H3 in the CA1 region of
the hippocampus show that the activation of NMDA receptors increased
the acetylation of H3 and conversely inhibition of the ERK pathway in
the CA1 region resulted in a decrease in acetylation of H3. In summary:
NMDA-R activation increases phosphorylation of ERK and Acetylation of Histone H3
Memory requires proper NMDA-R function
Memory conditioning increases phosphorylation of ERK and acetylation of Histone H3
HDAC inhibitors enhance LTP, this is dependent on rate of transcription
HDAC inhibitors do not affect NMDA-R
Histone deacetylation
HDACs' role in CREBP-dependent transcriptional activation
Figure 1 HDAC inhibition enhances memory and synaptic plasticity through CREB:CBP. Figure adapted from Vecsey et al. (2007)
Histone deacetylases (HDAC) remove acetyl groups (-COCH3) from histones altering chromatin structures
and decreasing accessibility of transcriptional factors to DNA, thereby
reducing transcription of genes. HDACs have shown to play a role in
learning and memory through their regulation in the CREB-CBP pathway.
Studies conclude that HDAC inhibitors such as trichostatin A (TSA) increase histone acetylation and improve synaptic plasticity and long-term memory (Fig 1A). CREB, a cAMP response element-binding protein and transcriptional activator, binds CBP
forming the CREBP complex. This complex activates genes involved in
synaptic formation and long-term memory.(Fig 1B) TSA treatments in the
hippocampal CA1 region of mice increased acetylation levels and enhanced
long-term potentiation (LTP), a mechanism involved in learning and
memory (Fig 1B). However, TSA treatments in CBP mutants lacking KIX domains
did not effect LTP in mice (Fig 1D). The KIX domain allows for
interaction between CREB and CBP, so knocking out this region disrupts
formation of the CREBP complex. Knock outs of CREB produced similar
results to those of mutant CBP mice (Fig 1C). Therefore, HDAC inhibition
and CREBP association are both necessary for memory development. TSA
treatments showed increased expression levels of Nr4a1 and Nra2
genes while other CREB regulated genes were unaffected. HDAC inhibitors
improve memory through activation of specific genes regulated by CREBP
complex.
HDAC2
The role of individual HDACs in learning and memory is not well understood, but HDAC2 has been shown to negatively regulate memory formation and synaptic plasticity.
Overexpression (OE) of HDAC1
and HDAC2 in mice resulted in decreased levels of acetylated lysines.
After exposing these mice to context and tone-dependent fear
conditioning experiments, HDAC1 OE mice did not change, but HDAC2 OE
mice showed a decrease in freezing behavior, suggesting impairment in
memory formation. On the other hand, mice with HDAC2 knockouts (KO)
illustrated increased freezing levels compared to wild-type (WT) mice
while HDAC1 displayed similar freezing behaviors to WTs. In summary,
Guan et al. have shown that:
Long term potentiation in CA1 neurons was not observed in
HDAC2 OE mice but was easily induced in HDAC2 KO mice. LTP was not
altered between HDAC1 KO and OE mice.
HDAC2 suppresses neuronal gene expression. HDAC2 interacted more than HDAC1 with specific memory-forming promoters such as Bdnf, Egr1, Fos, and GLUR1.
CoREST, a co-repressor, associates with HDAC2 not HDAC1.
SAHA,
a HDAC inhibitor, increased freezing of HDAC2 OE mice in contextual
fear and tone dependent experiments, but did not effect HDAC2 KO mice
suggesting HDAC2 is major target of SAHA
HDAC3
HDAC3 is also a negative regulator of long term potentiation formation. McQuown et al. have shown that:
RGFP136, HDAC3 inhibitor, enhances LTP for object recognition and location
RGFP136 enhances LTP through CBP-dependent mechanism
HDAC3 deletions showed increased Nr4a2 and c-Fos expression
HDAC3 interacts with NCoR and HDAC4 to perform its role in memory formation
HDACs' role in CNS disorders
Research has shown that HDACs and HATs play a crucial role in central nervous system (CNS) disorders such as Rett syndrome. Rubinstein-Tabyi syndrome causes intellectual disability through possible mutations in CREB-binding protein and p300.
However, enhancing expression of CREB-dependent genes or inhibition of
HDAC activity partially restore LTP loss and ameliorate late LTP
deficits. HDAC inhibitor like TSA may provide a possible therapy for
Rubinstein-Tabyi syndrome.
Other memory-deficit disorders which may involve HDAC inhibitors as
potential therapy are:
Role of DNA topoisomerase II beta in learning and memory
During a new learning experience, a set of genes is rapidly expressed in the brain. This induced gene expression is considered to be essential for processing the information being learned. Such genes are referred to as immediate early genes (IEGs). DNA topoisomerase II beta (TOP2B) activity is essential for the expression of IEGs in a type of learning experience in mice termed associative fear memory. Such a learning experience appears to rapidly trigger TOP2B to induce double-strand breaks in the promoter DNA of IEG genes that function in neuroplasticity. Repair of these induced breaks is associated with DNA demethylation of IEG gene promoters allowing immediate expression of these IEG genes.
Regulatory
sequence in a promoter at a transcription start site with a paused RNA
polymerase and a TOP2B-induced double-strand breakBrain regions involved in memory formation including medial prefrontal cortex (mPFC)
The double-strand breaks that are induced during a
learning experience are not immediately repaired. About 600 regulatory
sequences in promoters and about 800 regulatory sequences in enhancers appear to depend on double strand breaks initiated by topoisomerase 2-beta (TOP2B) for activation. The induction of particular double-strand breaks are specific with
respect to their inducing signal. When neurons are activated in vitro, just 22 of TOP2B-induced double-strand breaks occur in their genomes.
Such TOP2B-induced double-strand breaks are accompanied by at least four enzymes of the non-homologous end joining (NHEJ) DNA repair pathway
(DNA-PKcs, KU70, KU80 and DNA LIGASE IV) (see Figure). These enzymes
repair the double-strand breaks within about 15 minutes to two hours. The double-strand breaks in the promoter are thus associated with
TOP2B and at least these four repair enzymes. These proteins are
present simultaneously on a single promoter nucleosome
(there are about 147 nucleotides in the DNA sequence wrapped around a
single nucleosome) located near the transcription start site of their
target gene.
Contextual fear conditioning
in the mouse causes the mouse to have a long-term memory and fear of
the location in which it occurred. Contextual fear conditioning causes
hundreds of DSBs in mouse brain medial prefrontal cortex (mPFC) and
hippocampus neurons (see Figure: Brain regions involved in memory
formation). These DSBs predominately activate genes involved in
synaptic processes, that are important for learning and memory.
Roles of ROS and OGG1 in memory and learning
Initiation of DNA demethylation at a CpG site. In adult somatic cells DNA methylation typically occurs in the context of CpG dinucleotides (CpG sites), forming 5-methylcytosine-pG, or 5mCpG. Reactive oxygen species (ROS) may attack guanine at the dinucleotide site, forming 8-hydroxy-2'-deoxyguanosine (8-OHdG), and resulting in a 5mCp-8-OHdG dinucleotide site. The base excision repair enzyme OGG1 targets 8-OHdG and binds to the lesion without immediate excision. OGG1, present at a 5mCp-8-OHdG site recruits TET1 and TET1 oxidizes the 5mC adjacent to the 8-OHdG. This initiates demethylation of 5mC.Demethylation of 5-Methylcytosine (5mC) in neuron DNA. As reviewed in 2018, in brain neurons, 5mC is oxidized by the ten-eleven translocation (TET) family of dioxygenases (TET1, TET2, TET3) to generate 5-hydroxymethylcytosine
(5hmC). In successive steps TET enzymes further hydroxylate 5hmC to
generate 5-formylcytosine (5fC) and 5-carboxylcytosine (5caC). Thymine-DNA glycosylase (TDG) recognizes the intermediate bases 5fC and 5caC and excises the glycosidic bond resulting in an apyrimidinic site (AP site).
In an alternative oxidative deamination pathway, 5hmC can be
oxidatively deaminated by activity-induced cytidine
deaminase/apolipoprotein B mRNA editing complex (AID/APOBEC) deaminases to form 5-hydroxymethyluracil (5hmU) or 5mC can be converted to thymine (Thy). 5hmU can be cleaved by TDG, single-strand-selective monofunctional uracil-DNA glycosylase 1 (SMUG1), Nei-Like DNA Glycosylase 1 (NEIL1), or methyl-CpG binding protein 4 (MBD4). AP sites and T:G mismatches are then repaired by base excision repair (BER) enzymes to yield cytosine (Cyt).
As reviewed by Massaad and Klann in 2011 and by Beckhauser et al. in 2016, reactive oxygen species (ROS) are required for normal learning and memory functions.
One of the most frequent DNA oxidation products of ROS is 8-hydroxy-2'-deoxyguanosine (8-OHdG). Removal of oxidized bases in DNA usually occurs in a matter of minutes, with a half-life of 11 minutes for 8-OHdG. Steady-state levels of endogenous
DNA damages represent the balance between formation and repair.
8-OHdGs are among the most frequent DNA damages present in the
steady-state, with about 2,400 8-OHdG damaged nucleotides in the average
mammalian cell. The steady state 8-OHdG level in the brain is similar to that in other tissues.
The occurrence of 8-OHdG in neurons appears to have a role in memory and learning. The DNA glycosylase oxoguanine glycosylase (OGG1) is the primary enzyme responsible for the excision of 8-OHdG in base excision repair.
However, OGG1, which targets and associates with 8-OHdG, also has a
role in adaptive behavior, which implies a physiologically relevant role
for 8-OHdG combined with OGG1 in cognition in the adult brain. In particular, heterozygous OGG1+/- mice, with about half the protein
level of OGG1, exhibit poorer learning performance in the Barnes maze
compared to wild-type animals.
In adult somatic cells, such as neurons, DNA methylation typically occurs in the context of CpG dinucleotides (CpG sites), forming 5-methylcytosine (5mC). Thus, a CpG site may be methylated to form 5mCpG. The presence of 5mC
at CpG sites in gene promoters is widely considered to be an epigenetic
mark that acts to suppress transcription. If the guanine at the 5mCpG site is attacked by ROS, leading to
8-OHdG formation, OGG1 binds to the 8-OHdG lesion without immediate
excision of the 8-OHdG. When OGG1 is present at a 5mCp-8-OHdG site, it
recruits TET1 to the 8-OHdG lesion and TET1 oxidizes the 5mC adjacent to 8-OHdG. This causes the 5mC to enter the DNA demethylation pathway (see Figure titled "Initiation of DNA demethylation at a CpG site"). This pathway is initiated by formation of 5-hydroxymethylcytosine,
which may remain in the DNA, or there may be further oxidative
reactions followed by base excision repair, to return the nucleoside at
that position to cytosine (see Figure "Demethylation of 5-Methylcytosine
(5mC) in neuron DNA").
