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Is child death the crucible of human evolution?

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Child death has played an important role in the evolution of humans. Of all stages of development, and at all historical times beyond Modern history, childhood has been associated with the highest levels of mortality. Compared to other evolutionary pressures such as surviving as an adult or finding a mate and having children, the odds of genetic failure (i.e., failure to directly contribute to one's genetic line) are greatest in childhood. The enormous potential evolutionary pressure exerted by child death should have significantly influenced human psychological adaptations. Despite this potential influence, child death may be one of the least studied influences on human evolutionary psychology. This paper discusses the historical rates of child mortality, the relative odds associated with passing on one's genes, adult mental adaptations to child mortality, and child mental adaptations to child mortality.
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Journal of Social, Evolutionary, and Cultural Psychology
www.jsecjournal.com - 2008, Proceedings of the 2
nd
Annual Meeting of the NorthEastern
Evolutionary Psychology Society
103
Original Article
IS CHILD DEATH THE CRUCIBLE OF HUMAN
EVOLUTION?
Tony Volk
.
Department of Child and Youth Studies
Brock University
Jeremy Atkinson
Department of Psychology
SUNY, Albany
Abstract
Child death has played an important role in the evolution of humans. Of all stages of
development, and at all historical times beyond Modern history, childhood has been
associated with the highest levels of mortality. Compared to other evolutionary pressures
such as surviving as an adult or finding a mate and having children, the odds of genetic
failure (i.e., failure to directly contribute to one’s genetic line) are greatest in childhood.
The enormous potential evolutionary pressure exerted by child death should have
significantly influenced human psychological adaptations. Despite this potential
influence, child death may be one of the least studied influences on human evolutionary
psychology. This paper discusses the historical rates of child mortality, the relative odds
associated with passing on one’s genes, adult mental adaptations to child mortality, and
child mental adaptations to child mortality.
Keywords: Infant mortality, Child mortality, Life history, Human evolution, Infant death,
Child death, Environment of Evolutionary Adaptedness, Ancient mortality, Historical
mortality
Introduction
The ultimate biological goal of human evolution is to pass on copies of one’s
genes to future generations (Dawkins, 1989). To maximize the likelihood of passing on
their genes, humans are believed to have evolved mental adaptations for solving
problems that were present in the Environment of Evolutionary Adaptedness (EEA-
Bowlby, 1982). The EEA is not a single place in time or location, but rather represents
the collective periods and locals of human evolution (Hagen & Symons, 2007; Silk,
2007). By examining the evolutionary pressures present across the EEA, evolutionary
psychology has identified plausible psychological adaptations that evolved to help solve
the problems our ancestors faced in passing on their genes (Tooby & Cosmides, 1992).
.
AUTHOR NOTE: Please direct all correspondence to: Tony Volk, Department of Child and
Youth Studies, Brock University,500 Glenridge Avenue, St. Catherines, Ontario, L2S 3A1,
Canada. E-mail: tvolk@brocku.ca.
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To further our understanding of the evolution of the human mind, it is important to
consider new, or underappreciated selective pressures, and their relationship to known, or
well-studied selective pressures.
The goal of this paper is to present one such underappreciated selective pressure:
child mortality. We define selective pressure as the degree to which a particular variable
(e.g., child mortality) increases or decreases the likelihood of an individual
passing their genes onto future generations (Dawkins, 1989). First, we will compare the
relative strength of child mortality as a selective force in relation to other more
commonly studied selective pressures (e.g., adult mortality, mating). Having made the
case that it is a strong selective pressure, we will then examine potential implications of
this selective pressure on human psychological adaptations, taking note of some potential
adaptations that would allow children to overcome this strong selective pressure.
