Environmental enrichment

From Wikipedia, the free encyclopedia

A rodent is not stimulated by the environment in a wire cage, and this affects its brain negatively, particularly the complexity of its synaptic connections

 

Environmental enrichment is the stimulation of the brain by its physical and social surroundings. Brains in richer, more stimulating environments have higher rates of synaptogenesis and more complex dendrite arbors, leading to increased brain activity. This effect takes place primarily during neurodevelopment, but also during adulthood to a lesser degree. With extra synapses there is also increased synapse activity, leading to an increased size and number of glial energy-support cells. Environmental enrichment also enhances capillary vasculation, providing the neurons and glial cells with extra energy. The neuropil (neurons, glial cells, capillaries, combined together) expands, thickening the cortex. Research on rodent brains suggests that environmental enrichment may also lead to an increased rate of neurogenesis. 

Research on animals finds that environmental enrichment could aid the treatment and recovery of numerous brain-related dysfunctions, including Alzheimer's disease and those connected to aging, whereas a lack of stimulation might impair cognitive development. Moreover, this research also suggests that environmental enrichment leads to a greater level of cognitive reserve, the brain's resilience to the effects of conditions such as aging and dementia.

Research on humans suggests that lack of stimulation delays and impairs cognitive development. Research also finds that attaining and engaging in higher levels of education, environments in which people participate in more challenging cognitively stimulating activities, results in greater cognitive reserve.

Early research

Donald O. Hebb in 1947 found that rats raised as pets performed better on problem solving tests than rats raised in cages. His research, however, did not investigate the brain nor use standardized impoverished and enriched environments. Research doing this first was started in 1960 at the University of California, Berkeley by Mark Rosenzweig, who compared single rats in normal cages, and those placed in ones with toys, ladders, tunnels, running wheels in groups. This found that growing up in enriched environments affected enzyme cholinesterase activity. This work led in 1962 to the discovery that environmental enrichment increased cerebral cortex volume. In 1964, it was found that this was due to increased cerebral cortex thickness and greater synapse and glial numbers.

Also starting around 1960, Harry Harlow studied the effects of maternal and social deprivation on rhesus monkey infants (a form of environmental stimulus deprivation). This established the importance of social stimulation for normal cognitive and emotional development.

Synapses

Synaptogenesis

Rats raised with environmental enrichment have thicker cerebral cortices (3.3–7%) that contain 25% more synapses. This effect of environmental richness upon the brain occurs whether it is experienced immediately following birth, after weaning, or during maturity. When synapse numbers increase in adults, they can remain high in number even when the adults are returned to impoverished environment for 30 days suggesting that such increases in synapse numbers are not necessarily temporary. However, the increase in synapse numbers has been observed generally to reduce with maturation. Stimulation affects not only synapses upon pyramidal neurons (the main projecting neurons in the cerebral cortex) but also stellate ones (that are usually interneurons). It also can affect neurons outside the brain in the retina.

Dendrite complexity

Environmental enrichment affects the complexity and length of the dendrite arbors (upon which synapses form). Higher-order dendrite branch complexity is increased in enriched environments, as can the length, in young animals, of distal branches.

Activity and energy consumption

Synapses in animals in enriched environments show evidence of increased synapse activation. Synapses tend to also be much larger. Gamma oscillations become larger in amplitude in the hippocampus. This increased energy consumption is reflected in glial and local capillary vasculation that provides synapses with extra energy.

• Glial cell numbers per neuron increase 12–14%
• The direct apposition area of glial cells with synapses expands by 19%
• The volume of glial cell nuclei for each synapse is higher by 37.5%
• The mean volume of mitochondria per neuron is 20% greater
• The volume of glial cell nuclei for each neuron is 63% higher
• Capillary density is increased.
• Capillaries are wider (4.35 μm compared to 4.15 μm in controls)
• Shorter distance exist between any part of the neuropil and a capillary (27.6 μm compared to 34.6 μm)

These energy related changes to the neuropil are responsible for increasing the volume of the cerebral cortex (the increase in synapse numbers contributes in itself hardly any extra volume).

Motor learning stimulation

Part of the effect of environmental enrichment is providing opportunities to acquire motor skills. Research upon “acrobatic” skill learning in the rat shows that it leads to increased synapse numbers.

Maternal transmission

Environmental enrichment during pregnancy has effects upon the fetus such as accelerating its retinal development.

Neurogenesis

Environmental enrichment can also lead to the formation of neurons (at least in rats) and reverses the loss of neurons in the hippocampus and memory impairment following chronic stress. However, its relevance has been questioned for the behavioral effects of enriched environments.

Mechanisms

Enriched environments affect the expression of genes in the cerebral cortex and the hippocampus that determine neuronal structure. At the molecular level, this occurs through increased concentrations of the neurotrophins NGF, NT-3, and changes in BDNF. This alters the activation of cholinergic neurons, 5-HT, and beta-adrenolin. Another effect is to increase proteins such as synaptophysin and PSD-95 in synapses. Changes in Wnt signaling have also been found to mimic in adult mice the effects of environmental enrichment upon synapses in the hippocampus. Increase in neurons numbers could be linked to changes in VEGF.

Rehabilitation and resilience

Research in animals suggests that environmental enrichment aids in recovery from an array of neurological disorders and cognitive impairments. There are two mains areas of focus: neurological rehabilitation and cognitive reserve, the brain's resistance to the effects of exposure to physical, natural, and social threats. Although most of these experiments used animal subjects, mainly rodents, researchers have pointed to the affected areas of animal brains to which human brains are most similar and used their findings as evidence to show that humans would have comparable reactions to enriched environments. The tests done on animals are thus meant to represent human simulations for the following list of conditions.

Neurological rehabilitation

Autism

A study conducted in 2011 led to the conclusion that environmental enrichment vastly improves the cognitive ability of children with autism. The study found that autistic children who receive olfactory and tactile stimulation along with exercises that stimulated other paried sensory modalities clinically improved by 42 percent while autistic children not receiving this treatment clinically improved by just 7 percent. The same study also showed that there was significant clinical improvement in autistic children exposed to enriched sensorimotor environments, and a vast majority of parents reported that their child's quality of life was much better with the treatment. A second study confirmed its effectiveness. The second study also found after 6 months of sensory enrichment therapy, 21% of the children who initially had been given an autism classification, using the Autism Diagnostic Observation Schedule, improved to the point that, although they remained on the autism spectrum, they no longer met the criteria for classic autism. None of the standard care controls reached an equivalent level of improvement. The therapy using the methodologies is titled Sensory Enrichment Therapy.

Alzheimer's disease

Through environmental enrichment, researchers were able to enhance and partially repair memory deficits in mice between ages of 2 to 7 months with characteristics of Alzheimer's disease. Mice in enriched environments performed significantly better on object recognition tests and the Morris Water Maze than they had when they were in standard environments. It was thus concluded that environmental enrichment enhances visual and learning memory for those with Alzheimer's. Furthermore, it has been found that mouse models of Alzheimer's disease that were exposed to enriched environment before amyloid onset (at 3 months of age) and then returned to their home cage for over 7 months, showed preserved spatial memory and reduced amyloid deposition at 13 months old, when they are supposed to show dramatic memory deficits and amyloid plaque load. These findings reveal the preventive, and long-lasting effects of early life stimulating experience on Alzheimer-like pathology in mice and likely reflect the capacity of enriched environment to efficiently stimulate the cognitive reserve.