The total number of CpG sites
in the human genome is approximately 28 million and the average
frequency of CpG sites in the genome is about 1 per hundred base pairs. An intense learning situation can be applied to rats, referred to as contextual fear conditioning. This can result in a life-long fearful memory after a single training event. While the long-term memory of this event appears to be first stored in the hippocampus, this storage is transient and does not remain in the hippocampus. Much of the long-term storage of contextual fear conditioning memory appears to take place in the anterior cingulate cortex. (See Figure: Brain regions involved in memory formation, and also this reference.) When contextual fear conditioning is applied to a rat, more than 5,000 differentially methylated regions (DMRs) of 500 nucleotides each occur in the rat hippocampus neural genome both one hour and 24 hours after the conditioning in the hippocampus. This causes about 500 genes to be up-regulated (often due to
hypomethylation of CpG sites) and about 1,000 genes to be down-regulated
(often due to newly formed 5mC at CpG sites in a promoter region). The
pattern of induced and repressed genes within neurons appears to provide
a molecular basis for forming this first transient memory of this
training event in the hippocampus of the rat brain. When similar contextual fear conditioning is applied to a mouse, one
hour after contextual fear conditioning there were 675 demethylated
genes and 613 hypermethylated genes in the hippocampus region of the
mouse brain. These changes were transient in the hippocampal neurons, and almost
none were present after four weeks. However, in mice subjected to
conditional fear conditioning, after four weeks there were more than
1,000 differentially methylated genes and more than 1,000 differentially
expressed genes in the anterior cingulate cortex, where long-term memories are stored in the mouse brain.
The Big Bounce hypothesis is a cosmological model for the origin of the known universe. It was originally suggested as a phase of the cyclic model or oscillatory universe interpretation of the Big Bang, where the first cosmological event was the result of the collapse of a previous universe. It receded from serious consideration in the early 1980s after inflation theory emerged as a solution to the horizon problem, which had arisen from advances in observations revealing the large-scale structure of the universe.
Inflation was found to be inevitably eternal,
creating an infinity of different universes with typically different
properties, suggesting that the properties of the observable universe
are a matter of chance. An alternative concept that included a Big Bounce was conceived as a
predictive and falsifiable possible solution to the horizon problem. Investigation continued as of 2022.
Expansion and contraction
The concept of the Big Bounce envisions the Big Bang as the beginning of a period of expansion that followed a period of contraction. In this view, one could talk of a "Big Crunch"
followed by a "Big Bang" or, more simply, a "Big Bounce". This concept
suggests that we could exist at any point in an infinite sequence of
universes, or conversely, the current universe could be the very first
iteration. However, if the condition of the interval phase "between
bounces"—considered the "hypothesis of the primeval atom"—is taken into
full contingency, such enumeration may be meaningless because that condition could represent a singularity in time at each instance if such perpetual repeats (cycles) were absolute and undifferentiated.
The main idea behind the quantum theory of a Big Bounce is that, as density approaches infinity, the behavior of quantum foam changes. All the so-called fundamental physical constants,
including the speed of light in vacuum, need not remain constant during
a Big Crunch, especially in the time interval smaller than that in
which measurement may never be possible (one unit of Planck time, roughly 10−43 seconds) spanning or bracketing the point of inflection.
By the early 1980s, the advancing precision and scope of observational cosmology had revealed that the large-scale structure of the universe is flat, homogeneous, and isotropic, a finding later accepted as the cosmological principle to apply at scales beyond roughly 300 million light-years. This led cosmologists to seek an explanation to the horizon problem,
which questioned how distant regions of the universe could have
identical properties without ever being in light-like communication. A
solution was proposed to be a period of exponential expansion of space
in the early universe, which formed the basis of what became known as inflation theory. Following the brief inflationary period, the universe continues to expand at a slower rate.
Various formulations of inflation theory and their detailed
implications became the subject of intense theoretical study. Without a
compelling alternative, inflation became the leading solution to the
horizon problem.
The phrase "Big Bounce" appeared in scientific literature
in 1987, when it was first used in the title of a pair of articles (in
German) in Stern und Weltraum by Wolfgang Priester and Hans-Joachim Blome. It reappeared in 1988 in Iosif Rozental's Big Bang, Big Bounce,
a revised English-language translation of a Russian-language book (by a
different title), and in a 1991 English-language article by Priester
and Blome in Astronomy and Astrophysics. The phrase originated as the title of a novel by Elmore Leonard in 1969, shortly after increased public awareness of the Big Bang model with of the discovery of the cosmic microwave background by Penzias and Wilson in 1965.
The idea of the existence of a big bounce in the very early universe has found diverse support in works based on loop quantum gravity. In loop quantum cosmology,
a branch of loop quantum gravity, the big bounce was first discovered
in February 2006 for isotropic and homogeneous models by Abhay Ashtekar, Tomasz Pawlowski, and Parampreet Singh at Pennsylvania State University. This result has been generalized to various other models by different
groups, and includes the case of spatial curvature, cosmological
constant, anisotropies, and Fock quantized inhomogeneities.
Martin Bojowald,
an assistant professor of physics at Pennsylvania State University,
published a study in July 2007 detailing work related to loop quantum
gravity that claimed to mathematically solve the time before the Big
Bang, which would give new weight to the oscillatory universe and Big
Bounce theories.
One of the main problems with the Big Bang theory is that there is a singularity
of zero volume and infinite energy at the moment of the Big Bang. This
is normally interpreted as a breakdown of physics as we know it; in this
case, of the theory of general relativity. This is why one expects quantum effects to become important and avoid a singularity.
However, research in loop quantum cosmology purported to
show that a previously existing universe collapses not to a singularity,
but to a point where the quantum effects of gravity become so strongly
repulsive that the universe rebounds back out, forming a new branch.
Throughout this collapse and bounce, the evolution is unitary.
Bojowald also claimed that some properties of the universe
that collapsed to form ours can be determined; however, other properties
are not determinable due to some uncertainty principle.
This result has been disputed by different groups, which show that due
to restrictions on fluctuations stemming from the uncertainty principle,
there are strong constraints on the change in relative fluctuations
across the bounce.
While the existence of the Big Bounce has still to be
demonstrated from loop quantum gravity, the robustness of its main
features has been confirmed using exact results and several studies involving numerical simulations using high performance computing in loop quantum cosmology.
In 2006, it was proposed that the application of loop
quantum gravity techniques to Big Bang cosmology can lead to a bounce
that need not be cyclic.
In 2010, Roger Penrose advanced a general relativity-based theory which he called the "conformal cyclic cosmology".
The theory explains that the universe will expand until all matter
decays and ultimately turns to light. Since nothing in the universe
would have any time or distance scale associated with it, the universe
becomes identical with the Big Bang, resulting in a type of Big Crunch
that becomes the next Big Bang, thus perpetuating the next cycle.
In 2011, Nikodem Popławski showed that a nonsingular Big Bounce appears naturally in the Einstein–Cartan–Sciama–Kibble theory of gravity. This theory extends general relativity by removing a constraint of the symmetry of the affine connection and regarding its antisymmetric part, the torsion tensor, as a dynamical variable. The minimal coupling between torsion and Dirac spinors
generates a spin-spin interaction which is significant in fermionic
matter at extremely high densities. Such an interaction avoids the
unphysical Big Bang singularity, replacing it with a cusp-like bounce at
a finite minimum scale factor, before which the universe was
contracting. This scenario also explains why the present Universe at the
largest scales appears spatially flat, homogeneous, and isotropic,
providing a physical alternative to cosmic inflation.
In 2012, a new theory of a nonsingular Big Bounce was constructed within the frame of standard Einstein gravity. This theory combines the benefits of matter bounce and ekpyrotic cosmology. Particularly, in the homogeneous and isotropic background cosmological solution, the BKL instability
is unstable to the growth of anisotropic stress, which is resolved in
this theory. Moreover, curvature perturbations seeded in matter
contraction can form a nearly scale-invariant primordial power spectrum
and thus provide a consistent mechanism to explain the cosmic microwave background (CMB) observations.
Critics
According to a study published in Physical Review Letters in May 2023, the Big Bounce should have left marks in the primordial light, known as the cosmic microwave background (CMB), but comparing observations conducted by the Planck satellite with the simulated CMB in the case the Universe bounced on itself only once, that particular bounce signature was not found.
Proponents of democratic peace theory argue that electoraldemocracies are hesitant to engage in armed conflict
with other identified democracies. Different advocates of this theory
suggest that several factors are responsible for motivating peace
between democratic states.Individual theorists maintain "monadic" forms of this theory
(democracies are in general more peaceful in their international
relations); "dyadic" forms of this theory (democracies do not go to war
with other democracies); and "systemic" forms of this theory (more
democratic states in the international system makes the international
system more peaceful).
In terms of norms and identities, it is hypothesized that democracies are more dovish in their interactions with other democracies, and that democratically elected leaders are more likely to resort to peaceful resolution in disputes
(both in domestic politics and international politics). In terms of
structural or institutional constraints, it is hypothesized that
institutional checks and balances, accountability of leaders to the public, and larger winning coalitions make it harder for democratic leaders to go to war unless there are clearly favorable ratio of benefits to costs.
These structural constraints, along with the transparent
nature of democratic politics, make it harder for democratic leaders to
mobilize for war and initiate surprise attacks,
which reduces fear and inadvertent escalation to war. The transparent
nature of democratic political systems, as well as deliberative debates
(involving opposition parties, the media, experts, and bureaucrats), make it easier for democratic states to credibly signal their intentions. The concept of audience costs
entails that threats issued by democratic leaders are taken more
seriously because democratic leaders will be electorally punished by
their citizens from backing down from threats, which reduces the risk of
misperception and miscalculation by states.
The connection between peace and democracy has long been recognized, but theorists disagree about the direction of causality. The democratic peace theory posits that democracy causes peace, while the territorial peace theory makes the opposite claim that peace causes democracy. Other theories argue that omitted variables
explain the correlation better than democratic peace theory.