Relative Selective Pressures
Human psychological evolution should have been most strongly influenced by
the evolutionary pressures that had the greatest chance of affecting the successful
transmission of genes from one generation to the next in the EEA. This suggests that
stronger selective pressures (e.g., survival) can supersede weaker selective pressures (e.g.,
preferences for eye-lash length) as the latter may be inconsequential if the former are not
overcome (Darwin, 1859; Dawkins, 1989). The same logic also suggests that pre-
reproductive pressures may, all else being equal, trump
ensuing
reproductive pressures
(Bjorklund & Pellegrini, 2000; Bjorklund & Yunger, 2001). Having the potential to be an
extremely fit adult does nothing to aid genetic transmission if an individual is incapable
of surviving to adulthood. Therefore, strong, pre-reproductive selective pressures had the
opportunity to significantly influence human evolution, and by extension, human
psychological adaptations.
A potential strong pre-reproductive pressure may be child survival. Child
survival clearly precedes adult transmission of genes, and therefore meets the criteria of a
pre-reproductive selective pressure. But how important is its relative selective pressure
compared to other selective pressures present in the EEA? To provide a relative
comparison of selective strengths, we compare the probabilities of passing on genes to
future generations associated with the different selective pressures. The greater the
impact on the probability of passing on genes to a future generation, the greater the
selective pressure. Specifically, we compare the impact of child mortality rates to two
well-studied human selective pressures: adult mortality rates and mating success.
Adult Mortality
Average mortality rate curves for hunter-gatherers and ancient civilizations all
show that the odds of survival for adults are relatively high. Even in hunter-gather groups
with high levels of adult mortality, per-year survival odds for adults are greater than
97.7%, with most hunter-gatherer groups averaging closer to 99% (Hill, Hurtado, &
Walker, 2007). Once a hunter-gatherer is an adult, their modal lifespan is 68-78 years of
age (Gurven & Kaplan, 2007). This post-childhood lifespan is similar to the lifespan of
adults in 18
th
-century Sweden, where the modal lifespan was 72 (Human Mortality
Database, 2008). Human adult mortality is 2 to 5 times lower when compared to adult
chimpanzee mortality and average lifespan is 20-30 years longer lifespan than in
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chimpanzees or gorillas (Gurven & Kaplan, 2007; Hill et al. 2001; Hill et al. 2007;
Robbins & Robbins, 2004). Therefore, despite being a critical selection pressure in that
survival is required for reproduction, the actual evolutionary pressure exerted by adult
mortality may have been relatively low in the human EEA. We acknowledge that current
hunter-gatherer adult mortality rates may not reflect EEA adult mortality rates. However,
we feel justified in using them as a relative (not absolute) comparison.
Adult Mating
Assuming an adult has survived, their next challenge is to find a mate. Without a
mate, passing on genes is impossible, making it another critical selection pressure.
Accordingly, perhaps the majority of evolutionary psychology research has focused on
mental adaptations aimed at solving this problem. For women hunter-gatherers, the odds
of finding a mate are extremely high (approaching 100%; Lee, 1979), leaving infertility
as the major cause of childlessness (rates typically 2%, ranging from 1 to 5% - Wood,
1994). For men, the challenges and increased variance associated with polygamous
mating can greatly increase the risk of not finding a mate (Buss & Schmidt, 1993; Wilson
& Daly, 1992). These rates may approach 10% in some hunter-gather societies, but are
typically lower (e.g., 6% Xavante- Salzano, Neel, & Maybury_Lewis, 1967; 2% Dani-
Heider, 1991). Like women, men also face a similar infertility rate of 2% (Wood, 1994).
Added to this risk for men is the risk of false paternity. While the actual EEA rates of
false paternity are virtually impossible to determine, current evidence from a variety of
cultures (including some hunter-gatherers) suggests that it may have been a relatively low
risk, in the order of 1.9 – 3.3% on average (Anderson, K., 2006). Taken together, the
odds of an adult man or woman surviving long enough to find a mate, and having at least
one child, are quite high particularly when compared to great apes (Goodall, 1986;
Robbins & Robbins, 2004). An exact number is difficult to quantify, but by adding these
probabilities together, we may obtain a cumulative percentage for the likelihood of
passing one’s genes on to the next generation. For example, the yearly likelihood of a
man passing on his genes in a high-mortality, high polygyny, high false paternity group,
is equal to: 100% chance of passing one’s genes minus (2% yearly mortality x 10%
failure to find mate x 2% infertility x 3% false paternity), for a final likelihood of 84%.
