Environmental
journalists and advocates have in recent weeks made a number of
apocalyptic predictions about the impact of climate change.
Bill McKibben suggested climate-driven fires in Australia had made koalas “functionally extinct.”
Extinction Rebellion said: “Billions will die” and “Life on Earth is dying.” Viceclaimed the “collapse of civilization may have already begun.”
Few
have underscored the threat more than student climate activist Greta
Thunberg and Green New Deal sponsor Rep. Alexandria Ocasio-Cortez.
The latter said, “The world is going to end in 12 years if we don’t address climate change.” Says
Thunberg in her new book, “Around 2030 we will be in a position to set
off an irreversible chain reaction beyond human control that will lead
to the end of our civilization as we know it.”
Sometimes,
scientists themselves make apocalyptic claims. “It’s difficult to see
how we could accommodate a billion people or even half of that,” if
Earth warms four degrees, said one earlier this year.
“The potential for multi-breadbasket failure is increasing,” said another. If sea levels rise as much as the Intergovernmental Panel on Climate Change predicts, another scientist said, “It will be an unmanageable problem.”
Apocalyptic statements like these have real-world impacts. In September, a group of British psychologists said children are increasingly suffering from anxiety from the frightening discourse around climate change.
In
October, an activist with Extinction Rebellion (”XR”) — an
environmental group founded in 2018 to commit civil disobedience to draw
awareness to the threat its founders and supporters say climate change
poses to human existence — and a videographer, were kicked and beaten in a London Tube station by angry commuters.
And last week, an XR co-founder said a genocide like the Holocaust was “happening again, on a far greater scale, and in plain sight” from climate change.
Climate
change is an issue I care passionately about and have dedicated a
significant portion of my life to addressing. I have been politically active on the issue for over 20 years and have researched and written about it for 17 years.
Over
the last four years, my organization, Environmental Progress, has
worked with some of the world’s leading climate scientists to prevent
carbon emissions from rising. So far, we’ve helped prevent emissions
from increasing the equivalent of adding 24 million cars to the road.
I
also care about getting the facts and science right and have in recent
months corrected inaccurate and apocalyptic news media coverage of fires in the Amazon and fires in California, both of which have been improperly presented as resulting primarily from climate change.
Journalists
and activists alike have an obligation to describe environmental
problems honestly and accurately, even if they fear doing so will reduce
their news value or salience with the public.
There is good evidence that the catastrophist framing of climate change is self-defeating because it alienates and polarizes many people.
And
exaggerating climate change risks is distracting us from other
important issues including ones we might have more near-term control
over.
I feel the need to say this up-front because I want the
issues I’m about to raise to be taken seriously and not dismissed by
those who label anyone as “climate deniers” or “climate delayers” who
push back against exaggeration.
With that out of the way, let’s look at whether the science supports what’s being said.
First,
no credible scientific body has ever said climate change threatens the
collapse of civilization much less the extinction of the human species.
“‘Our children are going to die in the next 10 to 20 years.’
What’s the scientific basis for these claims?” BBC’s Andrew Neil asked a visibly uncomfortable XR spokesperson last month.
“These
claims have been disputed, admittedly,” she said. “There are some
scientists who are agreeing and some who are saying it’s not true. But
the overall issue is that these deaths are going to happen.”
“But
most scientists don’t agree with this,” said Neil. “I looked through
IPCC reports and see no reference to billions of people going to die, or
children in 20 years. How would they die?”
“Mass
migration around the world already taking place due to prolonged drought
in countries, particularly in South Asia. There are wildfires in
Indonesia, the Amazon rainforest, Siberia, the Arctic,” she said.
But
in saying so, the XR spokesperson had grossly misrepresented the
science. “There is robust evidence of disasters displacing people
worldwide,” notes IPCC, “but limited evidence that climate change or sea-level rise is the direct cause.”
What about “mass migration”? “The majority of resultant population movements tend to occur within the borders of affected countries,” says IPCC.
It’s not like climate doesn’t matter. It’s that climate change is outweighed by other factors.
Earlier this year, researchers found
that climate “has affected organized armed conflict within countries.
However, other drivers, such as low socioeconomic development and low
capabilities of the state, are judged to be substantially more
influential.”
Last January, after climate scientists criticized
Rep. Ocasio-Cortez for saying the world would end in 12 years, her
spokesperson said:
“We can quibble about the phraseology, whether it’s existential or
cataclysmic.” He added, “We’re seeing lots of [climate change-related]
problems that are already impacting lives.”
That last part may be
true, but it’s also true that economic development has made us less
vulnerable, which is why there was a 99.7% decline in the death toll from natural disasters since its peak in 1931.
In 1931, 3.7 million people died from natural disasters. In 2018, just 11,000 did. And that decline occurred over a period when the global population quadrupled.
What
about sea-level rise? IPCC estimates sea level could rise two feet (0.6
meters) by 2100. Does that sound apocalyptic or even “unmanageable”?
Consider that one-third of the Netherlands is below sea level, and some areas areseven meters below sea level. You might object that the Netherlands is rich while Bangladesh is poor.
But the Netherlands adapted to living below sea level 400 years ago. Technology has improved a bit since then.
What
about claims of crop failure, famine, and mass death? That’s science
fiction, not science. Humans today produce enough food for 10 billion
people or 25% more than we need, and scientific bodies predict increases
in that share, not declines.
The United Nations Food and Agriculture Organization (FAO) forecasts
crop yields increasing by 30% by 2050. And the poorest parts of the
world, like sub-Saharan Africa, are expected to see increases of 80 to
90%.
Nobody is suggesting climate change won’t negatively impact
crop yields. It could. But such declines should be put in perspective.
Wheat yields increased 100 to 300% around the world since the 1960s, while a study of 30 models found that yields would decline by 6% for every one degree Celsius increase in temperature.
Rates
of future yield growth depend far more on whether poor nations get
access to tractors, irrigation, and fertilizer than on climate change,
says FAO.
All of this helps explain why IPCC anticipates climate
change will have a modest impact on economic growth. By 2100, IPCC
projects the global economy will be 300 to 500% larger than it is today.
Both IPCC and the Nobel-winning Yale economist, William Nordhaus, predict that warming of 2.5°C and 4°C would reduce gross domestic product (GDP) by 2% and 5% over that same period.
Does this mean we shouldn’t worry about climate change? Not at all.
One of the reasons I work on climate change is because I worry about the impact it could have on endangered species.
Climate change may threaten one million species globally and half
of all mammals, reptiles, and amphibians in diverse places like the
Albertine Rift in central Africa, home to the endangered mountain
gorilla.
But it’s not the case that “we’re putting our own survival in danger” through extinctions, as Elizabeth Kolbert claimed in her book, Sixth Extinction. As tragic as animal extinctions are, they do not threaten human civilization.
If
we want to save endangered species, we need to do so because we care
about wildlife for spiritual, ethical, or aesthetic reasons, not
survival ones.
And exaggerating the risk and suggesting climate
change is more important than things like habitat destruction is
counterproductive.
For example, Australia’s fires are not driving koalas ‘extinct,’ as Bill McKibben suggested.
The main scientific body that tracks the species, the International Union for the Conservation of Nature, or IUCN, labels
the koala “vulnerable,” which is one level less threatened than
“endangered,” two levels less than “critically endangered,” and three
less than “extinct” in the wild.
Should we worry about koalas?
Absolutely! They are amazing animals and their numbers have declined to
around 300,000. But they face far bigger threats such as the destruction of habitat, disease, bushfires, and invasive species.
Think
of it this way. The climate could change dramatically — and we could
still save koalas. Conversely, the climate could change only modestly —
and koalas could still go extinct.