Alternative explanations for the correlation of peace among democracies
include arguments revolving around institutions, commerce, interdependence, alliances, US world dominance and political stability. There are instances in the historical record that serve as exceptions to the democratic peace theory.
Though the democratic peace theory was not rigorously or
scientifically studied until the 1960s, the basic principles of the
concept had been argued as early as the 18th century in the works of
philosopher Immanuel Kant and political theorist Thomas Paine. Kant foreshadowed the theory in his essay Perpetual Peace: A Philosophical Sketch
written in 1795, although he thought that a world with only
constitutional republics was only one of several necessary conditions
for a perpetual peace. In earlier but less cited works, Thomas Paine
made similar or stronger claims about the peaceful nature of republics.
Paine wrote in "Common Sense"
in 1776: "The Republics of Europe are all (and we may say always) in
peace." Paine argued that kings would go to war out of pride in
situations where republics would not. French historian and social scientist Alexis de Tocqueville also argued, in Democracy in America (1835–1840), that democratic nations were less likely to wage war. Herbert Spencer also argued for a relationship between democracy and peace. At the Geneva Congress of the League of Peace and Freedom in 1867, Giuseppe Garibaldi declared that "democracy alone can provide a guarantee against the evils of war".
Dean Babst,
a criminologist, was the first to do statistical research on this
topic. His academic paper supporting the theory was published in 1964 in
Wisconsin Sociologist; he published a slightly more popularized version, in 1972, in the trade journal Industrial Research. Both versions initially received little attention.
Melvin Small and J. David Singer
responded; they found an absence of wars between democratic states with
two "marginal exceptions", but denied that this pattern had statistical
significance. This paper was published in the Jerusalem Journal of International Relations which finally brought more widespread attention to the theory, and started the academic debate. A 1983 paper by political scientist Michael W. Doyle contributed further to popularizing the theory.
Maoz and Abdolali extended the research to lesser conflicts than wars. Bremer, Maoz and Russett found the correlation between democracy and
peacefulness remained significant after controlling for many possible
confounding variables.This moved the theory into the mainstream of social science. Supporters of realism in international relations
and others responded by raising many new objections. Other researchers
attempted more systematic explanations of how democracy might cause
peace, and of how democracy might also affect other aspects of foreign relations such as alliances and collaboration.
There have been numerous further studies in the field since these pioneering works. Most studies have found some form of democratic peace exists, although
neither methodological disputes nor doubtful cases are entirely
resolved.
Definitions
World population by political regime type
Research on the democratic peace theory has to define "democracy" and "peace" (or, more often, "war").
Defining democracy
Democracies have been defined differently by different
theorists and researchers; this accounts for some of the variations in
their findings. Some examples:
Small and Singer define democracy as a nation that (1)
holds periodic elections in which the opposition parties are as free to
run as government parties, (2) allows at least 10% of the adult
population to vote, and (3) has a parliament that either controls or
enjoys parity with the executive branch of the government.
Doyle requires (1) that "liberal regimes" have market or
private property economics, (2) they have policies that are internally
sovereign, (3) they have citizens with juridical rights, and (4) they
have representative governments. Either 30% of the adult males were able
to vote or it was possible for every man to acquire voting rights as by
attaining enough property. He allows greater power to hereditary
monarchs than other researchers; for example, he counts the rule of Louis-Philippe of France as a liberal regime.
Ray requires that at least 50% of the adult population is
allowed to vote and that there has been at least one peaceful,
constitutional transfer of executive power from one independent
political party to another by means of an election. This definition
excludes long periods often viewed as democratic. For example, the
United States until 1800, India from independence until 1979, and Japan
until 1993 were all under a dominant-party system, and thus would not be counted under this definition.
Rummel states that "By democracy is meant liberal
democracy, where those who hold power are elected in competitive
elections with a secret ballot and wide franchise (loosely understood as
including at least 2/3 of adult males); where there is freedom of
speech, religion, and organization; and a constitutional framework of
law to which the government is subordinate and that guarantees equal
rights."
Non-binary classifications
The above definitions are binary, classifying nations into
either democracies or non-democracies. Many researchers have instead
used more finely grained scales. One example is the Polity data series
which scores each state on two scales, one for democracy and one for
autocracy, for each year since 1800; as well as several others. The use of the Polity Data has varied. Some researchers have done
correlations between the democracy scale and belligerence; others have
treated it as a binary classification by (as its maker does) calling all
states with a high democracy score and a low autocracy score
democracies; yet others have used the difference of the two scores,
sometimes again making this into a binary classification.
Young democracies
Several researchers have observed that many of the possible
exceptions to the democratic peace have occurred when at least one of
the involved democracies was very young. Many of them have therefore
added a qualifier, typically stating that the peacefulness apply to
democracies older than three years. Rummel argues that this is enough time for "democratic procedures to be
accepted, and democratic culture to settle in." Additionally, this may
allow for other states to actually come to the recognition of the state
as a democracy.
Mansfield and Snyder, while agreeing that there have been
no wars between mature liberal democracies, state that countries in
transition to democracy are especially likely to be involved in wars.
They find that democratizing countries are even more warlike than stable
democracies, stable autocracies or even countries in transition towards
autocracy. So, they suggest caution in eliminating these wars from the
analysis, because this might hide a negative aspect of the process of
democratization. A reanalysis of the earlier study's statistical results emphasizes that
the above relationship between democratization and war can only be said
to hold for those democratizing countries where the executive lacks
sufficient power, independence, and institutional strength. A review cites several other studies finding that the increase in the
risk of war in democratizing countries happens only if many or most of
the surrounding nations are undemocratic. If wars between young democracies are included in the analysis, several
studies and reviews still find enough evidence supporting the stronger
claim that all democracies, whether young or established, go into war
with one another less frequently; while some do not.
Defining war
Quantitative research on international wars usually
defines war as a military conflict with more than 1000 killed in battle
in one year. This is the definition used in the Correlates of War Project
which has also supplied the data for many studies on war. It turns out
that most of the military conflicts in question fall clearly above or
below this threshold.
Some researchers have used different definitions. For example, Weart defines war as more than 200 battle deaths. Russett, when looking at Ancient Greece, only requires some real battle
engagement, involving on both sides forces under state authorization.
Militarized Interstate Disputes
(MIDs), in the Correlates of War Project classification, are lesser
conflicts than wars. Such a conflict may be no more than military
display of force with no battle deaths. MIDs and wars together are
"militarized interstate conflicts" or MICs. MIDs include the conflicts
that precede a war; the difference between MIDs and MICs may thus be
less than it appears, though this remains an area of investigation.
Statistical analysis and concerns about degrees of freedom
are the primary reasons for using MID's instead of actual wars. Wars
are relatively rare. An average ratio of 30 MIDs to one war provides a
richer statistical environment for analysis.
Monadic vs. dyadic peace
Most research is regarding the dyadic peace, that democracies do not fight one another. Very few researchers have supported the monadic
peace, that democracies are more peaceful in general. There are some
recent papers that find a slight monadic effect. Müller and Wolff, in
listing them, agree "that democracies on average might be slightly, but
not strongly, less warlike than other states," but general "monadic
explanations are neither necessary nor convincing." They note that
democracies have varied greatly in their belligerence against
non-democracies.
Exceptions
Some scholars support the democratic peace on
probabilistic grounds: since many wars have been fought since
democracies first arose, we might expect a proportionate number of wars
to have occurred between democracies, if democracies fought each other
as freely as other pairs of states; but proponents of democratic peace
theory claim that the number is much less than might be expected. However, opponents of the theory argue this is mistaken and claim there are numerous examples of wars between democracies.
Historically, troublesome cases for the democratic peace theory include the Sicilian Expedition, the War of 1812, the Mexican-American War, the U.S. Civil War, the Fashoda Crisis, conflicts between Ecuador and Peru, the Cod Wars, the Spanish–American War, and the Kargil War. Not all scholars agree on specific exceptions to the democratic peace theory, if any. Doyle cites the Paquisha War and the Lebanese air force's intervention in the Six-Day War. The total number of cases suggested in the literature is at least 50. The data set Bremer was using showed one exception, the French-Thai War of 1940; Gleditsch sees the state of war between Finland and United Kingdom during World War II,
as a special case, which should probably be treated separately: an
incidental state of war between democracies during large and complex war
with hundreds of belligerents and the constant shifting of geopolitical
and diplomatic boundaries. However, the British did conduct a few military actions of minor scope
against the Finns, more to demonstrate their alliance with the Soviets
than to actually engage in war with Finland. Page Fortna discusses the 1974 Turkish invasion of Cyprus and the Kargil War as exceptions, finding the latter to be the most significant.
Conflict initiation
According to a 2017 review study, "there is enough
evidence to conclude that democracy does cause peace at least between
democracies, that the observed correlation between democracy and peace
is not spurious".
Most studies have looked only at who is involved in the
conflicts and ignored the question of who initiated the conflict. In
many conflicts both sides argue that the other side was the initiator.
Several researchers have argued that studying conflict initiation is of
limited value, because existing data about conflict initiation may be
especially unreliable. Even so, several studies have examined this. Reitner and Stam argue
that autocracies initiate conflicts against democracies more frequently
than democracies do against autocracies. Quackenbush and Rudy, while confirming Reiter and Stam's results, find
that democracies initiate wars against non-democracies more frequently
than non-democracies do to each other. Several following studies have studied how different types of
autocracies with different institutions vary regarding conflict
initiation. Personalistic and military dictatorships may be particularly
prone to conflict initiation, as compared to other types of autocracy
such as one-party states, but also more likely to be targeted in a war having other initiators.
One 2017 study found that democracies are no more likely to settle border disputes peacefully than non-democracies.
Internal violence and genocide
As well as improved releations between states, there is
also evidence that democracies have less internal systematic violence.
For instance, one study finds that the most democratic and the most
authoritarian states have few civil wars,
and intermediate regimes the most. The probability for a civil war is
also increased by political change, regardless whether toward greater
democracy or greater autocracy. Intermediate regimes continue to be the
most prone to civil war, regardless of the time since the political
change. In the long run, since intermediate regimes are less stable than
autocracies, which in turn are less stable than democracies, durable
democracy is the most probable end-point of the process of democratization. Abadie's study finds that the most democratic nations have the least terrorism. Harff finds that genocide and politicide are rare in democracies. Rummel finds that the more democratic a regime, the less its democide. He finds that democide has killed six times as many people as battles.