Huntington's disease

Research has indicated that environmental enrichment can help relieve motor and psychiatric deficits caused by Huntington's disease. It also improves lost protein levels for those with the disease, and prevents striatal and hippocampal deficits in the BDNF, located in the hippocampus. These findings have led researchers to suggest that environmental enrichment has a potential to be a possible form of therapy for those with Huntington's.

Parkinson's disease

Multiple studies have reported that environmental enrichment for adult mice helps relieve neuronal death, which is particularly beneficial to those with Parkinson's disease. A more recent study shows that environmental enrichment particularly affects the nigrostriatal pathway, which is important for managing dopamine and acetylcholine levels, critical for motor deficits. Moreover, it was found that environmental enrichment has beneficial effects for the social implications of Parkinson's disease.

Stroke

Research done in animals has shown that subjects recovering in an enriched environment 15 days after having a stroke had significantly improved neurobehavioral function. In addition these same subjects showed greater capability of learning and larger infarct post-intervention than those who were not in an enriched environment. It was thus concluded that environmental enrichment had a considerable beneficial effect on the learning and sensorimotor functions on animals post-stroke. A 2013 study also found that environmental enrichment socially benefits patients recovering from stroke. Researchers in that study concluded that stroke patients in enriched environments in assisted-care facilities are much more likely to be engaging with other patients during normal social hours instead of being alone or sleeping.

Rett syndrome

A 2008 study found that environmental enrichment was significant in aiding recovery of motor coordination and some recovery of BDNF levels in female mice with conditions similar to those of Rett syndrome. Over the course of 30 weeks female mice in enriched environments showed superior ability in motor coordination to those in standard conditions. Although they were unable to have full motor capability, they were able to prevent a more severe motor deficit by living in an enriched environment. These results combined with increased levels of BDNF in the cerebellum led researchers to conclude that an enriched environment that stimulates areas of the motor cortex and areas of the cerebellum having to do with motor learning is beneficial in aiding mice with Rett syndrome.

Amblyopia

A recent study found that adult rats with amblyopia improved visual acuity two weeks after being placed into an enriched environment. The same study showed that another two weeks after ending environmental enrichment, the rats retained their visual acuity improvement. Conversely, rats in a standard environment showed no improvement in visual acuity. It was thus concluded that environmental enrichment reduces GABA inhibition and increases BDNF expression in the visual cortex. As a result, the growth and development of neurons and synapses in the visual cortex were much improved due to the enriched environment.

Sensory deprivation

Studies have shown that with the help of environmental enrichment the effects of sensory deprivation can be corrected. For example, a visual impairment known as "dark-rearing" in the visual cortex can be prevented and rehabilitated. In general, an enriched environment will improve, if not repair, the sensory systems animals possess.

Lead poisoning

During development, gestation is one of the most critical periods for exposure to any lead. Exposure to high levels of lead at this time can lead to inferior spatial learning performance. Studies have shown that environmental enrichment can overturn damage to the hippocampus induced by lead exposure. Learning and spatial memory that are dependent on the long-term potentiation of the hippocampus are vastly improve as subjects in an enriched environments had lower levels of lead concentration in their hippocampi. The findings also showed that enriched environments result in some natural protection of lead-induced brain deficits.

Chronic spinal cord injuries

Research has indicated that animals suffering from spinal cord injuries showed significant improvement in motor capabilities even with a long delay in treatment after the injury when exposed to environmental enrichment. Social interactions, exercise, and novelty all play major roles in aiding the recovery of an injured subject. This has led to some suggestions that the spinal cord has a continued plasticity and all efforts must be made for enriched environments to stimulate this plasticity in order to aid recovery.

Maternal deprivation stress

Maternal deprivation can be caused by the abandonment by a nurturing parent at a young age. In rodents or nonhuman primates, this leads to a higher vulnerability for stress-related illness. Research suggests that environmental enrichment can reverse the effects of maternal separation on stress reactivity, possibly by affecting the hippocampus and the prefrontal cortex.

Child neglect

In all children, maternal care is one of the significant influences for hippocampal development, providing the foundation for stable and individualized learning and memory. However, this is not the case for those who have experienced child neglect. Researchers determined that through environmental enrichment, a neglected child can partially receive the same hippocampal development and stability, albeit not at the same level as that of the presence of a parent or guardian. The results were comparable to those of child intervention programs, rendering environmental enrichment a useful method for dealing with child neglect.

Cognitive reserve

Aging

Decreased hippocampal neurogenesis is a characteristic of aging. Environmental enrichment increases neurogenesis in aged rodents by potentiating neuronal differentiation and new cell survival. As a result, subjects exposed to environmental enrichment aged better due to superior ability in retaining their levels of spatial and learning memory.

Prenatal and perinatal cocaine exposure

Research has shown that mice exposed to environmental enrichment are less affected by the consequences of cocaine exposure in comparison with those in standard environments. Although the levels of dopamine in the brains of both sets of mice were relatively similar, when both subjects were exposed to the cocaine injection, mice in enriched environment were significantly less responsive than those in standard environments. It was thus concluded that both the activating and rewarding effects are suppressed by environmental enrichment and early exposure to environmental enrichment can help prevent drug addiction.

Humans

Though environmental enrichment research has been mostly done upon rodents, similar effects occur in primates, and are likely to affect the human brain. However, direct research upon human synapses and their numbers is limited since this requires histological study of the brain. A link, however, has been found between educational level and greater dendritic branch complexity following autopsy removal of the brain.

Localized cerebral cortex changes

MRI detects localized cerebral cortex expansion after people learn complex tasks such as mirror reading (in this case in the right occipital cortex), three-ball juggling (bilateral mid-temporal area and left posterior intraparietal sulcus), and when medical students intensively revise for exams (bilaterally in the posterior and lateral parietal cortex). Such changes in gray matter volume can be expected to link to changes in synapse numbers due to the increased numbers of glial cells and the expanded capillary vascularization needed to support their increased energy consumption.

Institutional deprivation

Children that receive impoverished stimulation due to being confined to cots without social interaction or reliable caretakers in low quality orphanages show severe delays in cognitive and social development. 12% of them if adopted after 6 months of age show autistic or mildly autistic traits later at four years of age. Some children in such impoverished orphanages at two and half years of age still fail to produce intelligible words, though a year of foster care enabled such children to catch up in their language in most respects. Catch-up in other cognitive functioning also occurs after adoption, though problems continue in many children if this happens after the age of 6 months.

Such children show marked differences in their brains, consistent with research upon experiment animals, compared to children from normally stimulating environments. They have reduced brain activity in the orbital prefrontal cortex, amygdala, hippocampus, temporal cortex, and brain stem. They also showed less developed white matter connections between different areas in their cerebral cortices, particularly the uncinate fasciculus.