The monomaniacal focus on
climate distracts our attention from other threats to koalas and
opportunities for protecting them, like protecting and expanding their
habitat.
As for fire, one of Australia’s leading scientists on the issue says,
“Bushfire
losses can be explained by the increasing exposure of dwellings to
fire-prone bushlands. No other influences need to be invoked. So even if
climate change had played some small role in modulating recent
bushfires, and we cannot rule this out, any such effects on risk to the
property are clearly swamped by the changes in exposure.”
Nor
are the fires solely due to drought, which is common in Australia, and
exceptional this year. “Climate change is playing its role here,” said
Richard Thornton of the Bushfire and Natural Hazards Cooperative
Research Centre in Australia, “but it’s not the cause of these fires.”
The same is true for fires in the United States. In 2017, scientists modeled 37 different regions and found “humans may not only influence fire regimes but their presence can actually override, or swamp out, the effects of climate.”
Of
the 10 variables that influence fire, “none were as significant… as the
anthropogenic variables,” such as building homes near, and managing
fires and wood fuel growth within, forests.
Climate scientists are starting to push back against exaggerations by activists, journalists, and other scientists.
“While many species are threatened with extinction,” said
Stanford’s Ken Caldeira, “climate change does not threaten human
extinction… I would not like to see us motivating people to do the right
thing by making them believe something that is false.”
I asked
the Australian climate scientist Tom Wigley what he thought of the claim
that climate change threatens civilization. “It really does bother me
because it’s wrong,” he said. “All these young people have been
misinformed. And partly it’s Greta Thunberg’s fault. Not deliberately.
But she’s wrong.”
But don’t scientists and activists need to exaggerate in order to get the public’s attention?
“I’m
reminded of what [late Stanford University climate scientist] Steve
Schneider used to say,” Wigley replied. “He used to say that as a
scientist, we shouldn’t really be concerned about the way we slant
things in communicating with people out on the street who might need a
little push in a certain direction to realize that this is a serious
problem. Steve didn’t have any qualms about speaking in that biased way.
I don’t quite agree with that.”
Wigley started working on climate
science full-time in 1975 and created one of the first climate models
(MAGICC) in 1987. It remains one of the main climate models in use
today.
“When I talk to the general public,” he said, “I point out some of the things that might make projections of warming less and the things that might make them more. I always try to present both sides.”
Part
of what bothers me about the apocalyptic rhetoric by climate activists
is that it is often accompanied by demands that poor nations be denied
the cheap sources of energy they need to develop. I have found that many
scientists share my concerns.
“If you want to minimize carbon
dioxide in the atmosphere in 2070 you might want to accelerate the
burning of coal in India today,” MIT climate scientist Kerry Emanuel
said.
“It doesn’t sound like it makes sense. Coal is terrible for
carbon. But it’s by burning a lot of coal that they make themselves
wealthier, and by making themselves wealthier they have fewer children,
and you don’t have as many people burning carbon, you might be better
off in 2070.”
Emanuel and Wigley say the extreme rhetoric is making any political agreement on climate change harder.
“You’ve
got to come up with some kind of middle ground where you do reasonable
things to mitigate the risk and try at the same time to lift people out
of poverty and make them more resilient,” said Emanuel. “We shouldn’t be
forced to choose between lifting people out of poverty and doing
something for the climate.”
Happily, there is plenty of middle ground between climate apocalypse and climate denial.
Michael Shellenberger was Time Magazine’s “Hero of the Environment,” Green Book Award Winner, and President of Environmental Progress, a research and policy organization. He is also the author of several bestselling books.
In evolutionary psychology, the Cinderella effect is the phenomenon of higher incidence of different forms of child abuse and mistreatment by stepparents than by biological parents. It takes its name from the fairy tale character Cinderella,
which is about a girl who is mistreated by her stepsisters.
Evolutionary psychologists describe the effect as a byproduct of a bias
towards kin, and a conflict between reproductive partners of investing
in young that are unrelated to one partner. There is both supporting
evidence for this theory and criticism against it.
Background
In the early 1970s, a theory arose on the connection between stepparents and child maltreatment.
"In 1973, forensic psychiatrist P. D. Scott summarized information on a
sample of "fatal battered-baby cases" perpetrated in anger ... 15 of
the 29 killers – 52% – were stepfathers."
Although initially there was no analysis of this raw data, empirical
evidence has since been collected on what is now called the Cinderella
effect through official records, reports, and census.
For over 30 years, data has been collected regarding the validity
of the Cinderella effect, with a wealth of evidence indicating a direct
relationship between step-relationships and abuse. This evidence of child abuse and homicide
comes from a variety of sources including official reports of child
abuse, clinical data, victim reports, and official homicide data. Studies have concluded that "stepchildren in Canada, Great Britain, and the United States indeed incur greatly elevated risk of child maltreatment of various sorts, especially lethal beatings".
Powerful evidence in support of the Cinderella effect comes from
the finding that when abusive parents have both step and genetic
children, they generally spare their genetic children. In such families,
stepchildren were exclusively targeted 9 out of 10 times in one study
and in 19 of 22 in another.
In addition to displaying higher rates of negative behaviors (e.g.,
abuse) toward stepchildren, stepparents display fewer positive behaviors
toward stepchildren than do the genetic parents. For example, on
average, stepparents invest less in education, play with stepchildren
less, take stepchildren to the doctor less, etc.
This discrimination against stepchildren is unusual compared with abuse
statistics involving the overall population given "the following
additional facts: (1) when child abuse is detected, it is often found
that all the children in the home have been victimized; and (2)
stepchildren are almost always the eldest children in the home, whereas
the general ... tendency in families of uniform parentage is for the
youngest to be most frequent victims."
Evolutionary psychology theory
Evolutionary psychologistsMartin Daly and Margo Wilson propose that the Cinderella effect is a direct consequence of the modern evolutionary theory of inclusive fitness, especially parental investment theory.
They argue that human child rearing is so prolonged and costly that "a
parental psychology shaped by natural selection is unlikely to be
indiscriminate".
According to them, "research concerning animal social behaviour provide
a rationale for expecting parents to be discriminative in their care
and affection, and more specifically, to discriminate in favour of their
own young".
Inclusive fitness theory proposes a selective criterion for the
evolution of social traits, where social behavior that is costly to an
individual organism can nevertheless emerge when there is a statistical
likelihood that significant benefits of that social behavior accrue to
(the survival and reproduction of) other organisms whom also carry the
social trait (most straightforwardly, accrue to close genetic
relatives). Under such conditions, a net overall increase in
reproduction of the social trait in future generations can result.
The initial presentation of inclusive fitness theory (in the mid
1960s) focused on making the mathematical case for the possibility of
social evolution, but also speculated about possible mechanisms whereby a
social trait could effectively achieve this necessary statistical
correlation between its likely bearers. Two possibilities were
considered: One that a social trait might reliably operate
straightforwardly via social context in species where genetic relatives
are usually concentrated in a local home area where they were born
('viscous populations'); The other, that genetic detection mechanisms
('supergenes') might emerge that go beyond statistical correlations, and
reliably detect actual genetic relatedness between the social
actors using direct 'kin recognition'. The relative place of these two
broad types of social mechanisms has been debated,
but many biologists consider 'kin recognition' to be an important
possible mechanism. Martin Daly and Margo Wilson follow this second
mechanism, and expect that parents "discriminate in favour of their own
young", i.e. their actual genetic relatives.