These admittedly rough, and general estimates, lead to average likelihoods of 84-90%
(men vs. women) that may suffice for the comparative purposes of this paper. Thus, men
and women face a reproductive failure rate of 10-15%, on average. Again, we stress that
this range represents a rough estimate of average rates. These rates can, and do, vary
significantly across individuals, groups, and times (Anderson, 2006; Gurven & Kaplan,
2007; Kramer & Greaves, 2007). We do not claim that these rates are precisely accurate,
rather, we are interested in their relative size for the purpose of comparing their selective
pressure to that of child mortality.
Child Mortality
Childhood has changed in many ways over the course of human evolution.
However, there are a few universals that stand out over time and across cultures. One of
these universals of childhood is the very high mortality rates of infants and children prior
to the arrival of modern sanitation and medicine (Stearns, 2006). Before exploring the
mortality rates of infant and children, four points need to be addressed. First infants, and
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particularly children, are defined loosely in the literature. For the purposes of this paper
infancy ends at one year of age, and childhood ends at puberty (12-15 years of age).
Second, children are typically underrepresented in historical recordings, literature, burials,
and censuses, making complete and accurate estimation difficult (Perry, 2006; Woods,
2003). These factors may plausibly, if not likely, cause our estimates of child mortality to
be too low, rather than too high (Lewis & Gowland, 2007). Third, infant and child
mortality is believed to have varied significantly across and within times and cultures
(Woods, 2003). Therefore, the following mortality rates should generally be viewed as
probable estimates rather than absolute rates. Finally, infant mortality is measured by the
probability of dying within the first year of life, while child mortality is measured by the
cumulative probability of dying before adolescence/puberty. Thus, the two mortality rates
are not additive.
Table 1. Infant and Child Mortality Rates Across Time and Cultures
Time Culture Infant Mortality Child Mortality
500 – 300 BC Greece 25 – 35% n/a
200 BC - 200 AD Rome 30% 50%
1300 – 1400 AD Japan 28% 48%
1500 AD England 27% n/a
1600 – 1700 AD France/Sweden 25-40% 50%
1650-1800 AD Imperial China 26% 45%
1950 AD Afghanistan 28% 36%+
Modern Hunter-Gatherers 23% 46%
Modern Horticulturalists 21% 39%
Modern Pastoralists 21% 34%
Modern Developed Countries > 1% 1%
Modern Chimpanzee 20% 55-60%
Modern Gorilla 20-23% 50%+
With these caveats in mind, we can try to estimate infant and child mortality rates
in the EEA. The most ancient attempts at estimating child mortality come from ancient
Greece (circa 500 – 300 BC). Golden (1990) estimates a mortality rate of 25-35%
amongst infants in ancient Greece. In ancient Rome, infant mortality is similar at 30%,
with childhood mortality at 50% (Rawson, 2003). Jumping ahead a thousand years in
time, we find similar rates in Medieval and Renaissance Europe. In Medieval England,
infant death rates continued to be high (Lewis & Gowland, 2007; Paine, 2000), with
estimates of 27% mortality in the first year of life (Orme, 2001). In Renaissance France
and Sweden, average infant mortality rates varied between 25-40%, with childhood
mortality rates of approximately 50% (Cunningham, 2005; Human Mortality Database,
2008). During the same time period, we find similar mortality rates in the Far East.
Estimates of infant and child mortality in Medieval Japan are a very similar 28% and
44% respectively (Nagaoka, Hirata, Yokota, & Matsu’ura, 2006). The last Chinese
emperor Aixin Jueluo Puyi (1964) estimated infant mortality rates amongst the Imperial
Lineage in China from 1640-1911 as 23% for boys and 29% for girls, whereas childhood
mortality rates were 40% for boys and 50% for girls (note that Lee, Feng, & Campbell,
1994 suggest lower mortality rates). Thus, we see remarkable similarities in infant and
child mortality rates across different civilizations from different time periods. The first
changes in these rates occurred during the middle of the 19
th
century when improved
sanitation, diet, and medical practices caused infant and child mortality rates to plummet
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down to >1 – 1% in developed countries (Cunningham, 2005; Stearns, 2006; United
Nations Data, 2008). However, in non-developed countries that lacked these modern
developments, infant mortality remained high (e.g., Afghanistan 1950, 28% infant and
36% to age five mortality in 1950; United Nations Data, 2008).