Davenport and Armstrong II list several other studies and
states: "Repeatedly, democratic political systems have been found to
decrease political bans, censorship, torture, disappearances and mass
killing, doing so in a linear fashion across diverse measurements,
methodologies, time periods, countries, and contexts." It concludes:
"Across measures and methodological techniques, it is found that below a
certain level, democracy has no impact on human rights violations, but
above this level democracy influences repression in a negative and
roughly linear manner." They also state that thirty years worth of statistical research has
revealed that only two variables decrease human rights violations:
political democracy and economic development.
Abulof and Goldman add a caveat, focusing on the
contemporary Middle East and North Africa (MENA). Statistically, a MENA
democracy makes a country more prone to both the onset and incidence of
civil war, and the more democratic a MENA state is, the more likely it
is to experience violent intrastate strife. Moreover, anocracies
do not seem to be predisposed to civil war, either worldwide or in
MENA. Looking for causality beyond correlation, they suggest that
democracy's pacifying effect is partly mediated through societal
subscription to self-determination and popular sovereignty. This may
turn “democratizing nationalism” to a long-term prerequisite, not just
an immediate hindrance, to peace and democracy.
Explanations
These theories have traditionally been categorized into
two groups: explanations that focus on democratic norms and explanations
that focus on democratic political structures. They usually are meant to be explanations for little violence between
democracies, not for a low level of internal violence in democracies.
Several of these mechanisms may also apply to countries of similar systems. The book Never at War
finds evidence that the oligarchic republics common in ancient Greece
and medieval and early modern Europe hardly ever made war on one
another. One example is the Polish–Lithuanian Commonwealth, in which the Sejm resisted and vetoed most royal proposals for war, like those of Władysław IV Vasa.
A study by V-Dem Institute found both interbranch constraint on the executive and civil society activism as the mechanism for democratic peace but found accountability provided directly by elections not as crucial.
Democratic norms
One example from the first group is that liberal
democratic culture may make the leaders accustomed to negotiation and
compromise. Policy makers who have built their careers within a
political culture of non-violent accommodations with domestic rivals,
unlike autocrats who typically hold power through the threat of
coercion, will be inclined toward non-violent methods abroad. Another that a belief in human rights may make people in democracies
reluctant to go to war, especially against other democracies. The
decline in colonialism, also by democracies, may be related to a change
in perception of non-European peoples and their rights.
Bruce Russett also argues that the democratic culture
affects the way leaders resolve conflicts. In addition, he holds that a
social norm emerged toward the end of the nineteenth century; that
democracies should not fight each other, which strengthened when the
democratic culture and the degree of democracy increased, for example by
widening the franchise. Increasing democratic stability allowed
partners in foreign affairs to perceive a nation as reliably democratic.
The alliances between democracies during the two World Wars and the
Cold War also strengthened the norms. He sees less effective traces of
this norm in Greek antiquity.
Hans Köchler relates the question of transnational democracy to empowering the individual citizen by involving him, through procedures of direct democracy,
in a country's international affairs, and he calls for the
restructuring of the United Nations Organization according to democratic
norms. He refers in particular to the Swiss practice of participatory democracy.
Mousseau argues that it is market-oriented development
that creates the norms and values that explain both democracy and the
peace. In less developed countries individuals often depend on social
networks that impose conformity to in-group norms and beliefs, and
loyalty to group leaders. When jobs are plentiful on the market, in
contrast, as in market-oriented developed countries, individuals depend
on a strong state that enforces contracts equally. Cognitive routines
emerge of abiding by state law rather than group leaders, and, as in
contracts, tolerating differences among individuals. Voters in
marketplace democracies thus accept only impartial ‘liberal’
governments, and constrain leaders to pursue their interests in securing
equal access to global markets and in resisting those who distort such
access with force. Marketplace democracies thus share common foreign
policy interests in the supremacy—and predictability—of international
law over brute power politics, and equal and open global trade over
closed trade and imperial preferences. When disputes do originate
between marketplace democracies, they are less likely than others to
escalate to violence because both states, even the stronger one,
perceive greater long-term interests in the supremacy of law over power
politics.
Braumoeller argues that liberal norms of conflict
resolution vary because liberalism takes many forms. By examining survey
results from the newly independent states of the former Soviet Union,
the author demonstrates that liberalism in that region bears a stronger
resemblance to 19th-century liberal nationalism than to the sort of
universalist, Wilsonian liberalism described by democratic peace
theorists, and that, as a result, liberals in the region are more, not less, aggressive than non-liberals.
A 2013 study by Jessica Weeks
and Michael Tomz found through survey experiments that the public was
less supportive of war in cases involving fellow democracies.
Democratic political structures
The case for institutional constraints goes back to Immanuel Kant, who wrote:
[I]f
the consent of the citizens is required in order to decide that war
should be declared (and in this constitution it cannot but be the case),
nothing is more natural than that they would be very cautious in
commencing such a poor game, decreeing for themselves all the calamities
of war. Among the latter would be: having to fight, having to pay the
costs of war from their own resources, having painfully to repair the
devastation war leaves behind, and, to fill up the measure of evils,
load themselves with a heavy national debt that would embitter peace
itself and that can never be liquidated on account of constant wars in
the future.
Democracy thus gives influence to those most likely to be
killed or wounded in wars, and their relatives and friends (and to those
who pay the bulk of the war taxes.) This monadic theory must, however, explain why democracies do attack
non-democratic states. One explanation is that these democracies were
threatened or otherwise were provoked by the non-democratic states.
Doyle argued that the absence of a monadic peace is only to be expected:
the same ideologies that cause liberal states to be at peace with each
other inspire idealistic wars with the illiberal, whether to defend
oppressed foreign minorities or avenge countrymen settled abroad. Doyle also notes liberal states do conduct covert operations against
each other; the covert nature of the operation, however, prevents the
publicity otherwise characteristic of a free state from applying to the
question.
Charles Lipson
argues that four factors common in democracies give them a "contracting
advantage" that leads to a dyadic democratic peace: (1) Greater
transparency, (2) Greater continuity, (3) Electoral incentives for
leaders to keep promises, and (4) Constitutional governance.
Studies show that democratic states are more likely than
autocratic states to win the wars that they start. One explanation is
that democracies, for internal political and economic reasons, have
greater resources. This might mean that democratic leaders are unlikely
to select other democratic states as targets because they perceive them
to be particularly formidable opponents. One study finds that interstate
wars have important impacts on the fate of political regimes, and that
the probability that a political leader will fall from power in the wake
of a lost war is particularly high in democratic states.
As described by Gelpi and Griesdorf, several studies have
argued that liberal leaders face institutionalized constraints that
impede their capacity to mobilize the state's resources for war without
the consent of a broad spectrum of interests. Survey results that compare the attitudes of citizens and elites in the
Soviet successor states are consistent with this argument. Moreover, these constraints are readily apparent to other states and
cannot be manipulated by leaders. Thus, democracies send credible
signals to other states of an aversion to using force. These signals
allow democratic states to avoid conflicts with one another, but they
may attract aggression from non-democratic states. Democracies may be
pressured to respond to such aggression—perhaps even
preemptively—through the use of force. Also as described by Gelpi and
Griesdorf, studies have argued that when democratic leaders do choose to
escalate international crises, their threats are taken as highly
credible, since there must be a relatively large public opinion for
these actions. In disputes between liberal states, the credibility of
their bargaining signals allows them to negotiate a peaceful settlement
before mobilization. A 2017 study by Jeff Carter found evidence that democratic states are slower to mobilize for war.
An explanation based on game theory
similar to the last two above is that the participation of the public
and the open debate send clear and reliable information regarding the
intentions of democracies to other states. In contrast, it is difficult
to know the intentions of non-democratic leaders, what effect
concessions will have, and if promises will be kept. Thus there will be
mistrust and unwillingness to make concessions if at least one of the
parties in a dispute is a non-democracy.
The risk factors for certain types of state have, however,
changed since Kant's time. In the quote above, Kant points to the lack
of popular support for war – first that the populace will directly or
indirectly suffer in the event of war – as a reason why republics will
not tend to go to war. The number of American troops killed or maimed
versus the number of Iraqi soldiers and civilians maimed and killed in
the American-Iraqi conflict is indicative. This may explain the
relatively great willingness of democratic states to attack weak
opponents: the Iraq war was, initially at least, highly popular in the
United States. The case of the Vietnam War
might, nonetheless, indicate a tipping point where publics may no
longer accept continuing attrition of their soldiers (even while
remaining relatively indifferent to the much higher loss of life on the
part of the populations attacked).
Coleman uses economic cost-benefit analysis to reach
conclusions similar to Kant's. Coleman examines the polar cases of
autocracy and liberal democracy. In both cases, the costs of war are
assumed to be borne by the people. In autocracy, the autocrat receives
the entire benefits of war, while in a liberal democracy the benefits
are dispersed among the people. Since the net benefit to an autocrat
exceeds the net benefit to a citizen of a liberal democracy, the
autocrat is more likely to go to war. The disparity of benefits and
costs can be so high that an autocrat can launch a welfare-destroying
war when his net benefit exceeds the total cost of war. Contrarily, the
net benefit of the same war to an individual in a liberal democracy can
be negative so that he would not choose to go to war. This disincentive
to war is increased between liberal democracies through their
establishment of linkages, political and economic, that further raise
the costs of war between them. Therefore, liberal democracies are less
likely to go war, especially against each other. Coleman further
distinguishes between offensive and defensive wars and finds that
liberal democracies are less likely to fight defensive wars that may
have already begun due to excessive discounting of future costs.
Brad LeVeck and Neil Narang argue that democratic states
are less likely to produce decision-making errors in crises due to a
larger and more diverse set of actors who are involved in the foreign
policy decision-making process.
Using selectorate theory,
Bruce Bueno de Mesquita, James D. Morrow, Randolph M. Siverson and
Alastair Smith argue that the democratic peace stems in part from the
fact that democratic leaders sustain their power through large winning
coalitions, which means that democratic leaders devote more resources to
war, have an advantage in war, and choose wars that they are highly
likely to win. These leads democratic states to avoid one another, but
war with weak non-democratic states.