Daly and Wilson research
The
most abundant data on stepchild mistreatment has been collected and
interpreted by psychologists Martin Daly and Margo Wilson, who study
with an emphasis in Neuroscience and Behavior at McMaster University. Their first measure of the validity of the Cinderella effect was based on data from the American Humane Association (AHA), an archive of child abuse reports in the United States holding over twenty thousand reports.
These records led Wilson and Daly to conclude that "a child under three
years of age who lived with one genetic parent and one stepparent in
the United States in 1976 was about seven times more likely to become a
validated child-abuse case in the records than one who dwelt with two
genetic parents".
Their overall findings demonstrate that children residing with
stepparents have a higher risk of abuse even when other factors are
considered.
Explanation
All organisms face trade-offs as to how to invest their time, energy, risk, and other resources, so investment in one domain (e.g., parental investment) generally takes away from their ability to invest in other domains (e.g. mating effort, growth, or investment in other offspring).
Investment in non-genetic children therefore reduces an individual's
ability to invest in itself or its genetic children, without directly
bringing reproductive benefits. Thus, from an evolutionary biology perspective, one would not expect organisms to regularly and deliberately care for unrelated offspring.
Daly and Wilson point out that infanticide is an extreme form of biasing parental investment that is widely practiced in the animal world. For example, when an immigrant male lion enters a pride, it is not uncommon for him to kill the cubs fathered by other males. Since the pride can only provide support for a limited number of cubs to survive to adulthood, the killing of the cubs in competition with the new male's potential offspring increases the chances of his progeny surviving to maturity. In addition, the act of infanticide speeds the return to sexual receptivity in the females, allowing for the male to father his own offspring in a timelier manner.
These observations indicate that in the animal world, males employ
certain measures in order to ensure that parental investment is geared
specifically toward their own offspring.
Unlike the lion, however, humans in a stepparenting situation
face a more complicated tradeoff since they cannot completely disown
their partner's offspring from a previous relationship, as they would
risk losing sexual access to their partner and any chance of producing
potential offspring. Thus, according to Daly and Wilson, stepparental
investment can be viewed as mating effort to ensure the possibility of
future reproduction with the parent of their stepchild.
This mating effort hypothesis suggests that humans will tend to invest
more in their genetic offspring and invest just enough in their
stepchildren. It is from this theoretical framework that Daly and Wilson
argue that instances of child abuse towards non-biological offspring
should be more frequent than towards biological offspring.
One would therefore expect greater parental responsiveness
towards one's own offspring than towards unrelated children, and this
will result in more positive outcomes and fewer negative outcomes
towards one's own children than towards other children in which one is
expected to invest (i.e., stepchildren). "If child abuse is a behavioral
response influenced by natural selection, then it is more likely to
occur when there are reduced inclusive fitness payoffs owing to uncertain or low relatedness".
Owing to these adaptations from natural selection, child abuse is more
likely to be committed by stepparents than genetic parents—both are
expected to invest heavily in the children, but genetic parents will
have greater child-specific parental love that promotes positive
caretaking and inhibits maltreatment.
Daly and Wilson report that this parental love can explain why genetic offspring are more immune to lashing out by parents.
They assert that, "Child-specific parental love is the emotional
mechanism that permits people to tolerate—even to rejoice in—those long
years of expensive, unreciprocated parental investment".
They point to a study comparing natural father and stepfather families
as support for the notion that stepparents do not view their
stepchildren the same as their biological children, and likewise,
children do not view their stepparents the same as their biological
parents.
This study, based on a series of questionnaires which were then
subjected to statistical analyses, reports that children are less likely
to go to their stepfathers for guidance and that stepfathers rate their
stepchildren less positively than do natural fathers.
Daly and Wilson's reports on the overrepresentation of stepparents in child homicide and abuse statistics support the evolutionary principle of maximizing one's inclusive fitness, formalized under Hamilton's Rule, which helps to explain why humans will preferentially invest in close kin. Adoption statistics also substantiate this principle, in that non-kin adoptions represent a minority of worldwide adoptions. Research into the high adoption rates of Oceania shows that childlessness
is the most common reason for adopting, and that in the eleven
populations for which data was available, a large majority of adoptions
involved a relative with a coefficient of relatedness greater than or equal to 0.125 (e.g., genetic cousins).
It is also observed that parents with both biological and adopted
children bias the partitioning of their estates in favor of the
biological children, demonstrating again that parental behavior
corresponds to the principles of kin selection.
Methods
In
their 1985 Canadian sample, Daly and Wilson classify the frequencies of
different living arrangements (two natural parents, one natural parent,
one natural parent with one stepparent, or other) according to child
age. This was accomplished by administering a randomized telephone
survey.
Records of child abuse from children's aid organizations as well
as police reports on runaways and juvenile offenders were then used to
determine whether children from stepparental living situations were
overrepresented as abuse victims when compared to the demographic data
gathered from the telephone survey data. The results indicate that the
only living situation that has a significant correlation to increased
child abuse is one natural parent and one stepparent in the same
household. While rates of running away and crime were comparable for
children living with stepparents and children of single-parents, abuse
rates for children living with stepparents were much higher.
Daly and Wilson examined several potentially confounding variables in their research, including socioeconomic status,
family size, and maternal age at childbirth, however only minor
differences between natural-parent and stepparent families with respect
to these factors were found, indicating that none of these are major
contributing factors to the observed Cinderella effect.
Attachment theory
Evolutionary psychologists have also suggested that one of the causes of stepchild abuse may be the lack of a parental attachment bond that the mother would normally form with her own child.
An attachment bond will, in general, be more secure if formed before
the age of two, and adoption can often disrupt the development of this
bond. An infant who is fed by the primary parental figure, usually the
mother, and has the mother present during severely physically painful
events will have form a stronger parental attachment bond, and either a
consistent omission of the mother from this process or an alteration
between two people (the original mother and the adoptive mother) can
cause either an insecure attachment or disorganized attachment from the
parent to the child.
As a result, it is highly recommended by most psychologists that the
adoptive mother be present very early in the infant's life, preferably
immediately after its birth, in order to avoid attachment disruptions
and attachment disorders.
This theory cannot be a whole explanation for the Cinderella effect, as
psychological research has shown that secure attachment bonds can be
developed between a parent and adopted child, and the quality of the
relationship between parent and child will more often depend on the
child's pre-adoption experiences, such as length of time in social care
and previous trauma, more than characteristics of the parents.
Misunderstandings
It
is sometimes argued that this evolutionary psychological account does
not explain why the majority of stepparents do not abuse their partners'
children, or why a significant minority of genetic parents do abuse
their own offspring. However, their argument is based on a
misunderstanding: the evolutionary psychological account is that (all
else equal) parents will love their own children more than other
people's children – it does not argue that stepparents will "want" to
abuse their partner's children, or that genetic parenthood is absolute
proof against abuse. Under this account, stepparental care is seen as
"mating effort" towards the genetic parent, such that most interactions
between stepparent and stepchildren will be generally positive or at
least neutral, just usually not as positive as interactions between the
genetic parent and the child would be.
Supportive evidence
Strong
support for the Cinderella effect as described by Daly and Wilson comes
from a study of unintentional childhood fatal injuries in Australia.
Tooley et al. follow the argument of Daly and Wilson to extend the
Cinderella effect from cases of abuse to incidences of unintentional
fatalities. Children are not only vulnerable to abuse by their parents,
but they are also dependent on their parents for supervision and
protection from a variety of other harms.
Given that parental supervision is fundamentally correlated to
incidences of unintentional childhood injury as shown by Wadsworth et
al. and Peterson & Stern, Tooley et al. posit that selective
pressures would favor an inclination towards parental vigilance against
threats to offspring well-being.