While we lack any concrete data for infant and child mortality rates prior to
ancient civilization, we may estimate their mortality rates by using those found in modern
hunter-gatherer and horticulturalist-pastoralist groups. Again, we note that modern
groups are not perfect replicates of ancient groups, and therefore these mortality rates
remain educated estimates for ancient mortality rates. Nevertheless, the figures once
again converge on similar values as those found throughout pre-Modern history.
Studying 9 hunter-gatherer, 10 horticulturalist and 5 pastoralist groups, Hewlett (1991)
found active hunter-gatherer infant and child mortality rates of 23% and 46%,
horticulturalist infant and child mortality rates of 21% and 39%, and pastoralist infant and
child mortality rates of 21% and 34%. These values are similar to those obtained by Hill
et al. (2007), but are slightly lower than those estimated by Gurven and Kaplan (2007).
While these rates are slightly lower than those found in pre-Modern civilizations, they are
much higher than modern infant and child mortality rates of less than 1% in developed
countries (Human Mortality Database, 2008). Furthermore, one must be wary of the
possible contaminating effect of expose to modern sanitation and medicine in even the
most remote groups (Kramer & Greaves, 2007). This may explain the lower
horticulturalist and pastoralist mortality rates as compared to the historical and hunter-
gatherer rates.
Taken as a whole then, one cannot help but be impressed by the similarity in
infant and child mortality rates across an enormous range of different cultures and times.
This is perhaps more striking given that the causes of these mortality rates are believed to
vary between individual groups (Hill et al. 2007) and times (Paine, 2000). How do these
mortality rates compare to mortality rates in other species? Juvenile mortality rates are
generally quite high across nature (Dawkins, 1989). Yet, when compared to our close
relatives, chimpanzees and gorillas, human infant and early childhood mortality are
higher than estimates for both species (see Table 1; Groves & Meder, 2001; Hill et al.
2001; Robbins, Bermejo, Cipoleletta, Magliocca, Parnell, & Stoke, 2004). However,
chimps and gorillas surpass human mortality by adolescence (Gurven & Kaplan, 2007;
Hill et al. 2001; Robbins & Robbins, 2004; Sugiyama, 2004). This unique high-infant
mortality pattern in humans is thus not shared by our close primate ancestors, opening the
door to entirely novel selection pressures.
Relative Selective Pressures of Child and Adult mortality
How then does child mortality compare to adult mortality and mating success as
a selective pressure on human evolution? We previously estimated the adult probability
of failure to directly pass one’s genes to lie between 10%-15%. When this range is
compared to either the average infant mortality rate of 25%, or the average child
mortality rate of 40-50% (respectively), it suggests that humans typically face a
significantly greater chance of failing to directly pass on any genes to future generations
(i.e., total genetic failure) during infancy (~1.7 to 2.5 times) and childhood (2.7 to 5 times)
than in adulthood! This may be especially true for women, whose lower reproductive
variance reduces their adult likelihood of genetic failure. However the pressure is still
very strong for men; even in an extreme hypothetical population with a high rate of
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cuckoldry (20%), a high homicide rate (20%) borne both before and between the
fathering of each child as well as a sizeable chance of never finding a mate (20%), a male
still has a 51% chance of successfully fathering one child and a 26% chance of fathering
two children. Compared with these very high male adult failure rates, the selection
pressures of surviving childhood in this extreme, hypothetical population are roughly
equal to those associated with reproduction in adulthood. And while they are equal in
likelihood, the childhood pressure is a necessary precursor to the adult selective pressures.