Audience costs
A prominent rational choice argument for the democratic
peace is that democracies carry greater audience costs than
authoritarian states, which makes them better at signaling their
intentions in interstate disputes.Arguments regarding the credibility of democratic states in disputes
has been subject to debate among international relations scholars. Two
studies from 2001, using the MID and ICB datasets, provided empirical support for the notion that democracies were more likely to issue effective threats. However, a 2012 study by Alexander B. Downes and Todd S. Sechser found
that existing datasets were not suitable to draw any conclusions as to
whether democratic states issued more effective threats. They constructed their own dataset specifically for interstate military
threats and outcomes, which found no relationship between regime type
and effective threats. A 2017 study which recoded flaws in the MID dataset ultimately
conclude, "that there are no regime-based differences in dispute
reciprocation, and prior findings may be based largely on poorly coded
data." Other scholars have disputed the democratic credibility argument, questioning its causal logic and empirical validity. Research by Jessica Weeks argued that some authoritarian regime types have similar audience costs as in democratic states.
A 2021 study found that Americans perceived democracies to
be more likely to back down in crises, which contradicts the
expectations of the audience costs literature.
Democracy differences
One general criticism motivating research of different
explanations is that actually the theory cannot claim that "democracy
causes peace", because the evidence for democracies being, in general,
more peaceful is very slight or nonexistent; it only can support the
claim that "joint democracy causes peace". According
to Rosato, this casts doubts on whether democracy is actually the cause
because, if so, a monadic effect would be expected.
Perhaps the simplest explanation to such perceived anomaly
(but not the one the Realist Rosato prefers, see the section on Realist
explanations below) is that democracies are not peaceful to each other
because they are democratic, but rather because they are similar in
democratic scores. This line of thought started with several independent
observations of an "Autocratic Peace" effect, a reduced probability of
war (obviously no author claims its absence) between states which are
both non-democratic, or both highly so. This has led to the hypothesis that democratic peace emerges as a
particular case when analyzing a subset of states which are, in fact,
similar. Or, that similarity in general does not solely affect the probability
of war, but only coherence of strong political regimes such as full
democracies and stark autocracies.
Autocratic peace and the explanation based on democratic
similarity is a relatively recent development, and opinions about its
value are varied. Henderson builds a model considering political
similarity, geographic distance and economic interdependence as its main
variables, and concludes that democratic peace is a statistical
artifact which disappears when the above variables are taken into
account. Werner finds a conflict reducing effect from political similarity in
general, but with democratic dyads being particularly peaceful, and
noting some differences in behavior between democratic and autocratic
dyads with respect to alliances and power evaluation. Beck, King, and Zeng use neural networks to show two distinct low
probability zones, corresponding to high democracy and high autocracy.Petersen uses a different statistical model and finds that autocratic
peace is not statistically significant, and that the effect attributed
to similarity is mostly driven by the pacifying effect of joint
democracy. Ray similarly disputes the weight of the argument on logical grounds,
claiming that statistical analysis on "political similarity" uses a main
variable which is an extension of "joint democracy" by linguistic
redefinition, and so it is expected that the war reducing effects are
carried on in the new analysis. Bennett builds a direct statistical model based on a triadic
classification of states into "democratic", "autocratic" and "mixed". He
finds that autocratic dyads have a 35% reduced chance of going into any
type of armed conflict with respect to a reference mixed dyad.
Democratic dyads have a 55% reduced chance. This effect gets stronger
when looking at more severe conflicts; for wars (more than 1000 battle
deaths), he estimates democratic dyads to have an 82% lower risk than
autocratic dyads. He concludes that autocratic peace exists, but
democratic peace is clearly stronger. However, he finds no relevant
pacifying effect of political similarity, except at the extremes of the
scale.
To summarize a rather complex picture, there are no less than four possible stances on the value of this criticism:
Political similarity, plus some
complementary variables, explains everything. Democratic peace is a
statistical artifact. Henderson subscribes to this view.
Political similarity has a pacifying effect, but democracy makes it stronger. Werner would probably subscribe to this view.
Political similarity in general has little or no effect,
except at the extremes of the democracy-autocracy scale: a democratic
peace and an autocratic peace exist separately, with the first one being
stronger, and may have different explanations. Bennett holds this view,
and Kinsella mentions this as a possibility
Political similarity has little or no effect and there is
no evidence for autocratic peace. Petersen and Ray are among defendants
of this view.
Interactive model of democratic peace
The interactive model of democratic peace is a combination
of democratic similarity with the traditional model of democratic peace
theory demonstrated on V-Dem Democracy Indices.
Criticism
There are several logically distinguishable classes of criticism. They usually apply to no wars or few MIDs between democracies, not to
little systematic violence in established democracies. In addition,
there have been a number of wars between democracies. The 1987–1989 JVP insurrection in Sri Lanka is an example in which politicide
was committed by a democratic regime, resulting in the deaths of at
least 13,000 and 30,000 suspected JVP members or alleged supporters.
Statistical significance
One study has argued that there have been as many wars
between democracies as one would expect between any other couple of
states. Its authors conclude that the argument for democratic peace
"rests in an ambiguity", since empirical evidence not confirm neither
deny democratic pacifism, and strongly relies upon what degree of
democracy makes a government democratic; according to them "because
perfect democracy is infeasible, one can always sidestep
counter-evidence by raising the bar of democracy".
Others state that, although there may be some evidence for
democratic peace, the data sample or the time span may be too small to
assess any definitive conclusions. For example, Gowa finds evidence for democratic peace to be
insignificant before 1939, because of the too small number of
democracies, and offers an alternate realist explanation for the following period. Gowa's use of statistics has been criticized, with several other studies and reviews finding different or opposing results. However, this can be seen as the longest-lasting criticism to the
theory; as noted earlier, also some supporters agree that the
statistical sample for assessing its validity is limited or scarce, at
least if only full-scale wars are considered.
According to one study, which uses a rather restrictive
definition of democracy and war, there were no wars between jointly
democratic couples of states in the period from 1816 to 1992. Assuming a
purely random distribution of wars between states, regardless of their
democratic character, the predicted number of conflicts between
democracies would be around ten. So, Ray argues that the evidence is
statistically significant, but that it is still conceivable that, in the
future, even a small number of inter-democratic wars would cancel out
such evidence.
The territorial peace theory
argues that peace leads to democracy more than democracy leads to
peace. This argument is supported by historical studies showing that
peace almost always comes before democracy and that states do not
develop democracy until all border disputes have been settled. These
studies indicate that there is strong evidence that peace causes
democracy but little evidence that democracy causes peace.
The hypothesis that peace causes democracy is supported by psychological and cultural theories. Christian Welzel's human empowerment theory posits that existential security leads to emancipative cultural values and support for a democratic political organization. This also follows from the so-called regality theory based on evolutionary psychology.
The territorial peace theory
explains why countries in conflict with their neighbor countries are
unlikely to develop democracy. The democratic peace theory is more
relevant for peace between non-neighbor countries and for relations
between countries that are already at peace with each other.
Critics of Democratic Peace theory note that liberal
states often engage in conflicts with non-liberal states they deem
"rogue," "failed," or "evil." Several studies fail to confirm that democracies are less likely to
wage war than autocracies if wars against non-democracies are included.
Edward Gibbon
stressed that the principal conquests of the Romans were achieved under
the republic; and the emperors, for the most part, were satisfied with
preserving those dominions which had been acquired by the policy of the
Senate, the active emulation of the consuls, and the martial enthusiasm
of the people.
Some authors criticize the definition of democracy by
arguing that states continually reinterpret other states' regime types
as a consequence of their own objective interests and motives, such as
economic and security concerns. For example, one study reports that Germany was considered a democratic
state by Western opinion leaders at the end of the 19th century; yet in
the years preceding World War I, when its relations with the United
States, France and Britain started deteriorating, Germany was gradually
reinterpreted as an autocratic state, in absence of any actual regime
change.Shimmin moves a similar criticism regarding the western perception of Milosevic's Serbia between 1989 and 1999. Rummel replies to this criticism by stating that, in general, studies
on democratic peace do not focus on other countries' perceptions of
democracy; and in the specific case of Serbia, by arguing that the
limited credit accorded by western democracies to Milosevic in the early
1990s did not amount to a recognition of democracy, but only to the
perception that possible alternative leaders could be even worse.
Some democratic peace researchers have been criticized for post hoc
reclassifying some specific conflicts as non-wars or political systems
as non-democracies without checking and correcting the whole data set
used similarly. Supporters and opponents of the democratic peace agree
that this is bad use of statistics, even if a plausible case can be made
for the correction. A military affairs columnist of the newspaper Asia Times has summarized the above criticism in a journalist's fashion describing the theory as subject to the no true Scotsman problem: exceptions are explained away as not being between "real" democracies or "real" wars.
Some democratic peace researchers require that the
executive result from a substantively contested election. This may be a
restrictive definition: For example, the National Archives of the United
States notes that "For all intents and purposes, George Washington was unopposed for election as President, both in 1789 and 1792". (Under the original provisions for the Electoral College,
there was no distinction between votes for president and
Vice-president: each elector was required to vote for two distinct
candidates, with the runner-up to be vice-president. Every elector cast
one of his votes for Washington, John Adams received a majority of the other votes; there were several
other candidates: so the election for vice president was contested.)
Spiro made several other criticisms of the statistical methods used. Russett and a series of papers described by Ray responded to this, for example with different methodology.
Sometimes the datasets used have also been criticized. For
example, some authors have criticized the Correlates of War data for
not including civilian deaths in the battle deaths count, especially in
civil wars. Cohen and Weeks argue that most fishing disputes, which include no
deaths and generally very limited threats of violence, should be
excluded even from the list of military disputes. Gleditsch made several criticisms to the Correlates of War data set, and produced a revised set of data. Maoz and Russett made several criticisms to the Polity I and II data sets, which have mostly been addressed in later versions.
The most comprehensive critique points out that
"democracy" is rarely defined, never refers to substantive democracy, is
unclear about causation, has been refuted in more than 100 studies,
fails to account for some 200 deviant cases, and has been promoted
ideologically to justify one country seeking to expand democracy abroad. Most studies treat the complex concept of "democracy" as a bivariate
variable rather than attempting to dimensionalize the concept. Studies
also fail to take into account the fact that there are dozens of types
of democracy, so the results are meaningless unless articulated to a
particular type of democracy or claimed to be true for all types, such
as consociational or economic democracy, with disparate datasets.
Microfoundations
Recent work into the democratic norms explanations shows
that the microfoundations on which this explanation rest do not find
empirical support. Within most earlier studies, the presence of liberal
norms in democratic societies and their subsequent influence on the
willingness to wage war was merely assumed, never measured. Moreover, it
was never investigated whether or not these norms are absent within
other regime-types. Two recent studies measured the presence of liberal
norms and investigated the assumed effect of these norms on the
willingness to wage war. The results of both studies show that liberal
democratic norms are not only present within liberal democracies, but
also within other regime-types. Moreover, these norms are not of
influence on the willingness to attack another state during an
interstate conflict at the brink of war.