Tooley et al. further argue that parental vigilance is not as highly
engaged in stepparents as genetic parents, therefore placing
stepchildren at greater risk for unintentional injury.
Based on data gathered from the Australia National Coroners'
Information System, stepchildren under five years of age are two to
fifteen times more likely to experience an unintentional fatal injury,
especially drowning, than genetic children.
Additionally, the study finds that the risks of unintentional fatal
injury are not significantly higher for genetic children in single
parent homes versus two-parent homes.
This difference suggests that removing one biological parent from the
home does not significantly increase risk to the children, but that
adding a nonbiological parent to the home results in a drastic increase
in the risk of unintentional fatal injury.
Despite the fact that adding a stepparent to the home increases the
available resources in terms of supervision in comparison to a
single-parent home, risk of unintentional fatal injury still
significantly rises.
This higher risk of injury for stepchildren can be attributed to the
fact that stepparents occupy the same supervisory role as a genetic
parent, yet they have a lower intrinsic commitment to protecting the
child and therefore are less likely to be adequately vigilant.
The authors conclude that the Cinderella effect applies not only to
purposeful abuse by stepparents, but is also relevant to explaining
increased rates of accidental fatalities among stepchildren.
Furthermore, a study of parental investment behaviors among American men living in Albuquerque, New Mexico,
reveals a trend of increasing financial expenditures on genetic
offspring in comparison to step-offspring, which also suggests that
parents are less inclined to preserve the well-being of stepchildren.
The study assesses paternal investment based on four measures: the
probability that a child attends college, the probability that the child
receives money for college, the total money spent on children, and the
amount of time per week spent with children.
Four different classifications of father-child relationships are
examined and compared, including fathers living with their genetic
children and stepfathers living with the stepchildren of their current
mates.
Though the study finds a clear trend of increasing investment in
genetic children, the data also shows that stepfathers do still invest
substantially in stepchildren.
The authors explain the parental investment exhibited by stepfathers
towards stepchildren as possibly motivated by the potential to improve
the quality or increase the duration of the man's relationship with the
stepchildren's mother. This studied corroborates the findings of Lynn White, that stepparents in general provide less social support to stepchildren than their genetic children.
Though the general trend of the data from this study supports the
Cinderella effect, Anderson and colleagues note that the observed
differences between investment in children and stepchildren might be
slightly reduced by a few confounding factors. For example, the authors point out that stepparenting is a self-selective
process, and that when all else is equal, men who bond with unrelated
children are more likely to become stepfathers, a factor that is likely
to be a confounding variable in efforts to study the Cinderella effect. Anderson and colleagues also conducted a similar study of Xhosa students in South Africa
that analyzes the same four classifications of adult-child
relationships, and this study offers similar results to those observed
among men in Albuquerque.
Additionally, a study of Hadza foragers in Tanzania by Marlowe also finds evidence of decreased care provided by men to stepchildren when compared with genetic children. The author uses the Mann-Whitney U-tests
to evaluate most of the observed differences in care exhibited towards
children and stepchildren, and finds that Hadza men spend less time with
(U=96), communicate less with (U=94.5), nurture less, and never play
with their stepchildren.
Marlowe further argues that any care that is provided towards
stepchildren is likely attributable to the man's mating efforts and not
parental interest in the well-being of the stepchildren.
In further support of the Cinderella effect as elaborated by Daly and Wilson, a study conducted in a rural village in Trinidad
demonstrates that in households containing both genetic children and
stepchildren, fathers devote approximately twice as much time to
interaction with genetic offspring in comparison to stepchildren.
Additionally, this study finds that the duration of the relationship
between the stepfather and stepchildren is negatively correlated with
the relative proportion of interaction time and positively correlated
with the relative proportion of antagonistic interactions between the
two.
As a proportion of total time spent interacting with genetic and
stepchildren, stepfathers are shown to have approximately 75 percent
more antagonistic interactions with stepchildren.
In this study, antagonistic interactions are defined as involving
physical or verbal combat or an expression of injury. This includes, for
example, spanking, screaming, crying, and arguing. The duration of the
relationship between genetic fathers and children shows a positive
correlation with both relative proportion of interaction time and
antagonistic interaction.
The author argues that these results show that in terms of time
invested, men favor their children over stepchildren, and this
preference is not attributable to the duration of the adult-child
relationship, a factor which is sometimes believed to be a confounding
variable in the Cinderella effect.
Though this study does claim a significant increase in antagonistic
behavior between stepparents and stepchildren and therefore supports the
Cinderella effect, it also notes that only six percent of all the
observed parent-child interactions were considered antagonistic, and
that the researchers never noticed any blatant physical child abuse.
Criticism
David Buller
Philosopher of science David Buller, as a part of his general critique of evolutionary psychology has reviewed Daly and Wilson's data. He argues that evolutionary psychology (EP) mistakenly attempts to discover human psychological adaptations
rather than "the evolutionary causes of psychological traits." Buller
also argues that Daly and Wilson's 1985 Canadian sample included cases
of sexual abuse as well as cases of unintentional omission, such as not
buckling a child's seatbelt in the car. Buller asserts that
unintentional omission does not fall under the realm of dangerous acts,
and rather should be designated "maltreatment". He argues that since
sexual abuse is not often accompanied by physical abuse, it is
unreasonable to assume that it is motivated by the same kind of
psychological mechanism as child homicide. Buller also points out that
the conclusion that non-biological parents are more likely to abuse
children is contradicted by the fact that even if the rate of abuse
among stepparents was disproportionate, most child abuse is in fact
committed by biological parents, and that the lowest rate of child abuse
is found among adoptive parents.
Daly and Wilson respond to Buller's criticism by stating that Buller
confuses the empirical statistical findings, which define the Cinderella
effect, with the proposed theoretical framework, which offers an
evolutionary explanation for the data.
Buller also argues that Daly and Wilson's findings are inherently
biased since they use data from official documents, and the officials
collecting that data are trained to take special notice of stepparents
versus biological parents.
Furthermore, Buller states that since Daly and Wilson rely on official
reports (such as death certificates) for their data, and that this data
is inherently biased against stepparents. He cites a Colorado
study, in which it was found that maltreatment fatalities were more
likely to be correctly reported on death certificates when an unrelated
individual was the perpetrator rather than when a parent was the
perpetrator, suggesting that the data is empirically skewed to support
the Cinderella effect.
According to this study, by Crume et al., when the perpetrator of the
murder was a parent, maltreatment was correctly noted on the death
certificate only 46 percent of the time. Furthermore, they found that
when the perpetrator was an "Other unrelated (including boyfriend)"
individual, maltreatment was reported on the death certificate 86
percent of the time, significantly higher than for parents.
Although these statistics seem to provide evidence of bias against
stepparents, further review of the data undermines this conclusion. As
Crume et al. and Daly and Wilson note, maltreatment was only likely to
be reported on the death certificates 47 percent of the time in the case
of "Other relatives (including step-parents)," which represents a
marginal increase from the amount of parental maltreatment.
Therefore, as Daly and Wilson respond to Buller's critique, this does
not seem to be a significant source of error in studying the Cinderella
effect and does not provide evidence for inherent bias in their data.
Temrin et al. Sweden study
The findings of Daly and Wilson have been called into question by one study of child homicides in Sweden
between 1975 and 1995, which found that children living in households
with a non-genetic parent were not at an increased risk of homicide when
compared to children living with both genetic parents. The study,
published in 2000 and conducted by Temrin and colleagues argued that
when Daly and Wilson classified homicides according to family situation,
they did not account for the genetic relatedness of the parent who
actually committed the crime. In the Swedish sample, in two out of the
seven homicides with a genetic and non-genetic parent, the offender was
actually the genetic parent and thus these homicides do not support Daly
and Wilson's definition of the Cinderella effect.