Conversely, enriching the experience of preterm infants with massage quickens the maturating of their electroencephalographic activity and their visual acuity. Moreover, as with enrichment in experimental animals, this associates with an increase in IGF-1.

Cognitive reserve and resilience

Another source of evidence for the effect of environment stimulation upon the human brain is cognitive reserve (a measure of the brain’s resilience to cognitive impairment) and the level of a person’s education. Not only is higher education linked to a more cognitively demanding educational experience, but it also correlates with a person’s general engagement in cognitively demanding activities. The more education a person has received, the less the effects of aging, dementia, white matter hyperintensities, MRI-defined brain infarcts, Alzheimer's disease, and traumatic brain injury. Also, aging and dementia are less in those that engage in complex cognitive tasks. The cognitive decline of those with epilepsy could also be affected by the level of a person’s education.

Environment and intelligence

From Wikipedia, the free encyclopedia

 

Environment and intelligence research investigates the impact of environment on intelligence. This is one of the most important factors in understanding human group differences in IQ test scores and other measures of cognitive ability. It is estimated that genes contribute about 20–40% of the variance in intelligence in childhood and about 80% in old age. Thus the environment and its interaction with genes account for a high proportion of the variation in intelligence seen in groups of young children, and for a small proportion of the variation observed in groups of mature adults. Historically, there has been great interest in the field of intelligence research to determine environmental influences on the development of cognitive functioning, in particular, fluid intelligence, as defined by its stabilization at 16 years of age. Despite the fact that intelligence stabilizes in early adulthood it is thought that genetic factors come to play more of a role in our intelligence during middle and old age and that the importance of the environment dissipates.

Neurological theory

As babies, our neuronal connections are completely undifferentiated. Neurons make connections with neighboring neurons, and these become more complex and more idiosyncratic as the child ages, up until the age of 16, when this process halts. This is also the time frame for development of what is defined in psychometric studies as the general factor of intelligence, or g, as measured by IQ tests. A person's IQ is supposed to be relatively stable after they have reached maturity. It is likely that the growth in neuronal connections is largely due to an interaction with the environment, as there is not even enough genetic material to code for all the possible neural connections. Even if there was enough genetic material to code neural connections, it is unlikely that they could produce such fine tuned connections. In contrast the environment causes meaningful processing as the neurons adapt to stimuli presented.

The capacity of the brain to adapt its connections to environmental stimuli diminishes over time, and therefore it would follow that there is a critical period for intellectual development as well. While the critical period for the visual cortex ends in early childhood, other cortical areas and abilities have a critical period that lasts up through maturity (age 16), the same time frame for the development of fluid intelligence. In order for a person to develop certain intellectual abilities, they need to be provided with the appropriate environmental stimuli during childhood, before the critical period for adapting their neuronal connections ends. The existence of a critical period of language development is well established. A case illustrating this critical period is that of E.M., a young man who was born profoundly deaf and did not have any interaction with the deaf community. At the age of 15 he was fitted with hearing aids and taught Spanish; however, after 4 years he still had severe difficulties in verbal comprehension and production.

Some researchers believe that the critical period effect is a result of the manner by which intellectual abilities are acquired—that changes in neuronal connections inhibit or prevent possible future changes. However, the critical period is observed at approximately the same age in all people, no matter what level of intellectual ability is achieved.

Environmental influence

Sociocultural

Family

Having access to resources of the home, and having a home life conducive to learning is definitely associated with scores on intelligence tests. However, it is difficult to disentangle possible genetic factors from a parent's attitude or use of language, for example.

A child's ordinal position in their family has also been shown to affect intelligence. A number of studies have indicated that as birth order increases IQ decreases with first borns having especially superior intelligence. Many explanations for this have been proposed but the most widely accepted idea is that first borns receive more attention and resources from parents and are expected to focus on task achievement, whereas later borns are more focused on sociability.

The type and amount of praise received from family can also affect how intelligence develops. Research by Dweck and colleagues suggests that feedback to a child on their academic achievements can alter their future intelligence scores. Telling a child that they are intelligent and praising them for this 'intrinsic' quality indicates that intelligence is fixed, known as entity theory. Children holding the entity theory of ability have been reported as performing less well after a failure, perhaps because they believe that failure on a task indicates that they are not intelligent, and that therefore there is no point in trying to challenge themselves after failure. Dweck contrasts this with incremental theory beliefs – the idea that intelligence can be improved upon with effort. Children who hold this theory are more likely to develop a love for learning rather than for achievement. Parents who praise the child's effort at a task rather than the result are more likely to instill this incremental theory of intelligence in their children and thus to improve their intelligence.

Peer group

JR Harris suggested in The Nurture Assumption that an individual's peer group influences their intelligence greatly over time, and that different peer group characteristics may be responsible for the black-white IQ gap. Several longitudinal studies support the conjecture that peer groups significantly affect scholastic achievement, but relatively few studies have examined the effect on tests of cognitive ability. 

The peer group an individual identifies with can also influence intelligence through the stereotypes associated with that group. The stereotype threat, first introduced by Claude Steele, is the idea that people belonging to a stereotyped group may perform poorly in a situation where the stereotype is relevant. This has been shown to be a factor in differences in intelligence test scores between different ethnic groups, men and women, people of low and high social status and young and old participants. For example, females who were told that women are worse at chess than men, performed worse in a game of chess than females who were not told this.

Education

IQ and educational attainment are strongly correlated (estimates range form .40 to over .60.) There is controversy, however, as to whether education affects intelligence – it may be both a dependent and independent variable with regard to IQ. A study by Ceci illustrates the numerous ways in which education can affect intelligence. It was found that; IQ decreases during summer breaks from schooling, children with delayed school entry have lower IQ's, those who drop out of education earlier have lower IQ's and children of the same age but of one years less schooling have lower IQ scores. Thus it is difficult to unravel the interconnected relationship of IQ and education where both seem to affect one another.

Those who do better on childhood intelligence tests tend to have a lower drop out rate, and complete more years of school and are predictive of school success. For instance, one of the largest ever studies found a correlation of 0.81 between the general intelligence or g-factor and GCSE results. On the other hand, education has been shown to improve performance on intelligence tests. Research controlling for childhood IQ and treating years of education as a causal variable suggests that education causes an increase in total IQ score, although general intelligence was not affected.

For instance a natural experiment in Norway where the school leaving age was changed suggested that IQ was raised by additional year of school. School may alter specific knowledge, rather than general ability or biological speed. In terms of what matters about school, it appears that simple quantity or years-in-school may be what underpins the linkage of education with performance on IQ tests.

Training and interventions

Research on the effectiveness of interventions, and the degree to which fluid intelligence can be increased, especially after age 16, is somewhat controversial. Fluid intelligence is typically thought of as something more innate, and defined as immutable after maturity. One recent article however, demonstrates that, at least for a period of time, fluid intelligence can be increased through training in increasing an adult's working memory capacity. Working memory capacity is defined as the ability to remember something temporarily, like remembering a phone number just long enough to dial it.