Sebastian Rosato argues that democratic peace theory makes
several false assumptions. Firstly, it assumes that democratic
populaces will react negatively to the costs of war upon them. However,
in modern wars casualties tend to be fairly low and soldiers are largely
volunteers, meaning they accept the risks of fighting, so their
families and friends, whom the cost of their death falls on heaviest,
are less likely to criticise the government than the families and
friends of conscripted soldiers. Secondly, democratic peace theory
ignores the role of nationalism; democratic populaces are just as likely
to be influenced by nationalist sentiment as anyone else and if a
democratic populace believes that a war is necessary for their nation,
the populace will support it. Lastly, democratic leaders are as likely
to guide public opinion as they are to follow it. Democratic leaders are
often aware of the power of nationalist sentiment and thus seek to
encourage it when it comes to war, arguing that war is necessary to
defend or spread the nation's way of life. Democratic leaders may even
have an advatange over authoritarians in this regard, as they can be
seen as more legitimately representative. Rosato argues that this does
not just apply to wars of defence but also aggression; democratic
populaces can be roused by nationalist feelings to support aggressive
wars if they are seen as in the national interest.
Rosato also argues that authoritarian leaders have a
reduced incentive to go to war because civilian control over the
military is less guaranteed in autocracies; there is always the risk the
military could subvert civilian leadership and a war which results in
defeat could swiftly result in a coup. Even military dictators run the
risk of internal dissent within the armed forces. Autocratic leaders in
general also risk unleashing political and social turmoil that could
destroy them if they go to war. Conversely, bellicose democratic leaders
can rely on the acknowledgement of the legitimacy of the democratic
process, as pacifist actors in democracies will need to respect the
legitimacy of a democratically elected government. If pro-war groups can
capture the organs of the state in a democracy legitimately, then
anti-war groups will have little means of opposing them outside of
extra-constitutional means, which would likely backfire and cause the
anti-war groups to lose legitimacy.
A 2017 study found that public opinion in China showed the
same reluctance in going to war as publics in democratic states, which
suggests that publics in democratic states are not generally more
opposed to war than publics in authoritarian states.
Limited consequences
The peacefulness may have various limitations and qualifiers and may not actually mean very much in the real world.
Democratic peace researchers do in general not count as
wars conflicts which do not kill a thousand on the battlefield; thus
they exclude for example the bloodless Cod Wars. However, research has also found a peacefulness between democracies when looking at lesser conflicts.
Liberal democracies have less of these wars than other
states after 1945. This might be related to changes in the perception of
non-European peoples, as embodied in the Universal Declaration of Human Rights.
Related to this is the human rights violations committed against native people,
sometimes by liberal democracies. One response is that many of the
worst crimes were committed by non-democracies, like in the European
colonies before the nineteenth century, in King Leopold II of Belgium's privately owned Congo Free State, and in Joseph Stalin's Soviet Union. The United Kingdom abolished slavery in British territory in 1833, immediately after the Reform Act 1832
had significantly enlarged the franchise. (Of course, the abolition of
the slave trade had been enacted in 1807; and many DPT supporters would
deny that the UK was a liberal democracy in 1833 when examining
interstate wars.)
Hermann and Kegley Jr. argue that interventions between
democracies are more likely to happen than projected by an expected
model. They further argue that democracies are more likely to intervene in
other liberal states than against countries that are non-democracies. Finally, they argue that these interventions between democracies have
been increasing over time and that the world can expect more of these
interventions in the future. The methodology used has been criticized and more recent studies have found opposing results.
Rummel argues that the continuing increase in democracy worldwide will soon lead to an end to wars and democide, possibly around or even before the middle of this century. The fall of Communism
and the increase in the number of democratic states were accompanied by
a sudden and dramatic decline in total warfare, interstate wars, ethnic wars, revolutionary wars, and the number of refugees and displaced persons. One report claims that the two main causes of this decline in warfare are the end of the Cold War itself and decolonization; but also claims that the three Kantian factors have contributed materially.
Historical periods
Economic historians Joel Mokyr and Hans-Joachim Voth argue
that democratic states may have been more vulnerable to conquest
because the rulers in those states were too heavily constrained.
Absolutist rulers in other states could, however, operate more freely.
Adamson et al (2025) finds evidence against democratic
peace theory in the ancient world, with evidence instead indicating more
intense conflicts between two polities when both were democratic in
ancient Greece (Athens-Syracuse) and the Mediterranean (Roman
Republic-Carthage) and that ancient democracies were also more
belligerent in general. They argue this suggests that democracy's
relationship with peace is contingent rather than inherent. Eric Robinson makes a similar proposition, arguing that Greek
democracies fought each other even outside of the Athens-Syracuse
rivalry (such as in Greek Sicily)
and thus democratic peace in the modern day may be due to contextual or
structural differences between ancient and modern democracies.
Covert operations and proxy wars
Critics of the democratic peace theory have pointed to
covert operations and military interventions between democracies, and
argued that these interventions indicate that democracies do not
necessarily trust and respect each other. Alexander B. Downes and Lary Lauren Lilley argue that covert operations
conducted by democratic states has different implications depending on
which version of democratic peace theory one adheres to. They argue that
covert operations are inconsistent with variants of democratic peace
theory that emphasize norms and checks-and-balances, but that covert
operations may be more consistent with versions of democratic peace
theory that rely on selectorate theory's notion of large versus small winning coalitions.
A 2015 study by Michael Poznansky reconciles findings that
democracies engage in covert interventions against one another by
arguing that democracies do so when they expect another state's
democratic character to break down or decay.
A 2022 study found that democracies rarely wage proxy wars
against fellow democracies: "strong democratic institutions prevent
elected leaders from engaging in proxy war against sister regimes, and
embargo violations tend to occur when democratic institutions are weak."
Information manipulation
Chaim Kaufmann argues that the lead-up to the Iraq War
demonstrates that constraints on war in democracies may hinge on whether
democratic governments can control and manipulate information, and
suppress intelligence findings that run counter to administration
rhetoric, as well as whether there is a strong opposition party and
powerful media.
Coup by provoking a war
Many democracies become non-democratic by war, as being
aggressed or as aggressor (quickly after a coup), sometimes the coup
leader worked to provoke that war.
Carl Schmitt wrote on how to overrule a Constitution: "Sovereign is he who decides on the exception." Schmitt, again on the need for internal (and foreign) enemies because
they are useful to persuade the people not to trust anyone more than the
Leader: "As long as the state is a political entity this requirement
for internal peace compels it in critical situations to decide also upon
the domestic enemy. Every state provides, therefore, some kind of
formula for the declaration of an internal enemy." Whatever opposition
will be pictured and intended as the actual foreign enemy's puppet.
World GDP/capita
1–2003 AD. The increase in the number of democratic nations has
occurred at the same time as the increase in economic wealth.
The capitalist peace, or capitalist peace theory, posits
that according to given criteria for economic development (capitalism),
developed economies have not engaged in war with each other, and rarely
enter into low-level disputes. These theories have been proposed as an
explanation for democratic peace by accounting for both democracy and
the peace among democratic nations. The exact nature of the causality
depends upon both the proposed variable and the measure of the indicator
for the concept used.
A majority of researchers on the determinants of democracy
agree that economic development is a primary factor which allows the
formation of a stable and healthy democracy. Thus, some researchers have argued that economic development also plays a factor in the establishment of peace.
Mousseau argues that a culture of contracting in advanced
market-oriented economies may cause both democracy and peace. These
studies indicate that democracy, alone, is an unlikely cause of the
democratic peace. A low level of market-oriented economic development
may hinder development of liberal institutions and values. Hegre and Souva confirmed these expectations. Mousseau finds that democracy is a significant factor only when both
democracies have levels of economic development well above the global
median. In fact, the poorest 21% of the democracies studied, and the
poorest 4–5% of current democracies, are significantly more likely than other kinds of countries to fight each other. Mousseau, Hegre, and Oneal confirm that if at least one of the
democracies involved has a very low level of economic development,
democracy is ineffective in preventing war; however, they find that when
also controlling for trade, 91% of all the democratic pairs had high
enough development for the pacifying effect of democracy to be important
during the 1885–1992 period and all in 1992. The difference in results of these two studies may be due to sampling:
Mousseau's 2005 study observed only neighboring states where poor
countries actually can fight each other. In fact, fully 89% of
militarized conflicts between less developed countries from 1920 and
2000 were among directly contiguous neighbors. He argues that it is not likely that the results can be explained by
trade: Because developed states have large economies, they do not have
high levels of trade interdependence.In fact, the correlation of developed democracy with trade interdependence is a scant 0.06 (Pearson's r – considered substantively no correlation by statisticians.)
Both World Wars
were fought between countries which can be considered economically
developed. Mousseau argues that both Germany and Japan – like the USSR
during the Cold War and Saudi Arabia today – had state-managed economies
and thus lacked his market norms. Hegre finds that democracy is correlated with civil peace only for
developed countries, and for countries with high levels of literacy.
Conversely, the risk of civil war decreases with development only for
democratic countries.
Gartzke argues that economic freedom
(a quite different concept from Mousseau's market norms) or financial
dependence explains the developed democratic peace, and these countries
may be weak on these dimensions too. Rummel criticizes Gartzke's methodology and argues that his results are invalid.
Allan Dafoe, John R. Oneal, and Bruce Russett have challenged Gartzke and Mousseau's research.
Several studies find that democracy, more trade causing greater economic interdependence, and membership in more intergovernmental organizations
reduce the risk of war. This is often called the Kantian peace theory
since it is similar to Kant's earlier theory about a perpetual peace; it
is often also called "liberal peace" theory, especially when one
focuses on the effects of trade and democracy. (The theory that free trade can cause peace is quite old and referred to as Cobdenism.)
Many researchers agree that these variables positively affect each
other but each has a separate pacifying effect. For example, in
countries exchanging a substantial amount of trade, economic interest
groups may exist that oppose a reciprocal disruptive war, but in
democracy such groups may have more power, and the political leaders be
more likely to accept their requests. Weede argues that the pacifying effect of free trade and economic
interdependence may be more important than that of democracy, because
the former affects peace both directly and indirectly, by producing
economic development and ultimately, democracy. Weede also lists some other authors supporting this view. However, some
recent studies find no effect from trade but only from democracy.