Daly and Wilson attribute the contrasting findings of the Swedish
study to an analytical oversight. Temrin and colleagues neglect to
consider the fact that the proportion of children in living situations
with a stepparent is not constant for all child age groups, but rather
increases with age. After correcting for age differences, the Swedish
data set produces results in accordance with the previous findings of
Daly and Wilson. The Swedish sample does show, however, decreased risk
to children living with a stepparent compared to the North American
samples collected by Daly and Wilson, suggesting that there is some
degree of cross-cultural variation in the Cinderella effect.
Alternative hypotheses
It has been noted by multiple researchers that child abuse is an intricate issue and is affected by other factors.
Daly and Wilson state, however, that even if evolutionary psychology
cannot account for every instance of stepparental abuse, this does not
invalidate their empirical findings.
Burgess and Drais propose that child maltreatment is too complex
to be explained fully by genetic relatedness alone and cite other
reasons for child maltreatment, such as social factors, ecological
factors and child traits such as disability and age. However, they also note that these traits are simply indicative, and do not inevitably lead to child maltreatment.
Temrin and colleagues also suggest that there may be other factors
involved with child homicide, such as prior convictions, drug abuse
problems, lost custody battles and mental health problems.
In 1984, Giles-Sims and David Finkelhor
categorized and evaluated five possible hypotheses that could explain
the Cinderella effect: "social-evolutionary theory", "normative theory",
"stress theory", "selection factors", and "resource theory". The
social-evolutionary theory is based on the proposal that non-genetically
related parents will invest less in costly parental duties, due to the
fact that their genes are not being passed on by that individual. The
normative theory proposes that, due to genetic repercussions, incest
among genetically related individuals is a widespread taboo and would
thus be less common among biological relatives. They propose that incest
among stepfamilies would be less taboo, since there is no risk of
genetic degradation. The stress theory proposes that increased
stressors, which are inherently more common among stepfamilies, cause an
increased risk of abuse. The selection factors theory proposes that
individuals who are likely to be stepparents (divorcees) are likely to
be inherently more violent due to emotional disturbances, aggressive
impulses, and self-esteem issues. Due to this, stepparents as a group
would have a higher proportion of individuals with violent-prone
characteristics, which would suggest that the abuse is happening due to
personality factors, rather than the stepparental relationship directly.
Finally, according to resource theory, individuals who contribute
resources are granted authority, while individuals that lack resources
are denied authority and more likely to resort to violence to obtain
authority. It is therefore hypothesized that stepparents who are able to
contribute resources to a family and have those resources be accepted
by the family are less likely to be abusive. However, this hypothesis
had yet to be tested directly on stepfamilies. This paper of Giles-Sims and Finkelhor predates however practically all empirical studies on the Cinderella effect.
Ethical issues
Discussing
the implications of this line of research, Australian psychologist Greg
Tooley, author of a 2006 study confirming the existence of the effect, confessed that "it is certainly difficult to talk about because it is such a hot issue".
Parental investment, in evolutionary biology and evolutionary psychology, is any parental expenditure (e.g. time, energy, resources) that benefits offspring.
Parental investment may be performed by both males and females
(biparental care), females alone (exclusive maternal care) or males
alone (exclusive paternal care).
Care can be provided at any stage of the offspring's life, from
pre-natal (e.g. egg guarding and incubation in birds, and placental
nourishment in mammals) to post-natal (e.g. food provisioning and
protection of offspring).
Parental investment theory, a term coined by Robert Trivers
in 1972, predicts that the sex that invests more in its offspring will
be more selective when choosing a mate, and the less-investing sex will
have intra-sexual competition for access to mates. This theory has been
influential in explaining sex differences in sexual selection and mate preferences, throughout the animal kingdom and in humans.
History
In 1859, Charles Darwin published On the Origin of Species. This introduced the concept of natural selection to the world, as well as related theories such as sexual selection.
For the first time, evolutionary theory was used to explain why females
are "coy" and males are "ardent" and compete with each other for
females' attention. In 1930, Ronald Fisher wrote The Genetical Theory of Natural Selection, in which he introduced the modern concept of parental investment, introduced the sexy son hypothesis, and introduced Fisher's principle. In 1948, Angus John Bateman
published an influential study of fruit flies in which he concluded
that because female gametes are more costly to produce than male
gametes, the reproductive success of females was limited by the ability
to produce ovum, and the reproductive success of males was limited by
access to females. In 1972, Trivers
continued this line of thinking with his proposal of parental
investment theory, which describes how parental investment affects
sexual behavior. He concludes that the sex that has higher parental
investment will be more selective when choosing a mate, and the sex with
lower investment will compete intra-sexually for mating opportunities.
In 1974, Trivers extended parental investment theory to explain
parent-offspring conflict, the conflict between investment that is
optimal from the parent's versus the offspring's perspective.
Parental care
Parental investment theory is a branch of life history theory. The earliest consideration of parental investment is given by Ronald Fisher in his 1930 book The Genetical Theory of Natural Selection,
wherein Fisher argued that parental expenditure on both sexes of
offspring should be equal. Clutton-Brock expanded the concept of
parental investment to include costs to any other component of parental
fitness.
Male dunnocks tend to not discriminate between their own young and those of another male in polyandrous or polygynandrous systems. They increase their own reproductive success
through feeding the offspring in relation to their own access to the
female throughout the mating period, which is generally a good predictor
of paternity. This indiscriminative parental care by males is also observed in redlip blennies.
In some insects, male parental investment is given in the form of a nuptial gift. For instance, ornate moth
females receive a spermatophore containing nutrients, sperm and
defensive toxins from the male during copulation. This gift, which can
account for up to 10% of the male's body mass, constitutes the total
parental investment the male provides.
In some species, such as humans and many birds, the offspring are altricial
and unable to fend for themselves for an extended period of time after
birth. In these species, males invest more in their offspring than do
the male parents of precocial species, since reproductive success would otherwise suffer.
The benefits of parental investment to the offspring are large and
are associated with the effects on condition, growth, survival, and
ultimately on reproductive success of the offspring. For example, in the
cichlid fish Tropheus moorii, a female has very high parental investment in her young because she mouthbroods
the young and while mouthbrooding, all nourishment she takes in goes to
feed the young and she effectively starves herself. In doing this, her
young are larger, heavier, and faster than they would have been without
it. These benefits are very advantageous since it lowers their risk of
being eaten by predators and size is usually the determining factor in
conflicts over resources.
However, such benefits can come at the cost of parent's ability to
reproduce in the future e.g., through increased risk of injury when
defending offspring against predators, loss of mating opportunities
whilst rearing offspring, and an increase in the time interval until the
next reproduction.
A special case of parental investment is when young do need
nourishment and protection, but the genetic parents do not actually
contribute in the effort to raise their own offspring. For example, in Bombus terrestris,
oftentimes sterile female workers will not reproduce on their own, but
will raise their mother's brood instead. This is common in social Hymenoptera due to haplodiploidy,
whereby males are haploid and females are diploid. This ensures that
sisters are more related to each other than they ever would be to their
own offspring, incentivizing them to help raise their mother's young
over their own.
Overall, parents are selected to maximize the difference between
the benefits and the costs, and parental care will be likely to evolve
when the benefits exceed the costs.
Parent-offspring conflict
Reproduction is costly. Individuals are limited in the degree to
which they can devote time and resources to producing and raising their
young, and such expenditure may also be detrimental to their future
condition, survival, and further reproductive output.