In an experiment, groups of adults were first assessed using standard tests for fluid intelligence. Then they trained groups for four different numbers of days, for half an hour each day, using an n-back exercise that worked on improving one's working memory. It supposedly does so through a few different components, involving having to ignore irrelevant items, manage tasks simultaneously, and monitor performance on exercise, while connecting related items. After this training, the groups were tested again and those with training (compared against control groups who did not undergo training) showed significant increases in performance on the fluid intelligence tests.

A study by Blackwell et al. found that they could improve a child's mathematics achievement depending on which theory of intelligence they were taught; incremental or entity theory. Entity theory supposes that intelligence is fixed and cannot be altered by working harder. Incremental theory on the other hand assumes that intelligence is malleable and can be developed and improved with effort. Over the course of a year they found that students who had been taught the incremental theory of intelligence showed an upward trajectory in grades in mathematics throughout the year whereas those who had been taught entity theory showed no improvement. This indicates that teaching incremental theory may improve performance on academic tasks, though further research is needed to investigate whether the same results can be found for general intelligence.

Other studies have looked at improving intelligence and preventing cognitive decline by using cognition enhancing substances known as nootropics. One such study gave participants a number of known nootropics in combination in the hopes of targeting numerous cellular mechanisms and increasing the effects on cognition that each would have if administered individually. They conducted a double blind test and administered the combination treatment or placebo to adults for 28 days. They administered Raven's Advanced Progressive Matrices as a measure of intelligence on the first day and after 28 days. The results indicated a significant improvement in performance for those who had taken the treatment compared to those taking the placebo. The effect was equivalent to an increase in IQ of around 6 points.

Environmental enrichment

Environmental enrichment affects cognition and intellectual development from a neurobiological perspective. More stimulating environments can increase the number of synapses in the brain which increases synaptic activity. In humans this is most likely to occur during the development of the brain but can also occur in adults. Most of the research on environmental enrichment has been carried out on non human animals. In an experiment, four different habitats were set up to test how environmental enrichment or relative impoverishment affected rats' performance on various measures of intelligent behavior. First, rats were isolated, each to its own cage. In a second condition, the rats were still in isolation, but this time they had some toy, or enriching object in the cage with them. The third condition placed the rats in cages with each other, so they were receiving social enrichment, without any enriching object. The fourth and final condition exposed the rats to both social interaction and some form of object enrichment.

In measuring intellectual capacity, the rats who had both forms of enrichment performed best, the ones with social enrichment performed second best, and the ones with a toy in their cage performed still better than the rats with no toy or other rats. When the volume of the rat's cortices was measured the amount of enrichment again correlated with larger volume, which is an indicator of more synaptic connections, and greater intelligence. Attaining this sort of information in humans would be difficult as it requires histological research. 

However, studies where environmental deprivation has occurred provide insight and indicate that a lack of stimulation can lead to cognitive impairment. Further research using educational attainment as an indicator of cognitive stimulation have found that those with higher levels of education show fewer signs of cognitive aging and that stimulating environments could be used in the treatment of cognitive aging dysfunctions such as dementia.

Biological influences

Nutrition

Nutrition has been shown to affect intelligence prenatally and postnatally. The idea that prenatal nutrition may affect intelligence comes from Barker's hypothesis of fetal programming, which states that during critical stages of development the intrauterine environment affects or 'programmes' how the child will develop. Barker cited nutrition as being one of the most important intrauterine influences affecting development and that under-nutrition could permanently change the physiology and development of the child. It has been shown that under-nutrition, particularly protein malnutrition, can lead to irregular brain maturation and learning disabilities.

As prenatal nutrition is difficult to measure, birth weight has been used as a surrogate marker of nutrition in many studies. Birth weight needs to be corrected for gestational length to ensure that the effects are due to nutrition and not prematurity. The first longitudinal study looking at the effects of under-nutrition, as measured by birth weight, and intelligence focused on males who were born during the Dutch famine. The results indicated that there were no effects of under-nutrition on intellectual development. However, many studies since have found a significant relationship and a meta-analysis by Shenkin and colleagues indicates that birth weight is associated with scores on intelligence tests in childhood.

Post-natal malnutrition can also have a significant influence on intellectual development. This relationship has been harder to establish because the issue of malnutrition is often conflated with socioeconomic issues. However, it has been demonstrated in a few studies where pre-schoolers in two Guatemalan villages (where undernourishment is common) were given protein nutrition supplements for several years, and even in the lowest socioeconomic class, those children showed an increase in performance on intelligence tests, relative to controls with no dietary supplement.

Malnutrition has been shown to affect organizational processes of the brain such as neurogenesis, synaptic pruning, cell migration and cellular differentiation. This thus results in abnormalities in the formation of neural circuits and the development of neurotransmitter systems. However, some of these effects of malnutrition have been shown to be improved upon with a good diet and environment. Early nutrition can also affect brain structures that are actually correlated to IQ levels. Specifically, the caudate nucleus is particularly affected by early environmental factors and its volume correlates with IQ. In an experiment by Isaacs et al., infants born prematurely were either assigned a standard or high-nutrient diet during the weeks directly after birth. When the individuals were assessed later in adolescence, it was found that the high-nutrient group had significantly larger caudate volumes and scored significantly higher on verbal IQ tests. This study also found that the extent to which the caudate volume size related selectively to verbal IQ was much greater in male participants, and not very significant in females. This may help explain the finding in other earlier research that the effects of early diet on intelligence are more predominant in males.

Another study done by Lucas et al. confirms the conclusions about the importance of nutrition in the cognitive development of individuals born prematurely. It also found that the cognitive function of males was significantly more impaired by poorer postnatal nutrition. A unique finding however, was that there was a higher incidence of cerebral palsy in the individuals who were fed the non-nutrient enhanced formula.

Breast feeding has long been purported to supply important nutrients to infants and has been correlated with increased cognitive gains later in childhood. The link between intelligence and breast feeding has even been shown to persist into adulthood. However, this view has been challenged in recent times by studies which have found no such link between breast feeding and cognitive abilities. A meta-analysis by Der, batty and Deary concluded that there was no link between IQ and breastfeeding when maternal intelligence had been accounted for and that mothers' intelligence is likely to be the link between breastfeeding and intelligence.

Other studies have indicated that breast feeding may be particularly important for children born Small for Gestational Age (SGA). A study by Slykerman et al. found that there was no association between breast feeding and higher intelligence in their full sample but that when looking only at SGA babies there was a significant increase in intelligence for those who had been breastfed over those who had not.

A 2007 study provides a possible resolution to the different results found across studies investigating breastfeeding's effect on intelligence. Caspi et al. found that whether breast feeding increased IQ was linked to whether the infant had a certain variant of the FADS2 gene. Children with the C variant of the gene showed an IQ advantage of 7 points when breastfed, whereas those with the GG variant showed no IQ advantages with breastfeeding. However, other studies have failed to replicate this result.