None of the authors listed argues that free trade alone
causes peace. Even so, the issue of whether free trade or democracy is
more important in maintaining peace may have potentially significant
practical consequences, for example on evaluating the effectiveness of
applying economic sanctions and restrictions to autocratic countries.
It was Michael Doyle who reintroduced Kant's three
articles into democratic peace theory. He argued that a pacific union of
liberal states has been growing for the past two centuries. He denies
that a pair of states will be peaceful simply because they are both
liberal democracies; if that were enough, liberal states would not be
aggressive towards weak non-liberal states (as the history of American
relations with Mexico shows they are). Rather, liberal democracy is a
necessary condition for international organization and hospitality
(which are Kant's other two articles)—and all three are sufficient to
produce peace. Other Kantians have not repeated Doyle's argument that all three in the
triad must be present, instead stating that all three reduce the risk
of war.
Immanuel Wallerstein
has argued that it is the global capitalist system that creates shared
interests among the dominant parties, thus inhibiting potentially
harmful belligerence.
Toni Negri and Michael Hardt
take a similar stance, arguing that the intertwined network of
interests in the global capitalism leads to the decline of individual nation states, and the rise of a global Empire
which has no outside, and no external enemies. As a result, they write,
"The era of imperialist, interimperialist, and anti-imperialist wars is
over. (...) we have entered the era of minor and internal conflicts.
Every imperial war is a civil war, a police action".
Other explanations
Many studies supporting the theory have controlled for
many possible alternative causes of the peace. Examples of factors
controlled for are geographic distance, geographic contiguity, power
status, alliance ties, militarization, economic wealth and economic
growth, power ratio, and political stability. These studies have often
found very different results depending on methodology and included
variables, which has caused criticism. DPT does not state democracy is
the only thing affecting the risk of military conflict. Many of the
mentioned studies have found that other factors are also important.
Several studies have also controlled for the possibility
of reverse causality from peace to democracy. For example, one study
supports the theory of simultaneous causation, finding that dyads
involved in wars are likely to experience a decrease in joint democracy,
which in turn increases the probability of further war. So they argue
that disputes between democratizing or democratic states should be
resolved externally at a very early stage, in order to stabilize the
system. Another study finds that peace does not spread democracy, but spreading
democracy is likely to spread peace. A different kind of reverse
causation lies in the suggestion that impending war could destroy or
decrease democracy, because the preparation for war might include
political restrictions, which may be the cause for the findings of
democratic peace. However, this hypothesis has been statistically tested in a study whose
authors find, depending on the definition of the pre-war period, no
such effect or a very slight one. So, they find this explanation
unlikely. This explanation would predict a monadic effect, although weaker than the dyadic one.
Weart argues that the peacefulness appears and disappears
rapidly when democracy appears and disappears. This in his view makes it
unlikely that variables that change more slowly are the explanation. Weart, however, has been criticized for not offering any quantitative analysis supporting his claims.
Wars tend very strongly to be between neighboring states.
Gleditsch showed that the average distance between democracies is about
8000 miles, the same as the average distance between all states. He
believes that the effect of distance in preventing war, modified by the
democratic peace, explains the incidence of war as fully as it can be
explained.
A 2020 study in International Organization
found that it was not democracy per se that reduces the prospects for
conflict, but whether women's suffrage was ensured. The study argued,
"women's more pacific preferences generate a dyadic democratic peace
(i.e., between democracies), as well as a monadic peace."
According to Azar Gat's War in Human Civilization,
there are several related and independent factors that contribute to
democratic societies being more peaceful than other forms of
governments:
Wealth and comfort: Increased prosperity
in democratic societies has been associated with peace because civilians
are less willing to endure hardship of war and military service due to a
more luxurious life at home than in pre-modern times. Increased wealth
has worked to decrease war through comfort.
Metropolitan service society: The majority of army
recruits come from the countryside or factory workers. Many believe that
these types of people are suited for war. But as technology progressed
the army turned more towards advanced services in information that rely
more on computerized data which urbanized people are recruited more for
this service.
Sexual revolution: The availability of sex due to the
pill and women joining the labor market could be another factor that has
led to less enthusiasm for men to go to war. Young men are more
reluctant leave behind the pleasures of life for the rigors and chastity
of the army.
Fewer young males: There is greater life expectancy which
leads to fewer young males. Young males are the most aggressive and the
ones that join the army the most. With fewer younger males in developed
societies could help explain more pacificity.
Fewer children per family (lower fertility rate):
During pre modern times it was always hard for families to lose a child
but in modern times it has become more difficult due to more families
having only one or two children. It has become even harder for parents
to risk the loss of a child in war. However, Gat recognizes that this
argument is a difficult one because during pre modern times the life
expectancy was not high for children and bigger families were necessary.
Women's franchise: Women are less overtly aggressive than
men. Therefore, women are less inclined to serious violence and do not
support it as much as men do. In liberal democracies women have been
able to influence the government by getting elected. Electing more women
could have an effect on whether liberal democracies take a more
aggressive approach on certain issues.
Nuclear weapons: Nuclear weapons could be the reason for
not having a great power war. Many believe that a nuclear war would
result in mutually assured destruction
(MAD) which means that both countries involved in a nuclear war have
the ability to strike the other until both sides are wiped out. This
results in countries not wanting to strike the other for fear of being
wiped out.
Realist explanations
Supporters of realism in international relations
in general argue that not democracy or its absence, but considerations
and evaluations of power, cause peace or war. Specifically, many realist
critics claim that the effect ascribed to democratic, or liberal,
peace, is in fact due to alliance ties between democratic states which
in turn are caused, one way or another, by realist factors.
For example, Farber and Gowa find evidence for peace
between democracies to be statistically significant only in the period
from 1945 on, and consider such peace an artifact of the Cold War, when the threat from the communist states forced democracies to ally with one another. Mearsheimer offers a similar analysis of the Anglo-American peace before 1945, caused by the German threat. Spiro finds several instances of wars between democracies, arguing that
evidence in favor of the theory might be not so vast as other authors
report, and claims that the remaining evidence consists of peace between
allied states with shared objectives. He acknowledges that democratic
states might have a somewhat greater tendency to ally with one another,
and regards this as the only real effect of democratic peace. Rosato argues that most of the significant evidence for democratic
peace has been observed after World War II; and that it has happened
within a broad alliance, which can be identified with NATO and its
satellite nations, imposed and maintained by American dominance as part
of Pax Americana.
One of the main points in Rosato's argument is that, although never
engaged in open war with another liberal democracy during the Cold War,
the United States intervened openly or covertly in the political affairs
of democratic states several times, for example in the Chilean coup of 1973, the Operation Ajax (1953 coup in Iran) and Operation PBSuccess
(1954 coup in Guatemala); in Rosato's view, these interventions show
the United States' determination to maintain an "imperial peace".
The most direct counter arguments to such criticisms have
been studies finding peace between democracies to be significant even
when controlling for "common interests" as reflected in alliance ties. Regarding specific issues, Ray objects that explanations based on the
Cold War should predict that the Communist bloc would be at peace within
itself also, but exceptions include the Soviet Invasion of Afghanistan, the Cambodian-Vietnamese War, and the Sino-Vietnamese War.
Ray also argues that the external threat did not prevent conflicts in
the Western bloc when at least one of the involved states was a
non-democracy, such as the Turkish Invasion of Cyprus (against Greek Junta supported Cypriot Greeks), the Falklands War, and the Football War. Also, one study notes that the explanation "goes increasingly stale as
the post-Cold War world accumulates an increasing number of peaceful
dyad-years between democracies". Rosato's argument about American dominance has also been criticized for not giving supporting statistical evidence.
Some realist authors also criticize in detail the
explanations first by supporters of democratic peace, pointing to
supposed inconsistencies or weaknesses.
Rosato criticizes most explanations to how democracy might
cause peace. Arguments based on normative constraints, he argues, are
not consistent with the fact that democracies do go to war no less than
other states, thus violating norms preventing war; for the same reason
he refutes arguments based on the importance of public opinion.
Regarding explanations based on greater accountability of leaders, he
finds that historically autocratic leaders have been removed or punished
more often than democratic leaders when they get involved in costly
wars. Finally, he also criticizes the arguments that democracies treat
each other with trust and respect even during crises; and that democracy
might be slow to mobilize its composite and diverse groups and
opinions, hindering the start of a war, drawing support from other
authors. Another realist, Layne, analyzes the crises and brinkmanship
that took place between non-allied democratic great powers, during the
relatively brief period when such existed. He finds no evidence either
of institutional or cultural constraints against war; indeed, there was
popular sentiment in favor of war on both sides. Instead, in all cases,
one side concluded that it could not afford to risk that war at that
time, and made the necessary concessions.
Rosato's objections have been criticized for claimed
logical and methodological errors, and for being contradicted by
existing statistical research. Russett replies to Layne by re-examining some of the crises studied in
his article, and reaching different conclusions; Russett argues that
perceptions of democracy prevented escalation, or played a major role in
doing so. Also, a recent study finds that, while in general the outcome of
international disputes is highly influenced by the contenders' relative
military strength, this is not true if both contenders are democratic
states; in this case the authors find the outcome of the crisis to be
independent of the military capabilities of contenders, which is
contrary to realist expectations. Finally, both the realist criticisms here described ignore new possible
explanations, like the game-theoretic one discussed below.
Nuclear deterrent
A different kind of realist criticism stresses the role of
nuclear weapons in maintaining peace. In realist terms, this means
that, in the case of disputes between nuclear powers, respective
evaluation of power might be irrelevant because of Mutual assured destruction preventing both sides from foreseeing what could be reasonably called a "victory". The 1999 Kargil War between India and Pakistan has been cited as a counterexample to this argument, though this was a small, regional conflict and the threat of WMDs being used contributed to its de-escalation.
Some supporters of the democratic peace do not deny that realist factors are also important. Research supporting the theory has also shown that factors such as
alliance ties and major power status influence interstate conflict
behavior.
Statistical difficulties due to newness of democracy
Number of nations 1800–2018 scoring 8 or higher on Polity5 scale.
There have been no wars and in Wayman's listing of interliberal MIDs no
conflict causing any battle deaths between these nations.
One problem with the research on wars is that, as the RealistJohn Mearsheimer
put it, "democracies have been few in number over the past two
centuries, and thus there have been few opportunities where democracies
were in a position to fight one another". Democracies have been very rare until recently. Even looser definitions
of democracy, such as Doyle's, find only a dozen democracies before the
late nineteenth century, and many of them short-lived or with limited
franchise. Freedom House finds no independent state with universal suffrage in 1900.