However, such expenditure is typically beneficial to the offspring,
enhancing their condition, survival, and reproductive success. These
differences may lead to parent-offspring conflict.
Parents are naturally selected to maximize the difference between the
benefits and the costs, and parental care will tend to exist when the
benefits are substantially greater than the costs.
Parents are equally related to all offspring, and so in order to
optimize their fitness and chance of reproducing their genes, they
should distribute their investment equally among current and future
offspring. However, any single offspring is more related to themselves
(they have 100% of their DNA in common with themselves) than they are to
their siblings (siblings usually share 50% of their DNA), it is best
for the offspring's fitness if the parent(s) invest more in them. To
optimize fitness, a parent would want to invest in each offspring
equally, but each offspring would want a larger share of parental
investment. The parent is selected to invest in the offspring up until
the point at which investing in the current offspring is costlier than
investing in future offspring.
In iteroparous
species, where individuals may go through several reproductive bouts
during their lifetime, a tradeoff may exist between investment in
current offspring and future reproduction. Parents need to balance their
offspring's demands against their own self-maintenance. This potential
negative effect of parental care was explicitly formalized by Trivers in
1972, who originally defined the term parental investment to mean any investment by the parent in an individual offspring that increases the offspring's chance of surviving (and hence reproductive success) at the cost of the parent's ability to invest in other offspring.
King penguin and a chick
Penguins are a prime example of a species that drastically sacrifices
their own health and well-being in exchange for the survival of their
offspring. This behavior, one that does not necessarily benefit the
individual, but the genetic code from which the individual arises, can
be seen in the King Penguin. Although some animals do exhibit altruistic
behaviors towards individuals that are not of direct relation, many of
these behaviors appear mostly in parent-offspring relationships. While
breeding, males remain in a fasting-period at the breeding site for five
weeks, waiting for the female to return for her own incubation shift.
However, during this time period, males may decide to abandon their egg
if the female is delayed in her return to the breeding grounds.
It shows that these penguins initially show a trade-off of their
own health, in hopes of increasing the survivorship of their egg. But
there comes a point where the male penguin's costs become too high in
comparison to the gain of a successful breeding season. Olof Olsson
investigated the correlation between how many experiences in breeding an
individual has and the duration an individual will wait until
abandoning his egg. He proposed that the more experienced the
individual, the better that individual will be at replenishing his
exhausted body reserves, allowing him to remain at the egg for a longer
period of time.
The males' sacrifice of their body weight and possible
survivorship, in order to increase their offspring's chance of survival
is a trade-off between current reproductive success and the parents'
future survival.
This trade-off makes sense with other examples of kin-based altruism
and is a clear example of the use of altruism in an attempt to increase
overall fitness of an individual's genetic material at the expense of
the individual's future survival.
Maternal-offspring conflict in investment
The
maternal-offspring conflict has also been studied in animals species
and humans. One such case has been documented in the mid-1970s by
ethologist Wulf Schiefenhövel.
Eipo women of West New Guinea engage in a cultural practice in which
they give birth just outside the village. Following the birth of their
child, each woman weighed whether or not she should keep the child or
leave the child in the brush nearby, inevitably ending in the death of
the child.
Likelihood of survival and availability of resources within the village
were factors that played into this decision of whether or not to keep
the baby. During one illustrated birth, the mother felt the child was
too ill and would not survive, so she wrapped the child up, preparing to
leave the child in the brush; however, upon seeing the child moving,
the mother unwrapped the child and brought it into the village,
demonstrating a shift of life and death.
This conflict between the mother and the child resulted in detachment
behaviors in Brazil, seen in Scheper-Hughes work as "many Alto babies
remain[ed] not only unchristened but unnamed until they begin to walk or
talk", or if a medical crisis arose and the baby needed an emergency baptism.
This conflict between survival, both emotional and physical, prompted a
shift in cultural practices, thus resulting in new forms of investment
from the mother towards the child.
Alloparental care
Alloparental care
also referred to as 'Allomothering,' is when a member of a community,
apart from the biological parents of the infant, partake in offspring
care provision.
A range of behaviors fall under the term alloparental care, some of
which are: carrying, feeding, watching over, protecting, and grooming.
Through alloparental care stress on parents, especially the mother, can
be reduced, therefore reducing the negative effects of the
parent-offspring conflict on the mother.
In While the apparent altruistic nature of the behavior may seem at
odds with Darwin's theory of natural selection, as taking care of
offspring which are not one's own would not increase one's direct
fitness, while taking time, energy and resources away from raising one's
own offspring, the behavior can be explained evolutionarily as
increasing indirect fitness, as the offspring is likely to be
non-descendent kin, therefore carrying some of the genetics of the
alloparent.
Offspring and situation direction
Parental
investment behavior enhances the chances of survival of offspring, and
it does not require underlying mechanisms to be compatible with empathy
applicable to adults, or situations involving unrelated offspring, and
it does not require the offspring to reciprocate the altruistic behavior
in any way. Parentally investing individuals are not more vulnerable to being exploited by other adults.
Trivers' parental investment theory
Parental investment as defined by Trivers in 1972 is the investment in offspring by the parent that increases the offspring's chances of surviving and hence reproductive success
at the expense of the parent's ability to invest in other offspring. A
large parental investment largely decreases the parents' chances of
investing in other offspring. Parental investment can be split into two
main categories: mating investment and rearing investment.
Mating investment consist of the sexual act and the sex cells invested.
The rearing investment is the time and energy expended to raise the
offspring after conception.
Women's parental investment in both mating and rearing efforts greatly
surpasses that of the male. In terms of sex cells (egg and sperms
cells), the female's investment is a lot larger, while males produce
thousands of sperm cells which are supplied at a rate of twelve million per hour.
Women have a fixed supply of around 400 ova. Also, the acts of fertilization and gestation
occur in the women, which compared to the male's investment of just one
cell outweighs it. Furthermore, each intercourse could result in a
nine-month commitment such as gestation (the act of breastfeeding)
for the woman. From Trivers' theory of parental investment several
implications follow. The first is that women are often but not always
the more investing sex. The fact that they are the more investing sex
has meant that evolution has favored females who are more selective of
their mates to ensure that intercourse would not result in unnecessary
costs. The third implication is that because women invest more and are
essential for the reproductive success of their offspring they are a
valuable resource for men; as a result, males often compete for sexual
access to them.
Males as the more investing sex
For
many species the only type of male investment received is that of sex
cells. In those terms, the female investment greatly exceeds that of
male investment as previously mentioned. However, there are other ways
in which males invest in their offspring. For example, the male can
find food as in the example of balloon flies. He may find a safe environment for the female to feed or lay her eggs as exemplified in many birds.
He may also protect the young and provide them with opportunities
to learn as in many young as in wolves. Overall, the main role that
males overtake is that of protection of the female and their young. That
often can decrease the discrepancy of investment caused by the initial
investment of sex cells. There are some species such as the Mormon
cricket, pipefish seahorse and Panamanian poison arrow frog males invest
more. Among the species where the male invests more, the male is also
the pickier sex, placing higher demands on their selected female. For
example, the female that they often choose usually contain 60% more eggs
than rejected females.
This links Parental Investment Theory (PIT) with sexual selection:
where parental investment is bigger for a male than a female, it's
usually the female who competes for a mate, as shown by Phalaropidae and
polyandrous bird species. In these species females are usually more
aggressive, brightly colored, and larger than males, suggesting the more investing sex has more choice while selecting a mate compared to the sex engaged in intra-sexual selection.
Females as a valuable resource for males
The
second prediction that follows from Trivers' theory is that the fact
that women invest more heavily in offspring makes them a valuable
resource for males as it ensures the survival of their offspring which
is the driving force of natural selection.