Stress

Maternal stress levels may affect the developing child's intelligence. The timing and duration of stress can greatly alter the fetus' brain development which can have long-term effects on intelligence. Maternal reactions to stress such as increased heart rate are dampened during pregnancy in order to protect the fetus. The impact of stress can be seen across many different species and can be an indicator of the outside environment which can help the fetus to adapt for surviving in the outside world. However, not all maternal stress has been perceived as bad as some has been seen to induce advantageous adaptions.

Stress during early childhood may also affect the child's development and have negative consequences on neural systems underlying fluid intelligence. A 2006 study found that IQ scores were related to the number of traumas and symptoms of post traumatic stress disorder (PTSD) in children and adults. Similarly, another study found that exposure to violence in the community and the subsequent distress, were related to a significant decrease in intelligence scores and reading abilities in children aged 6–7 years. Exposure to violence in the community had similar cognitive effects as experiencing childhood maltreatment or trauma.

Maternal age

Maternal age has been shown to be related to intelligence with younger mothers tending have children of lower intelligence than older mothers. However, this relationship may be non-linear with older mothers being at increased risk of giving birth to children with down syndrome which greatly affects cognitive abilities.

Exposure to toxic chemicals and other substances

Lead exposure has been proven to have significant effects on the intellectual development of a child. In a long-term study done by Baghurst et al. 1992, children who grew up next to a lead-smelting plant had significantly lower intelligence test scores, negatively correlated with their blood-lead level exposure. Even though lead levels have been reduced in our environment, some areas in the United States, particularly inner cities, are still at risk for exposing their children.

Furthermore, prenatal exposure to alcohol can greatly affect a child's performance on intelligence tests, and their intellectual growth. At high doses, fetal alcohol syndrome can develop, which causes mental retardation, as well as other physical symptoms, such as head and face deformities, heart defects and slow growth. It is estimated that 1 in 1,000 babies born in the general population are born with fetal alcohol syndrome, as a result of heavy use of alcohol during pregnancy.

However, studies have shown that even at slightly less severe doses, prenatal exposure to alcohol can still affect the intelligence of the child in development, without having the full syndrome. Through a study done by Streissguth, Barr, Sampson, Darby, and Martin in 1989, it was shown that moderate prenatal doses of alcohol, defined as the mother ingesting 1.5 oz. daily, lowered children's test scores by 4 point below control levels, by the age of four. They also showed that prenatal exposure to aspirin and antibiotics is correlated with lower performance on intelligence tests as well. However, more recent studies have found that low to moderate alcohol consumption is not associated with children's intelligence scores. This contradictory evidence could perhaps be explained by findings that the effects of alcohol may depend on the genetic makeup of the fetus. In a recent study Lewis et al., looked at alcohol dehydrogenase genes and their mutations, which humans can have between 0 and 10. These mutations slow the breakdown of alcohol so the more mutations the fetus has the slower they will breakdown alcohol. They found that in children whose mothers had drank moderately, those children with four or more mutations performed worse on an intelligence test than those with two or less mutations.

In another study, prenatal drug exposure was shown to have significantly negative effects on cognitive functioning, as measured at the age of five, compared again controls matched for socioeconomic status and inner-city environment. The researchers concluded that prenatally drug-exposed children are at greater risk for learning difficulties and attention problems in school, and therefore should be the subject of interventions to support educational success. It could be hypothesized that the effect of these drugs on the development of the brain prenatally, and axon guidance could be the root of the negative consequences on later deficits in intellectual development.

Specifically, prenatal exposure to marijuana affects development of intelligence later in childhood, in a nonlinear fashion, with the degree of exposure. Heavy use by the mother within the first trimester is associated with lower verbal reasoning scores on the Stanford-Binet Intelligence Scale; heavy use during the second trimester is associated with deficits in composite, short-term memory as well as lower quantitative scores on the test; high exposure in third trimester associated with lower quantitative scores as well. A study by Fried and Smith indicated that marijuana exposure did not lead to a decrease in global intelligence but that it did lead to problems with executive functions in childhood. However, another study found that when influences such as maternal age, mother's personality and home environment, there was no longer a difference between children exposed to marijuana and those not exposed with relation to executive functions.

Exposure to tobacco smoking has been associated with diminished intelligence and attentional problems. One study indicated that children whose mothers had smoked 10 or more cigarettes a day were between 3 and 5 months behind schoolmates in reading, maths and general ability. However, other studies have found no direct link between IQ and tobacco smoking with maternal intelligence accounting entirely for the relationship.

Perinatal factors

There is also evidence that birth complications and other factors around the time of birth (perinatal) can have serious implications on intellectual development. For example, a prolonged period of time without access to oxygen during the delivery can lead to brain damage and mental retardation. Also, low birth weights have been linked to lower intelligence scores later in lives of the children. There are two reasons for low birth weight, either premature delivery or the infant's size is just lower than average for its gestational age; both contribute to intellectual deficits later in life. A meta analysis of low birth weight babies found that there is a significant relationship between low birth weight and impaired cognitive abilities; however, the relationship is small, and they concluded that, although it may not be relevant at an individual level, it may instead be relevant at a population level. Other studies have also found that the correlations are relatively small unless the weight is extremely low (less than 1,500 g) – in which case the effects on intellectual development are more severe and often result in mental retardation.

Development of genius

It has been hypothesized that the development of genius in an area results from early environmental exposure to the topic in which the "genius" has prodigious knowledge or skill. This is utilizing the definition of genius that is not just a significantly higher than average IQ score, but also having some type of exceptional understanding or ability in a specific field. Einstein is often used as an example of genius; he did not demonstrate generalized exceptional intelligence as a child; however, there is evidence that he started exploring the ideas of physics and the universe at a young age.

This fits with the model of development of fluid intelligence before age of maturity because the neuronal connections are still being made in childhood. The idea is if you expose a child to concepts of, for example theoretical physics, before their brain stops responding to the environment in a plastic way, then you get exceptional understanding of that field in adulthood, because there was a framework developed for it in early childhood. However, Garlick proposes that early environmental experience with their field of genius, is necessary but not sufficient to the development of genius.

Intelligence alone is not enough for the development of genius but the pathways and neural connections for divergent thinking are also necessary. Thus the home must encourage creativity. The parents of gifted children tend to supply enriching environments with intellectually and culturally stimulating materials thus increasing the child's likelihood to engage in creative activities.

There are many environmental influences on intelligence, typically divided into biological and non-biological factors, often involving social or cultural factors. The commonality between these two divisions is the exposure in early childhood. It seems as though exposure to these various positive or negative influences on intelligence levels needs to happen early on in the development of the brain, before the neuronal connections have ceased forming.

Parents of gifted children also tend to have above average educational achievement and at least one tends to work in an intellectual profession. There is also evidence that the probability of a gifted child becoming a genius may be increased if the child has had to face adversity or trauma and that a traditional upbringing may encourage conformity and discourage the necessary divergent thinking.

Training

Training at an early age reduces synaptic pruning, which helps save neurons.