Child mortality therefore appears to fit our criteria of a strong, pre-reproductive selective
pressure for both sexes. The purpose of this comparison is not to suggest that adult
mortality and mating were unimportant in the EEA- indeed, we believe they were both
very important. Nor do we wish to suggest that child mortality always supersedes adult
selective pressures. Rather, we wish to demonstrate that the well-studied adult selective
pressures are relatively equal to, or perhaps even weaker than, the poorly-studied
selective pressure of child mortality.
Why then, has the relatively high rate of child mortality received so little
attention from evolutionary psychologists? Searching the Journal of Evolution and
Human Behavior (as well as Ethology and Sociobiology), Evolutionary Psychology, the
Journal of Social, Evolutionary and Cultural Psychology and Human Nature returned no
articles that specifically investigated childhood or infancy mortality rates in relation to
adult mortality rates and addressed potential adaptations in children and infants. A
number of papers, such as Sear and Mace (2008), investigated the role of kin support but
did not focus on relative mortality rates (adult versus child) nor investigate potential
adaptations in children to secure investment (e.g., child agency). Several articles
investigated maternal investment strategies in relation to adult offspring reproductive
strategies (Belsky 1997; Belsky, Stienberg and Draper 1991) but likewise did not
mention the high rate of child mortality nor child adaptations to secure care and thus
increase the chances of survival to adulthood. Other articles that investigated potential
adaptations in children to secure care such as Lumaa et al (1998) do not mention the high
level of child mortality. Examining the all articles (n=365) published in the Journal of
Evolution and Human Behavior from 1997 to present uncovered 41 articles (11%)
investigating factors that would directly affect child mortality in a meaningful manner,
with only a handful specifically investigating adaptive mechanisms in either parent or
child to decrease mortality rates. Most of the 41 articles focused exclusively on 2
mechanisms affecting infant and child mortality; paternal resemblance and step-parenting.
Thus a gap in the current literature appears to exist, centered on the relatively high rates
of child mortality.
One reason for the dearth of studies that specifically address the importance of
child mortality may be that evolutionary psychologists are primarily social psychologists,
not developmental psychologists, and therefore child-related aspects of human behavior
and evolution are often overlooked. Furthermore, the vast majority of participants in
evolutionary psychological studies are over the age of eighteen, as children are rarely the
focus of non-developmental studies. Additionally, developmental psychologists, even if
they are adept at evolutionary theory, are not historians or anthropologists and would not
necessarily be cognizant of the large discrepancy between child and adult mortality rates
in the EEA. This explanation may also be extended to evolutionary psychologists; they
may simply be unaware of the impact of child death on human evolution because it is so
far removed from their daily existence. Mating, adult survival, adult social interactions,
and even child development are omnipresent in modern cultures, whereas child mortality
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is virtually absent. Indeed, what is striking about pre-Modern versus Modern lifespans is
the tremendous decrease in child mortality as compared to very modest decrease in
adulthood mortality (Gurven & Kaplan, 2007). Additionally few professors are
themselves parents: male professors are 21% less likely than doctors and 12% less likely
than lawyers to have children, while female professors are 41% and 24% respectively less
likely to have children (Wolfinger 2008)! As a result, child mortality may very well be
out of sight and therefore out of mind, for many modern evolutionary (or developmental)
psychologists. Thus, the lack of developmental theories, the paucity of younger research
study participants, and the everyday absence of child mortality may have hidden the
strong, pre-reproductive selective pressure of child mortality from evolutionary
psychologists. Having ascertained that child mortality is indeed a relatively strong
selective pressure that has perhaps been under-appreciated, we now examine some of the
potential consequences of this selective pressure on our evolved mental mechanisms.
Consequences of Child Mortality
From our conclusion that infant and child mortality represent strong, pre-
reproductive pressures, it follows that there should be a correspondingly strong degree of
adaptation to this pressure in human minds. It is well beyond the scope of this article to
detail all such possible adaptations. Instead, we discuss and provide examples of three
categories of such mental adaptations: adult adaptations to provide care, child adaptations
to actively secure adult care, and child adaptations for individual survival.