Therefore, the sex that invests less in offspring will compete among
themselves to breed with the more heavily investing sex. In other words,
males will compete for females.
It has been argued that jealousy has developed to avert the risk of potential loss of parental investment in offspring.
If a male redirects his resources to another female it is a
costly loss of time, energy and resources for her offspring. However,
the risks for males are higher because although women invest more in
their offspring, they have bigger maternity certainty because they
themselves have carried out the child. However, males can never have
100% paternal certainty and therefore risk investing resources and time
in offspring that is genetically unrelated. Evolutionary psychology views jealousy as an adaptive response to this problem.
Application of Trivers' theory in real life
Trivers'
theory has been very influential as the predictions it makes correspond
to differences in sexual behaviors of men and women, as demonstrated by
a variety of research. Cross-cultural study from Buss (1989)
shows that males are tuned into physical attractiveness as it signals
youth and fertility and ensures male reproductive success, which is
increased by copulating with as many fertile females as possible. Women
on the other hand are tuned into resources provided by potential mates,
as their reproductive success is increased by ensuring their offspring
will survive, and one way they do so is by getting resources for them.
Alternatively, another study shows that men are more promiscuous than
women, giving further support to this theory. Clark and Hatfield
found that 75% of men were willing to have sex with a female stranger
when propositioned, compared to 0% of women. On the other hand, 50% of
women agreed to a date with a male stranger. This suggests males seek
short term relationships, while women show a strong preference for
long-term relationships.
However, these preferences (male promiscuity and female
choosiness) can be explained in other ways. In Western cultures, male
promiscuity is encouraged through the availability of pornographic
magazines and videos targeted to the male audience. Alternatively, both
Western and Eastern cultures discourage female promiscuity through
social checks such as slut-shaming.
PIT (Parental Investment Theory) also explains patterns of sexual jealousy.
Males are more likely to show a stress response when imagining their
partners showing sexual infidelity (having sexual relations with someone
else), and women showed more stress when imagining their partner being
emotionally unfaithful (being in love with another woman). PIT explains
this, as woman's sexual infidelity decreases the male's paternal
certainty, thus he will show more stress due to fear of cuckoldry.
On the other hand, the woman fears losing the resources her partner
provides. If her partner has an emotional attachment to another female
it's likely that he won't invest into their offspring as much, thus a
greater stress response is shown in this circumstance.
A heavy criticism of the theory comes from Thornhill and Palmer's analysis of it in A Natural History of Rape: Biological Bases of Sexual Coercion, as it seems to rationalise rape and sexual coercion
of females. Thornhill and Palmer claimed rape is an evolved technique
for obtaining mates in an environment where women choose mates. As PIT
claims males seek to copulate with as many fertile females as possible,
the choice women have could result in a negative effect on the male's
reproductive success. If women didn't choose their mates, Thornhill and
Palmer claim there would be no rape. This ignores a variety of
sociocultural factors, such as the fact that not only fertile females
are raped – 34% of underage rape victims are under 12,
which means they are not of fertile age, thus there is no evolutionary
advantage in raping them. 14% of rapes in England are committed on
males,
who cannot increase a man's reproductive success as there will be no
conception. Thus, what Thornhill and Palmer called an 'evolved
machinery' might not be very advantageous.
Versus sexual strategies
Trivers'
theory overlooks that women do have short-term relationships such as
one-night stands, while not all men behave promiscuously. An alternative
explanation to PIT (Parental Investment Theory) and mate preferences
would be Buss and Schmitt's sexual strategies theory.
SST argues that both sexes pursue short-term and long-term
relationships, but seek different qualities in their short- and
long-term partners. For a short-term relationship women will prefer an
attractive partner, but in a long-term relationship they might be
willing to trade-off that attractiveness for resources and commitment.
On the other hand, men might be accepting of a sexually willing partner
in a short-term relationships, but to ensure their paternal certainty
they will seek a faithful partner instead.
International politics
Parental
investment theory is not only used to explain evolutionary phenomena
and human behavior but describes recurrences in international politics
as well. Specifically, parental investment is referred to when
describing competitive behaviors between states and determining
aggressive nature of foreign policies. The parental investment
hypothesis states that the size of coalitions and the physical strengths
of its male members determines whether its activities with its foreign
neighbors are aggressive or amiable.
According to Trivers, men have had relatively low parental investments,
and were therefore forced into fiercer competitive situations over
limited reproductive resources. Sexual selection
naturally took place and men have evolved to address its unique
reproductive problems. Among other adaptations, men's psychology has
also developed to directly aid men in such intra-sexual competition.
One essential psychological developments involved decision-making
of whether to take flight or actively engage in warfare with another
rivalry group. The two main factors that men referred to in such
situations were (1) whether the coalition they are a part of is larger
than its opposition and (2) whether the men in their coalition have
greater physical strength than the other. The male psychology conveyed
in the ancient past has been passed on to modern times causing men to
partly think and behave as they have during ancestral wars. According to
this theory, leaders of international politics were not an exception.
For example, the United States expected to win the Vietnam war
due to its greater military capacity when compared to its enemies. Yet
victory, according to the traditional rule of greater coalition size,
did not come about because the U.S. did not take enough consideration to
other factors, such as the perseverance of the local population.
The parental investment hypothesis contends that male physical
strength of a coalition still determines the aggressiveness of modern
conflicts between states. While this idea may seem unreasonable upon
considering that male physical strength is one of the least determining
aspects of today's warfare, human psychology has nevertheless evolved to
operate on this basis. Moreover, although it may seem that mate seeking
motivation is no longer a determinant, in modern wars sexuality, such
as rape, is undeniably evident in conflicts even to this day.
Pair of crested auklets
Sexual selection
In many species, males can produce a larger number of offspring over
the course of their lives by minimizing parental investment in favor of
investing time impregnating any reproductive-age female who is fertile.
In contrast, a female can have a much smaller number of offspring during
her reproductive life, partly due to higher obligate parental
investment. Females will be more selective ("choosy") of mates than
males will be, choosing males with good fitness (e.g., genes, high
status, resources, etc.), so as to help offset any lack of direct
parental investment from the male, and therefore increase reproductive
success. Robert Trivers' theory of parental investment predicts that the sex making the largest investment in lactation, nurturing, and protecting offspring will be more discriminating in mating; and that the sex that invests less in offspring will compete via intrasexual selection for access to the higher-investing sex.
In species where both sexes invest highly in parental care,
mutual choosiness is expected to arise. An example of this is seen in crested auklets,
where parents share equal responsibility in incubating their single egg
and raising the chick. In crested auklets both sexes are ornamented.
Parental investment in humans
Humans
have evolved increasing levels of parental investment, both
biologically and behaviorally. The fetus requires high investment from
the mother, and the altricial newborn requires high investment from a
community. Species whose newborn young are unable to move on their own
and require parental care have a high degree of altriciality.
Human children are born unable to care for themselves and require
additional parental investment post-birth in order to survive.
Maternal investment
Trivers (1972)
hypothesized that greater biologically obligated investment will
predict greater voluntary investment. Mothers invest an impressive
amount in their children before they are even born. The time and
nutrients required to develop the fetus, and the risks associated with
both giving these nutrients and undergoing childbirth, are a sizable
investment. To ensure that this investment is not for nothing, mothers
are likely to invest in their children after they are born, to be sure
that they survive and are successful. Relative to most other species,
human mothers give more resources to their offspring at a higher risk to
their own health, even before the child is born. This is associated
with the evolution of a slower life history, in which fewer, larger
offspring are born after longer intervals, requiring increased parental
investment.