Musical

Early musical training in children is said to improve IQ. Schellenberg conducted a study in which children either received music lessons, drama lessons or no lessons and measured their intelligence scores. He found that children in the music group showed a greater overall increase in IQ scores than the children in the other groups. However, a study claimed that musical training improves verbal, but not visual memory. Significant differences in brain structure between musicians and non-musicians have been found. It was shown that there were significant differences in gray matter volume in motor, auditory and visual-spatial regions of the brain. The authors suggest that this could in part be because musicians from a young age translate visually perceived musical notes into motor commands whilst listening to the auditory output.

Studies have shown that listening to Mozart before taking an IQ test will improve scores. This is called the Mozart Effect. The Mozart Effect improves spatial-temporal reasoning. For example, one study found that college students scores on a spatial abilities test increased by 8-9 points after they had listened to Mozart whereas there was no increase when they listened to relaxation instructions or silence.

Chess

Studies have shown that chess requires auditory-verbal-sequential skills, not visuospatial skills. A German study found that Garry Kasparov, a Soviet / Russian former World Chess Champion, regarded by many as the greatest chess player of all time, has an IQ of 135 and an extremely good memory. Similarly, a study looking at young Belgian chess experts found that they have an average IQ of 121, a verbal IQ of 109 and a performance IQ of 129. However, a recent study looking at an elite group of young chess players found that intelligence was not a significant factor in chess skill.

One study found that students who were taking a chess class improved mathematical and comprehension performance. Despite this a recent study found that chess did not improve students' academic or cognitive abilities. At-risk students were put into 2 groups: one group was given a chess class once a week for 90 minutes, the other group was not. The results indicated no differences between the groups in changes in math, reading, writing or general intelligence.

Flynn effect

From Wikipedia, the free encyclopedia

 

The Flynn effect is the substantial and long-sustained increase in both fluid and crystallized intelligence test scores that were measured in many parts of the world over the 20th century. When intelligence quotient (IQ) tests are initially standardized using a sample of test-takers, by convention the average of the test results is set to 100 and their standard deviation is set to 15 or 16 IQ points. When IQ tests are revised, they are again standardized using a new sample of test-takers, usually born more recently than the first. Again, the average result is set to 100. However, when the new test subjects take the older tests, in almost every case their average scores are significantly above 100.

 

Test score increases have been continuous and approximately linear from the earliest years of testing to the present. For the Raven's Progressive Matrices test, a study published in the year 2009 found that British children's average scores rose by 14 IQ points from 1942 to 2008. Similar gains have been observed in many other countries in which IQ testing has long been widely used, including other Western European countries, Japan, and South Korea.

There are numerous proposed explanations of the Flynn effect, as well as some skepticism about its implications. Similar improvements have been reported for other cognitions such as semantic and episodic memory. Research suggests that there is an ongoing reversed Flynn effect, i.e. a decline in IQ scores, in Norway, Denmark, Australia, Britain, the Netherlands, Sweden, Finland, France and German-speaking countries, a development which appears to have started in the 1990s.

Origin of term

The Flynn effect is named for James R. Flynn, who did much to document it and promote awareness of its implications. The term itself was coined by Richard Herrnstein and Charles Murray, authors of The Bell Curve. Although the general term for the phenomenon—referring to no researcher in particular—continues to be "secular rise in IQ scores", many textbooks on psychology and IQ testing have now followed the lead of Herrnstein and Murray in calling the phenomenon the Flynn effect.

Rise in IQ

IQ tests are updated periodically. For example, the Wechsler Intelligence Scale for Children (WISC), originally developed in 1949, was updated in 1974, 1991, 2003 and again in 2014. The revised versions are standardized based on the performance of test-takers in standardization samples. A standard score of IQ 100 is defined as the median performance of the standardization sample. Thus one way to see changes in norms over time is to conduct a study in which the same test-takers take both an old and new version of the same test. Doing so confirms IQ gains over time. Some IQ tests, for example tests used for military draftees in NATO countries in Europe, report raw scores, and those also confirm a trend of rising scores over time. The average rate of increase seems to be about three IQ points per decade in the United States, as scaled by the Wechsler tests. The increasing test performance over time appears on every major test, in every age range, at every ability level, and in every modern industrialized country, although not necessarily at the same rate as in the United States. The increase was continuous and roughly linear from the earliest days of testing to the mid-1990s. Though the effect is most associated with IQ increases, a similar effect has been found with increases in attention and of semantic and episodic memory.

Ulric Neisser estimated that using the IQ values of 1997, the average IQ of the United States in 1932, according to the first Stanford–Binet Intelligence Scales standardization sample, was 80. Neisser states that "Hardly any of them would have scored 'very superior', but nearly one-quarter would have appeared to be 'deficient.'" He also wrote that "Test scores are certainly going up all over the world, but whether intelligence itself has risen remains controversial."

Trahan et al. (2014) found that the effect was about 2.93 points per decade, based on both Stanford–Binet and Wechsler tests; they also found no evidence the effect was diminishing. In contrast, Pietschnig and Voracek (2015) reported, in their meta-analysis of studies involving nearly 4 million participants, that the Flynn effect had decreased in recent decades. They also reported that the magnitude of the effect was different for different types of intelligence ("0.41, 0.30, 0.28, and 0.21 IQ points annually for fluid, spatial, full-scale, and crystallized IQ test performance, respectively"), and that the effect was stronger for adults than for children.

Raven (2000) found that, as Flynn suggested, data interpreted as showing a decrease in many abilities with increasing age must be re-interpreted as showing that there has been a dramatic increase of these abilities with date of birth. On many tests this occurs at all levels of ability.

Some studies have found the gains of the Flynn effect to be particularly concentrated at the lower end of the distribution. Teasdale and Owen (1989), for example, found the effect primarily reduced the number of low-end scores, resulting in an increased number of moderately high scores, with no increase in very high scores. In another study, two large samples of Spanish children were assessed with a 30-year gap. Comparison of the IQ distributions indicated that the mean IQ scores on the test had increased by 9.7 points (the Flynn effect), the gains were concentrated in the lower half of the distribution and negligible in the top half, and the gains gradually decreased as the IQ of the individuals increased. Some studies have found a reverse Flynn effect with declining scores for those with high IQ.

In 1987, Flynn took the position that the very large increase indicates that IQ tests do not measure intelligence but only a minor sort of "abstract problem-solving ability" with little practical significance. He argued that if IQ gains do reflect intelligence increases, there would have been consequent changes of our society that have not been observed (a presumed non-occurrence of a "cultural renaissance"). Flynn no longer endorses this view of intelligence and has since elaborated and refined his view of what rising IQ scores mean.

Precursors to Flynn's publications

Earlier investigators had discovered rises in raw IQ test scores in some study populations, but had not published general investigations of that issue in particular. Historian Daniel C. Calhoun cited earlier psychology literature on IQ score trends in his book The Intelligence of a People (1973). R. L. Thorndike drew attention to rises in Stanford-Binet scores in a 1975 review of the history of intelligence testing.