Wayman, a supporter of the theory, states that "If we rely
solely on whether there has been an inter-democratic war, it is going
to take many more decades of peace to build our confidence in the
stability of the democratic peace".
Studying lesser conflicts
Many researchers have reacted to this limitation by
studying lesser conflicts instead, since they have been far more common.
There have been many more MIDs than wars; the Correlates of War Project
counts several thousand during the last two centuries. A review lists
many studies that have reported that democratic pairs of states are less
likely to be involved in MIDs than other pairs of states.
Another study finds that after both states have become
democratic, there is a decreasing probability for MIDs within a year and
this decreases almost to zero within five years.
When examining the inter-liberal MIDs in more detail, one
study finds that they are less likely to involve third parties, and that
the target of the hostility is less likely to reciprocate, if the
target reciprocates the response is usually proportional to the
provocation, and the disputes are less likely to cause any loss of life.
The most common action was "Seizure of Material or Personnel".
Studies find that the probability that disputes between
states will be resolved peacefully is positively affected by the degree
of democracy exhibited by the lesser democratic state involved in that
dispute. Disputes between democratic states are significantly shorter
than disputes involving at least one undemocratic state. Democratic
states are more likely to be amenable to third party mediation when they
are involved in disputes with each other.
In international crises that include the threat or use of
military force, one study finds that if the parties are democracies,
then relative military strength has no effect on who wins. This is
different from when non-democracies are involved. These results are the
same also if the conflicting parties are formal allies. Similarly, a study of the behavior of states that joined ongoing
militarized disputes reports that power is important only to
autocracies: democracies do not seem to base their alignment on the
power of the sides in the dispute.
Academic relevance and derived studies
Democratic peace theory is a well established research
field with more than a hundred authors having published articles about
it. Several peer-reviewed studies mention in their introduction that most researchers accept the theory as an empirical fact. According to a 2021 study by Kosuke Imai and James Lo, "overturning the
negative association between democracy and conflict would require a
confounder that is forty-seven times more prevalent in democratic dyads
than in other dyads. To put this number in context, the relationship
between democracy and peace is at least five times as robust as that
between smoking and lung cancer. To explain away the democratic peace,
therefore, scholars would have to find far more powerful confounders
than those already identified in the literature."
Imre Lakatos
suggested that what he called a "progressive research program" is
better than a "degenerative" one when it can explain the same phenomena
as the "degenerative" one, but is also characterized by growth of its
research field and the discovery of important novel facts. In contrast,
the supporters of the "degenerative" program do not make important new
empirical discoveries, but instead mostly apply adjustments to their
theory in order to defend it from competitors. Some researchers argue
that democratic peace theory is now the "progressive" program in
international relations. According to these authors, the theory can
explain the empirical phenomena previously explained by the earlier
dominant research program, realism in international relations;
in addition, the initial statement that democracies do not, or rarely,
wage war on one another, has been followed by a rapidly growing
literature on novel empirical regularities.
Other examples are several studies finding that
democracies are more likely to ally with one another than with other
states, forming alliances which are likely to last longer than alliances
involving non-democracies; several studies showing that democracies conduct diplomacy differently and in a more conciliatory way compared to non-democracies; one study finding that democracies with proportional representation are in general more peaceful regardless of the nature of the other party involved in a relationship; and another study reporting that proportional representation system and
decentralized territorial autonomy is positively associated with
lasting peace in postconflict societies.
In the United States, presidents from both major parties have expressed support for the theory. In his 1994 State of the Union address, then-President Bill Clinton, a member of the Democratic Party,
said: "Ultimately, the best strategy to ensure our security and to
build a durable peace is to support the advance of democracy elsewhere.
Democracies don't attack each other". In a 2004 press conference, then-President George W. Bush, a member of the Republican Party,
said: "And the reason why I'm so strong on democracy is democracies
don't go to war with each other. And the reason why is the people of
most societies don't like war, and they understand what war means....
I've got great faith in democracies to promote peace. And that's why I'm
such a strong believer that the way forward in the Middle East, the
broader Middle East, is to promote democracy."
In a 1999 speech, Chris Patten, the then-European Commissioner
for External Relations, said: "Inevitable because the EU was formed
partly to protect liberal values, so it is hardly surprising that we
should think it appropriate to speak out. But it is also sensible for
strategic reasons. Free societies tend not to fight one another or to be
bad neighbours". The A Secure Europe in a Better World, European Security Strategy states: "The best protection for our security is a world of well-governed democratic states." Tony Blair has also claimed the theory is correct.
As justification for initiating war
Some fear that the democratic peace theory may be used to
justify wars against non-democracies in order to bring lasting peace, in
a democratic crusade. Woodrow Wilson in 1917 asked Congress to declare war against Imperial Germany, citing Germany's sinking of American ships due to unrestricted submarine warfare and the Zimmermann telegram,
but also stating that "A steadfast concert for peace can never be
maintained except by a partnership of democratic nations" and "The world
must be made safe for democracy." R. J. Rummel was a notable proponent of war for the purpose of spreading democracy, based on this theory.
Some point out that the democratic peace theory has been used to justify the 2003 Iraq War, others argue that this justification was used only after the war had already started. Furthermore, Weede has argued that the justification is extremely weak,
because forcibly democratizing a country completely surrounded by
non-democracies, most of which are full autocracies, as Iraq was, is at
least as likely to increase the risk of war as it is to decrease it
(some studies show that dyads formed by one democracy and one autocracy
are the most warlike, and several find that the risk of war is greatly
increased in democratizing countries surrounded by non-democracies).]
According to Weede, if the United States and its allies wanted to adopt
a rationale strategy of forced democratization based on democratic
peace, which he still does not recommend, it would be best to start
intervening in countries which border with at least one or two stable
democracies, and expand gradually. Also, research shows that attempts to
create democracies by using external force has often failed. Gleditsch,
Christiansen, and Hegre argue that forced democratization by
interventionism may initially have partial success, but often create an
unstable democratizing country, which can have dangerous consequences in
the long run. Those attempts which had a permanent and stable success, like democratization in Austria, West Germany and Japan after World War II,
mostly involved countries which had an advanced economic and social
structure already, and implied a drastic change of the whole political
culture. Supporting internal democratic movements and using diplomacy
may be far more successful and less costly. Thus, the theory and related
research, if they were correctly understood, may actually be an
argument against a democratic crusade.
Michael Haas
has written perhaps the most trenchant critique of a hidden normative
agenda. Among the points raised: Due to sampling manipulation, the
research creates the impression that democracies can justifiably fight
non-democracies, snuff out budding democracies, or even impose
democracy. And due to sloppy definitions, there is no concern that
democracies continue undemocratic practices yet remain in the sample as
if pristine democracies.
This criticism is confirmed by David Keen who finds that almost all historical attempts to impose democracy by violent means have failed.
Related theories
Republican liberalism
Republican liberalism is a variation of Democratic Peace Theory which claims that liberal
and republican democracies will rarely go to war with each other. It
argues that these governments are more peaceful than non-democracies and
will avoid conflict when possible. According to Micheal Doyle, there
are three main reasons for this: Democracies tend to have similar
domestic political cultures, they share common morals, and their
economic systems are interdependent. Liberal democracies (republics) that trade with each other, are
economically dependent on one another and therefore, will always attempt
to maintain diplomatic relations as to not disrupt their economies.
Liberalism, as an overarching theory, holds that diplomacy and cooperation is the most effective way to avoid war and maintain peace. This is contrasting to the theory of realism,
which states that conflict will always be recurrent in the
international system, whether due to human nature or the anarchic
international system.
The concept of Republican liberalism is thought to have initially originated from Immanuel Kant's book "Perpetual Peace: A Philosophical Sketch"
(1795). The term "Perpetual Peace" refers to the permanent
establishment of peace, and was made notorious by the book. Democratic
peace, commercial peace and institutional peace were all advanced in the
book as well. It takes a rather utopian view, that humanities' desire
for peace will out compete humanities' desire for war.
Kantian liberalism
Kant and the liberal school of thought view international
co-operation as a more rational option for states than resorting to war.
However, the neo-liberal approach concedes to the realist school of
thought, that when states cooperate it is simply because it is in their
best interest. Kant insisted that a world with only peace was possible,
and he offered three definitive articles that would create the pathway
for it. Each went on to become a dominant strain of post–World War II
liberal international relations theory.
I: "The Civil Constitution of Every State should be Republican"
Kant believed that every state should have Republican style form of government. As in, a state where "supreme power is held by the people and their elected representatives." Kant saw this in Ancient Rome, where they began to move away from Athenian democracy (direct democracy) and towards a representative democracy.
Kant believed giving the citizens the right to vote and decide for
themselves would lead to shorter wars and less wars. He also thought it
was important to "check the power of monarchs", to establish a system of checks and balances where no one person holds
absolute power. Peace is always dependent on the internal character of
governments. Republics, with a legislative body that will be able to
hold the executive leader in check and maintain the peace.
II: "The Law of Nations shall be founded on a Federation of Free States"
Kant argues nations, like individuals can be tempted to
harm each other at any given moment. So, rule of law should be
established internationally. Without international laws and courts of
judgement, then force would be the only way to settle disputes. States
ought to instead develop international organisations and rules that
facilitate cooperation. In any case, some kind of federation is necessary in order to maintain peace between nations. Contemporary examples include the United Nations and the European Union, which try to maintain peace and encourage cooperation among nations.
III: "The Law of World Citizenship shall be Limited to Conditions of Universal Hospitality"
Kant is referring to "the right of the stranger not to be treated as an enemy when he arrives in the land of another.". So long "stranger" is peaceful, he should not be treated with
hostility. However, this is not the right to be a "permanent visitor",
simply as a temporary stay. This is applicable in the contemporary world
when a country is receiving a world leader. The host country usually
holds a state welcoming ceremony which strengthens diplomatic relations.
European peace
There is significant debate over whether the lack of any
major European general wars since 1945 is due to cooperation and
integration of liberal-democratic European states themselves (as in the European Union or Franco-German cooperation),
an enforced peace due to the intervention of the Soviet Union and the
United States until 1989 and the United States alone thereafter, or a combination of both. The debate over this theory was thrust in the public eye when the 2012 Nobel Peace Prize was awarded to the European Union for its role in creating peace in Europe. Notable major wars in Europe after 1945 are Yugoslav Wars and Russian invasion of Ukraine, which follows the prediction of the interactive model of democratic peace.