Adult Adaptations to Provide Care
If approximately 50% of all children die before reaching reproductive age, adult
reproductive success may be greatly influenced by child survival. Adults who are able to
even partially mitigate the mortality rates of their offspring should have had a strong
fitness advantage over adults who were less successful in improving their offspring’s
survival (Trivers, 1972). Adults should therefore have mental adaptations that promote
the survival of their offspring (Geary & Flinn, 2001), presumably by increasing the
quality and/or quantity of investment that their offspring receives.
Perhaps the clearest adaptation to decrease child mortality is attachment.
Attachment is believed to be a mental adaptation that typically allows the formation of a
strong, enduring bond to one’s offspring that enhances their survival and success
(Bowlby, 1982). Indeed, attachment is the first adaptation explicitly studied in the context
of the human EEA (Bowlby, 1982). Numerous studies (see Cassidy & Shaver, 1999)
have shown that parents who have a strong, positive attachment to their child make
effective parents, whereas the lack of a strong, positive attachment to one’s child often
leads to neglectful parenting that can dramatically increase the likelihood of
developmental psychopathologies and reduce the child’s chances of survival (Dozier,
Stovall, & Albus, 1999; Greenberg 1999; Hrdy, 1999; Schepher-Hughes, 1985). Indeed,
it has been suggested that poor quality attachments may be adaptive parental mechanisms
for limiting and/or terminating investment in an unwanted child when scarce parenting
resources are better conserved for other purposes (Wiley & Carlin, 1998).
In addition to modulating attachment to form a greater bond with a
phenotypically fitter child, and a weaker bond with a less fit child, it might be adaptive
for parents to feel greater grief over the loss of a child with a larger reproductive value
(RV). Crawford et al (1989) found that the intensity of grief that would hypothetically be
experienced over a child’s death was highly correlated to the RV of the child in both
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the !Kung and Canadians. Grief felt was more highly correlated to RV for the !Kung than
for Canadians, suggesting an ecological moderating effect. In a related study that directly
measured the intensity of grief felt over child loss, Littlefield and Rushton (1986) found
that children who were deemed to be healthier were also grieved more by both parents.
Additionally, a parent reported more grieving if they reported the child as being more
similar to their own side of the family, as opposed to the other parent’s family. Suarez
and Gallup (1985) suggest that this may be one of the reasons that women are more prone
to depression then men- a greater potential activation of attachment-related loss that
represents the flip side of attachment (Bowlby, 1982).
Given the importance of proper parenting, adults may also be under pressure to
choose mates who exhibit the potential to be good parents. Recent studies show that
women in particular, are interested in men who show characteristics believed to be
associated with good parenting (Buss & Schmidt, 1993). Both sexes show an increased
preference for “good parent” traits in long-term, as opposed to short-term, partners (Buss
& Schmidt, 1993; Little, Burris, Jones, DeBruine, & Caldwell, 2008). For a woman the
consequences of mating with a “cad” who fails to provide paternal investment may be
much more significant than is commonly believed if her “cad” jeopardizes the survival of
her infants (Hurtado & Hill, 1992). Indeed, research suggests that a lack of paternal input
can not only affect a child’s development, but also their future mating patterns and
choices (Belsky, 1997; Moffitt, Caspi, Belsky, & Silva, 1992)
A related adaptation may be alloparenting- the provision of care by adults other
than the biological parents of the offspring. Grandparents and female friends and relatives
appear to be particularly important in this regard. Sear and Mace’s recent study (2008)
suggests that grandparents may be as effective as fathers in increasing the probability of
child survival. From a broader perspective, the potential benefit of female-female
cooperation for child survival through alloparenting (Hrdy, 1999; 2005; Turnbull 1961)
may be one reason why some behavioral aspects of humans more closely resemble
bonobos than chimpanzees (deWaal, 2005).