Intelligence

There is debate about whether the rise in IQ scores also corresponds to a rise in general intelligence, or only a rise in special skills related to taking IQ tests. Because children attend school longer now and have become much more familiar with the testing of school-related material, one might expect the greatest gains to occur on such school content-related tests as vocabulary, arithmetic or general information. Just the opposite is the case: abilities such as these have experienced relatively small gains and even occasional decreases over the years. Meta-analytic findings indicate that Flynn effects occur for tests assessing both fluid and crystallized abilities. For example, Dutch conscripts gained 21 points during only 30 years, or 7 points per decade, between 1952 and 1982. But this rise in IQ test scores is not wholly explained by an increase in general intelligence. Studies have shown that while test scores have improved over time, the improvement is not fully correlated with latent factors related to intelligence. Rushton has shown that the gains in IQ over time (the Lynn-Flynn effect) are unrelated to g. Other researchers have shown that the IQ gains described by the Flynn effect are due in part to increasing intelligence, and in part to increases in test-specific skills. In parallel with the measured gains in IQ scores, secular declines have been found for "mental speed, digit span backwards, the use of difficult words, and color acuity, all of which are related to intelligence".

Proposed explanations

A 2017 survey of 75 experts in the field of intelligence research suggested four key causes of the Flynn effect: Better health, better nutrition, more and better education, and rising standards of living. Genetic changes were seen as not important. The experts' views agreed with an independently performed meta-analysis on published Flynn effect data, except that the latter found life history speed to be the most important factor.

The expert survey explained the possible end or decline in the Flynn effect by asymmetric fertility by means of genetic effects, migration, asymmetric fertility by means of socialization effects, declines in education, and the influence of media.

Schooling and test familiarity

Duration of average schooling has increased steadily. One problem with this explanation is that if in the US comparing older and more recent subjects with similar educational levels, then the IQ gains appear almost undiminished in each such group considered individually.

Many studies find that children who do not attend school score drastically lower on the tests than their regularly attending peers. During the 1960s, when some Virginia counties closed their public schools to avoid racial integration, compensatory private schooling was available only for Caucasian children. On average, the scores of African-American children who received no formal education during that period decreased at a rate of about six IQ points per year.

Another explanation is an increased familiarity of the general population with tests and testing. For example, children who take the very same IQ test a second time usually gain five or six points. However, this seems to set an upper limit on the effects of test sophistication. One problem with this explanation and others related to schooling is that in the US, the groups with greater test familiarity show smaller IQ increases.

Early intervention programs have shown mixed results. Some preschool (ages 3–4) intervention programs like "Head Start" do not produce lasting changes of IQ, although they may confer other benefits. The "Abecedarian Early Intervention Project", an all-day program that provided various forms of environmental enrichment to children from infancy onward, showed IQ gains that did not diminish over time. The IQ difference between the groups, although only five points, was still present at age 12. Not all such projects have been successful. Also, such IQ gains can diminish until age 18.

Citing a high correlation between rising literacy rates and gains in IQ, David Marks has argued that the Flynn effect is caused by changes in literacy rates.

Generally more stimulating environment

Still another theory is that the general environment today is much more complex and stimulating. One of the most striking 20th-century changes of the human intellectual environment has come from the increase of exposure to many types of visual media. From pictures on the wall to movies to television to video games to computers, each successive generation has been exposed to richer optical displays than the one before and may have become more adept at visual analysis. This would explain why visual tests like the Raven's have shown the greatest increases. An increase only of particular form(s) of intelligence would explain why the Flynn effect has not caused a "cultural renaissance too great to be overlooked."

In 2001, Dickens and Flynn presented a model for resolving several contradictory findings regarding IQ. They argue that the measure "heritability" includes both a direct effect of the genotype on IQ and also indirect effects such that the genotype changes the environment, thereby affecting IQ. That is, those with a greater IQ tend to seek stimulating environments that further increase IQ. These reciprocal effects result in gene environment correlation. The direct effect could initially have been very small, but feedback can create large differences of IQ. In their model, an environmental stimulus can have a very great effect on IQ, even for adults, but this effect also decays over time unless the stimulus continues (the model could be adapted to include possible factors, like nutrition during early childhood, that may cause permanent effects). The Flynn effect can be explained by a generally more stimulating environment for all people. The authors suggest that any program designed to increase IQ may produce long-term IQ gains if that program teaches children how to replicate the types of cognitively demanding experiences that produce IQ gains outside the program. To maximize lifetime IQ, the programs should also motivate them to continue searching for cognitively demanding experiences after they have left the program.

Flynn in his 2007 book What Is Intelligence? further expanded on this theory. Environmental changes resulting from modernization—such as more intellectually demanding work, greater use of technology and smaller families—have meant that a much larger proportion of people are more accustomed to manipulating abstract concepts such as hypotheses and categories than a century ago. Substantial portions of IQ tests deal with these abilities. Flynn gives, as an example, the question 'What do a dog and a rabbit have in common?' A modern respondent might say they are both mammals (an abstract, or a priori answer, which depends only on the meanings of the words dog and rabbit), whereas someone a century ago might have said that humans catch rabbits with dogs (a concrete, or a posteriori answer, which depended on what happened to be the case at that time).

Nutrition

Improved nutrition is another possible explanation. Today's average adult from an industrialized nation is taller than a comparable adult of a century ago. That increase of stature, likely the result of general improvements of nutrition and health, has been at a rate of more than a centimeter per decade. Available data suggest that these gains have been accompanied by analogous increases of head size, and by an increase in the average size of the brain. This argument had been thought to suffer the difficulty that groups who tend to be of smaller overall body size (e.g. women, or people of Asian ancestry) do not have lower average IQs. Richard Lynn, however, claims that while people of East Asian origin may often have smaller bodies, they tend to have larger brains and higher IQs than average whites.

A 2005 study presented data supporting the nutrition hypothesis, which predicts that gains will occur predominantly at the low end of the IQ distribution, where nutritional deprivation is probably most severe. An alternative interpretation of skewed IQ gains could be that improved education has been particularly important for this group. Richard Lynn makes the case for nutrition, arguing that cultural factors cannot typically explain the Flynn effect because its gains are observed even at infant and preschool levels, with rates of IQ test score increase about equal to those of school students and adults. Lynn states that "This rules out improvements in education, greater test sophistication, etc. and most of the other factors that have been proposed to explain the Flynn effect. He proposes that the most probable factor has been improvements in pre-natal and early post-natal nutrition."

A century ago, nutritional deficiencies may have limited body and organ functionality, including skull volume. The first two years of life is a critical time for nutrition. The consequences of malnutrition can be irreversible and may include poor cognitive development, educability, and future economic productivity. On the other hand, Flynn has pointed to 20-point gains on Dutch military (Raven's type) IQ tests between 1952, 1962, 1972, and 1982. He observes that the Dutch 18-year-olds of 1962 had a major nutritional handicap. They were either in the womb, or were recently born, during the great Dutch famine of 1944—when German troops monopolized food and 18,000 people died of starvation. Yet, concludes Flynn, "they do not show up even as a blip in the pattern of Dutch IQ gains. It is as if the famine had never occurred." It appears that the effects of diet are gradual, taking effect over decades (affecting mother as well as child) rather than a few months.