The placenta attaches to the uterine wall, and the umbilical cord connects it to the fetus.
The
developing human fetus––and especially the brain––requires nutrients to
grow. In the later weeks of gestation, the fetus requires increasing
nutrients as the growth of the brain increases.
Rodents and primates have the most invasive placenta phenotype, the
hemochorial placenta, in which the chorion erodes the uterine epithelium
and has direct contact with maternal blood. The other placental
phenotypes are separated from the maternal bloodstream by at least one
layer of tissue. The more invasive placenta allows for a more efficient
transfer of nutrients between the mother and fetus, but it comes with
risks as well. The fetus is able to release hormones directly into the
mother’s bloodstream to “demand” increased resources. This can result in
health problems for the mother, such as pre-eclampsia. During childbirth, the detachment of the placental chorion can cause excessive bleeding.
The obstetrical dilemma
also makes birth more difficult and results in increased maternal
investment. Humans have evolved both bipedalism and large brain size.
The evolution of bipedalism altered the shape of the pelvis, and shrunk
the birth canal at the same time brains were evolving to be larger. The
decreasing birth canal size meant that babies are born earlier in
development, when they have smaller brains. Humans give birth to babies
with brains 25% developed, while other primates give birth to offspring
with brains 45-50% developed.
A second possible explanation for the early birth in humans is the
energy required to grow and sustain a larger brain. Supporting a larger
brain gestationally requires energy the mother may be unable to invest.
The obstetrical dilemma makes birth challenging, and a
distinguishing trait of humans is the need for assistance during
childbirth. The altered shape of the bipedal pelvis requires that babies
leave the birth canal facing away from the mother, contrary to all
other primate species. This makes it more difficult for the mother to
clear the baby’s breathing passageways, to make sure the umbilical cord
isn’t wrapped around the neck, and to pull the baby free without bending
its body the wrong way.
The human need to have a birth attendant also requires sociality.
In order to guarantee the presence of a birth attendant, humans must
aggregate in groups. It has been controversially claimed that humans
have eusociality,
like ants and bees, in which there is relatively high parental
investment, cooperative care of young, and division of labor. It is
unclear which evolved first; sociality, bipedalism, or birth attendance.
Bonobos, our closest living relatives alongside chimpanzees, have high
female sociality and births among bonobos are also social events.
Sociality may have been a prerequisite for birth attendance, and
bipedalism and birth attendance could have evolved as long as five
million years ago.
A baby, mother, grandmother, and great-grandmother. In humans, grandparents often help to raise a child.
As female primates age, their ability to reproduce decreases. The grandmother hypothesis describes the evolution of menopause, which may or may not be unique to humans among primates.
As women age, the costs of investing in additional reproduction
increase and the benefits decrease. At menopause, it is more beneficial
to stop reproduction and begin investing in grandchildren. Grandmothers
are certain of their genetic relation to their grandchildren, especially
the children of their daughters, because maternal certainty of their
own children is high, and their daughters are certain of their maternity
to their children as well. It has also been theorized that grandmothers
preferentially invest in the daughters of their daughters because X
chromosomes carry more DNA and their granddaughters are most closely
related to them.
Paternal investment
As altriciality increased, investment from individuals other than the
mother became more necessary. High sociality meant that female
relatives were present to help the mother, but paternal investment
increased as well. Paternal investment increases as it becomes more
difficult to have additional children, and as the effects of investment
on offspring fitness increase.
Men are more likely than women to give no parental investment to
their children, and the children of low-investing fathers are more
likely to give less parental investment to their own children. Father
absence is a risk factor for both early sexual activity and teenage
pregnancy.
Father absence raises children's stress levels, which are linked to
earlier onset of sexual activity and increased short-term mating
orientation.
Daughters of absent fathers are more likely to seek short-term
partners, and one theory explains this as a preference for outside
(non-partner) social support because of the perceived uncertain future
and uncertain availability of committing partners in a high-stress
environment.
Women can only get pregnant while ovulating. Human ovulation is
concealed, or not signaled externally. Concealed ovulation decreases
paternity certainty because men are unsure when women ovulate.
The evolution of concealed ovulation has been theorized to be a result
of altriciality and increased need for paternal investment. There are
two ways this could be true. First, if men are unsure of the time of
ovulation, the best way to successfully reproduce would be to repeatedly
mate with a woman throughout her cycle, which requires pair bonding,
which in turn increases paternal investment.
The second theory states that decreased paternity certainty would
increase paternal investment in polygamous groups, because more men may
invest in the offspring. The second theory is better regarded today,
because all mammals with concealed ovulation are promiscuous, and men
display relatively low mate-guarding behavior, as monogamy and the first
theory require.
Mating orientations
Sociosexuality was first described by Alfred Kinsey as a willingness to engage in casual and uncommitted sexual relationships.
Sociosexual orientation describes sociosexuality on a scale from
unrestricted to restricted. Individuals with an unrestricted sociosexual
orientation have higher openness to sex in less committed
relationships, and individuals with a restricted sociosexual orientation
have lower openness to casual sexual relationships.
However, today it is acknowledged that sociosexuality does not in
reality exist on a one-dimensional scale. Individuals who are less open
to casual relationships are not always seeking committed relationships,
and individuals who are less interested in committed relationships are
not always interested in casual relationships.
Short- and long-term mating orientations are the modern descriptors of
openness to uncommitted and committed relationships, respectively.
Parental investment theory, as proposed by Trivers, argues that
the sex with higher obligatory investment will be more selective in
choosing sex partners, and the sex with lower obligatory investment will
be less selective and more interested in "casual" mating opportunities.
The more investing sex cannot reproduce as frequently, causing the less
investing sex to compete for mating opportunities. In humans, women have higher obligatory investment (pregnancy and childbirth), than men (sperm production).
Women are more likely to have higher long-term mating orientations, and
men are more likely to have higher short-term mating orientations.
Short- and long-term mating orientations influence women's
preferences in men. Studies have found that women put great emphasis on
career-orientation, ambition and devotion only when considering a
long-term partner. When marriage is not involved, women put greater emphasis on physical attractiveness.
Generally, women prefer men who are likely to perform high parental
investment and have good genes. Women prefer men with good financial
status, who are more committed, who are more athletic, and who are
healthier.
Some inaccurate theories have been inspired by parental investment theory. The "structural powerlessness hypothesis"
proposes that women strive to find mates with access to high levels of
resources because as women, they are excluded from these resources
directly. However, this hypothesis has been disproved by studies which
found that financially successful women place an even greater importance
on financial status, social status, and possession of professional
degrees.
Humans are sexually dimorphic; the average man is taller than the average woman.
Sexual dimorphism
Sexual dimorphism is the difference in body size between male and
female members of a species as a result of intrasexual selection, which
is sexual selection that acts within a sex. High sexual dimorphism and larger body size in males is a result of male-male competition for females.
Primate species in which groups are formed of many females and one male
have higher sexual dimorphism than species that have both multiple
females and males, or one female and one male. Polygynous primates have
the highest sexual dimorphism, and polygamous and monogamous primates
have less.
Humans have the lowest levels of sexual dimorphism of any primate
species, indicating that we have evolved decreasing levels of polygyny.
Decreased polygyny is associated with increased paternal investment.
The demographic transition
The demographic transition
describes the modern decrease in both birth and death rates. From a
Darwinian perspective, it does not make sense that families with more
resources are having fewer children. One explanation for the demographic
transition is the increased parental investment required to raise
children who will be able to maintain the same level of resources as
their parents.
Environmental
journalists and advocates have in recent weeks made a number of
apocalyptic predictions about the impact of climate change.
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