Child Adaptations to Secure Care
Children are not simply passive receptors of parental care. Indeed, Bowlby’s
theory of attachment is explicitly dualistic, in that both the child and its caregiver are
responsible for the creation of the attachment bond between them (1982). It is the
dynamic of their mutual interactions that determines the strength and quality of their
bond (Bowlby, 1982). Infants may attempt to increase the strength of this bond by
engaging in behaviors that elicit (e.g., crying- Donavan & Leavitt, 1989; Zeifman, 2003;
Lumaa et al 1998), maintain (mutual eye gaze- Hains & Muir, 1996), or strengthen
(smiling- Strathearn, Li, Fonagy, & Montague, 2008; Volk & Quinsey, 2002) this bond.
Even at ages as young as 3 months, infants appear to be highly capable of initiating and
regulating person-to-person interactions (Gusella, Muir, & Tronick, 1988). From the
moment a child is born it is capable of crying. Although there are many hypothesized
adaptive reasons for an infant or child to cry (see Lummaa et al 1998), there is strong
evidence that crying is an honest signal of the vitality and health of the child. By crying a
child is able to accurately and honestly signal its phenotypic quality to its parents and
thus receive more parental care (Furlow 1997); Devries (1984) found that quieter babies
received fewer resources than fussier babies in the Masai which translated into higher
mortality rates for the quieter infants during a famine.
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Children may also have physical adaptations that elicit feelings of parental care
in adults. Lorenz hypothesized that infant facial cues evolved to serve that purpose.
Research shows that infant facial cues of health, cuteness, and resemblance appear to
significantly influence adult investment (real and hypothetical) in children (e.g., Langlois,
Ritter, Casey, & Sawin, 1995; Volk & Quinsey, 2002). Adults appear to have a unique
neural response to infant faces versus adult faces (Kringlebach et al. 2008), and
dopamine-associated reward-processing regions of
the brain are activated when mothers
viewed their own infant's
face compared with an unknown infant's face (Strathearn et al.
2008). Research has also shown that adults prefer to invest their hypothetical resources in
younger children, even if they appear to be less healthy than older children (Volk,
Lukjanczuk, & Quinsey, 2007). Taken together, these results suggest that children’s
facial features appear to physically equip children for soliciting parental care.
In addition to securing care, it would be adaptive for a child to be able to increase
the inter birth interval (IBI) and thus secure a high level of investment for a longer period
of time. It has been hypothesized that waking in the middle of the night is such an
adaptation; if a child is more likely to wake up (or cry) in the middle of the night he or
she is most likely to be soothed back to sleep via breastfeeding. This behavior could
easily increase the duration of ovulation suppression of the child’s mother and thus
directly increase the IBI (Jones and Costa 1987; Lumaa et al 1998). This is a good
example of potential parent-offpsring conflict, whereby the children actively seek to
boost their own fitness at the (limited) expense of their parent’s fitness (Trivers, 1974).
Presumably, human parents tolerate a degree of this behavior in order to avoid costly,
potentially mortality-inducing, conflict.
Child Adaptations for Individual Survival
Bowlby’s model of attachment once again provides us with another potential
adaptation to child death. According to attachment theory, children not only form a bond
with their caregiver, they use the quality and nature of this bond as a template for all
future social bonds (Belsky, Stienberg, & Draper 1991). The adaptive purpose of this
generalization is believed to be matching the child to the predominant levels of social
care they will receive throughout their lives. Children who receive nurturing, supportive,
parental care are more likely to continue receiving such care from other individuals
(Bowlby, 1982; Cassidy & Shaver, 1999). Whereas individuals who fail to receive such
care may often face social climates where they do not receive large amounts of social
care and hence have to take a more selfish, and self-reliant, view of human relationships
in order to maximize their chances of survival (Belsky 1997; Frodi, Dernevik, Sepa,
Philipson, & Bragesjö, 2001; Wiley & Carlin, 1998).