In support of the nutritional hypothesis, it is known that, in the United States, the average height before 1900 was about 10 cm (∼4 inches) shorter than it is today. Possibly related to the Flynn effect is a similar change of skull size and shape during the last 150 years. Though the idea that brain size is unrelated to race and intelligence was popularized in the 1980s, studies continue to show significant correlations. A Norwegian study found that height gains were strongly correlated with intelligence gains until the cessation of height gains in military conscript cohorts towards the end of the 1980s. Both height and skull size increases probably result from a combination of phenotypic plasticity and genetic selection over this period. With only five or six human generations in 150 years, time for natural selection has been very limited, suggesting that increased skeletal size resulting from changes in population phenotypes is more likely than recent genetic evolution.

It is well known that micronutrient deficiencies change the development of intelligence. For instance, one study has found that iodine deficiency causes a fall, on average, of 12 IQ points in China.

Scientists James Feyrer, Dimitra Politi, and David N. Weil have found in the U.S. that the proliferation of iodized salt increased IQ by 15 points in some areas. Journalist Max Nisen has stated that, with this type of salt becoming popular, that "the aggregate effect has been extremely positive."

Daley et al. (2003) found a significant Flynn effect among children in rural Kenya, and concluded that nutrition was one of the hypothesized explanations that best explained their results (the others were parental literacy and family structure).

Infectious diseases

Eppig, Fincher, and Thornhill (2009) argue that "From an energetics standpoint, a developing human will have difficulty building a brain and fighting off infectious diseases at the same time, as both are very metabolically costly tasks" and that "the Flynn effect may be caused in part by the decrease in the intensity of infectious diseases as nations develop." They suggest that improvements in gross domestic product (GDP), education, literacy, and nutrition may have an effect on IQ mainly through reducing the intensity of infectious diseases.

Eppig, Fincher, and Thornhill (2011) in a similar study instead looking at different US states found that states with a higher prevalence of infectious diseases had lower average IQ. The effect remained after controlling for the effects of wealth and educational variation.

Atheendar Venkataramani (2010) studied the effect of malaria on IQ in a sample of Mexicans. Exposure during the birth year to malaria eradication was associated with increases in IQ. It also increased the probability of employment in a skilled occupation. The author suggests that this may be one explanation for the Flynn effect and that this may be an important explanation for the link between national malaria burden and economic development. A literature review of 44 papers states that cognitive abilities and school performance were shown to be impaired in sub-groups of patients (with either cerebral malaria or uncomplicated malaria) when compared with healthy controls. Studies comparing cognitive functions before and after treatment for acute malarial illness continued to show significantly impaired school performance and cognitive abilities even after recovery. Malaria prophylaxis was shown to improve cognitive function and school performance in clinical trials when compared to placebo groups.

Heterosis

Heterosis, or hybrid vigor associated with historical reductions of the levels of inbreeding, has been proposed by Michael Mingroni as an alternative explanation of the Flynn effect. However, James Flynn has pointed out that even if everyone mated with a sibling in 1900, subsequent increases in heterosis would not be a sufficient explanation of the observed IQ gains.

Possible end of progression

Jon Martin Sundet and colleagues (2004) examined scores on intelligence tests given to Norwegian conscripts between the 1950s and 2002. They found that the increase of scores of general intelligence stopped after the mid-1990s and declined in numerical reasoning sub-tests.

Teasdale and Owen (2005) examined the results of IQ tests given to Danish male conscripts. Between 1959 and 1979 the gains were 3 points per decade. Between 1979 and 1989 the increase approached 2 IQ points. Between 1989 and 1998 the gain was about 1.3 points. Between 1998 and 2004 IQ declined by about the same amount as it gained between 1989 and 1998. They speculate that "a contributing factor in this recent fall could be a simultaneous decline in proportions of students entering 3-year advanced-level school programs for 16–18-year-olds." The same authors in a more comprehensive 2008 study, again on Danish male conscripts, found that there was a 1.5-point increase between 1988 and 1998, but a 1.5-point decrease between 1998 and 2003/2004. A possible contributing factor to the more recent decline may be changes in the Danish educational system. Another may be the rising proportion of immigrants or their immediate descendants in Denmark. This is supported by data on Danish draftees where first or second generation immigrants with Danish nationality score below average.

In Australia, the IQ of 6–12 year olds as measured by the Colored Progressive Matrices has shown no increase from 1975–2003.

In the United Kingdom, a study by Flynn (2009) found that tests carried out in 1980 and again in 2008 show that the IQ score of an average 14-year-old dropped by more than two points over the period. For the upper half of the results the performance was even worse. Average IQ scores declined by six points. However, children aged between five and 10 saw their IQs increase by up to half a point a year over the three decades. Flynn argues that the abnormal drop in British teenage IQ could be due to youth culture having “stagnated” or even dumbed down. He also states that the youth culture is more oriented towards computer games than towards reading and holding conversations. Researcher Richard Gray, commenting on the study, also mentions the computer culture diminishing reading books as well as a tendency towards teaching to the test.

Lynn and Harvey argued in 2008 that the causes of the above are difficult to interpret since these countries had had significant recent immigration from countries with lower average national IQs. Nevertheless, they expect that similar patterns will occur, or have occurred, first in other developed nations and then in the developing world as there is a limit to how much environmental factors can improve intelligence. Furthermore, during the last century there is a negative correlation between fertility and intelligence although there is not yet any conclusive evidence of the association between the two. They estimate that there has been a dysgenic decline in the world's genotypic IQ (masked by the Flynn effect for the phenotype) of 0.86 IQ points per decade for the years 1950–2000.

Bratsberg & Rogeberg (2018) present evidence that the Flynn effect in Norway has reversed, and that both the original rise in mean IQ scores and their subsequent decline were caused by environmental factors.

IQ group differences

If the Flynn effect has ended in developed nations, then this may possibly allow national differences in IQ scores to diminish if the Flynn effect continues in nations with lower average national IQs.

Also, if the Flynn effect has ended for the majority in developed nations, it may still continue for minorities, especially for groups like immigrants where many may have received poor nutrition during early childhood or have had other disadvantages. A study in the Netherlands found that children of non-Western immigrants had improvements for g, educational achievements, and work proficiency compared to their parents, although there were still remaining differences compared to ethnic Dutch.

There is a controversy as to whether the US racial gap in IQ scores is diminishing. If that is the case then this may or may not be related to the Flynn effect. Flynn has commented that he never claimed that the Flynn effect has the same causes as the black-white gap, but that it shows that environmental factors can create IQ differences of a magnitude similar to the gap. Research that has examined whether g factor and IQ gains from the Flynn effect are related have found there is a negative correlation between the two, which may indicate that group differences and the Flynn effect are possibly due to differing causes.

The Flynn effect has also been part of the discussions regarding Spearman's hypothesis, which states that differences in the g factor are the major source of differences between blacks and whites observed in many studies of race and intelligence.