Children also appear to have specific mental adaptations that promote survival at
distinct stages of their lives. For instance, infants are born with a variety of reflexes that
are believed to have aided their survival in ancestral environments (e.g., rooting, grasping,
startle reflex, etc.). As they grow, children appear to show adaptive fear responses that
promote their survival. A fear of heights would be inappropriate for an infant, who is
routinely carried several times their own height off the ground by caregivers. However,
once the child is able to move on their own, and therefore get themselves into trouble
with falling, a fear of heights develops (Campos, Bertenthal, & Kermoian, 1992; Gibson
& Walk, 1960). Other fears appear to adaptively manifest at age-appropriate levels
(Marks & Nesse, 1994) such as animals (age of exploration- 2 years) and agoraphobia
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(age of leaving home- adolescents). A fear of strangers is not present when infants are
young, and when it may be advantageous for infants to begin forming pre-attachment
bonds with a variety of caregivers (e.g., the communal care of the BaMbuti; Turnbull,
1961). However, once infants begin to form attachment bonds, it may be maladaptive for
them to respond to strangers who lack such a bond with them and/or who may interfere
with their development of an appropriate internal model of attachment (Bowlby, 1982).
Children also appear to have cognitive adaptations that reflect immaturity, but
may in reality be ways of maximizing learning at a particular stage (Bjorklund, 1997).
For instance, the shorter working memories of infants may automatically help them to
break language into more manageable strings of data (Elman, 1994). Rather than
understanding an entire paragraph, sentence, or even word, they are able to concentrate
on understanding the underlying phonemic structure. Similarly, children appear to have
unrealistic, immature meta-cognitions that may in fact represent potent learning
adaptations. For example, children often and repeatedly overestimate their ability to
complete a new or difficult task (e.g., riding a bicycle). This overestimation of their
ability appears to both increase their performance in the task (similar to self-efficacy in
adults) as well as their motivation to persevere on difficult tasks (Bjorklund & Pellegrini,
2000). Although it would be premature to call a child’s optimism ‘self-deception’, an
unrealistically optimistic outlook may at times be adaptive for a child. This behavior
could later underlie the similar, but lesser, tendency of adults to similarly overestimate
the performance on a variety of tasks (Williams & Gilovich, 2008).
Conclusion
We believe that the vastly reduced rates of modern child mortality have hidden
the true scope of this selective pressure. The literature on child history is relatively
underdeveloped, as is the literature on child anthropology in general (Hewlett & Lamb,
2005). As adult researchers, we may have a bias towards looking at ourselves as we are,
rather than as we were when we were children. Children are often viewed as immature
version of adults, whose only purpose is to grow out of maladaptive, sterile childhood
and into adaptive, reproductively capable, adulthood.
We have shown that such a belief may be missing a crucial part of human
evolution. Child death appears to be a strong pre-reproductive universal pressure in
human evolution. As such, it has likely exerted a strong influence on the evolution of the
human mind. Several potential adaptations have been discovered that appear to respond
directly to the pressure of child mortality. We believe that further attention to these
adaptations is warranted, particularly those that stress the child as an independent agent
of selection, and that there are many other adaptations, or aspects of existing adaptations,
that serve to respond to the ancestral crucible of child death. Although child mortality
may, or may not, in fact be the crucible of human evolution, it has undoubtedly played a
significant role in human evolution that researchers should be aware of when testing
human evolutionary hypotheses.
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Acknowledgements
We would like to thank Joseph Camilleri and Gordon G. Gallup Jr. for their valuable comments.
We would also like to thank the NEEPS 2008 audience for their insightful questions given during
the presentation of this article. Finally, we would like to thank our reviewers for their helpful
comments and suggestions.
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Az evolúciós pszichológia (EP) legnagyobb érdeme, hogy következetesen kifejtett formában mutatta be, hogy a darwini evolúcióelmélet alkalmas a pszichológiára mint tudományterületre vonatkozó metaelmélet szerepének betöltésére. Az irányzat által képviselt adaptácionista megközelítés mindmáig nagyszámú pszichológiai jelenségnek új szempontokon alapuló vizsgálatához és magyarázatához járult hozzá. Más tekintetben viszont az EP mint paradigma némileg egyoldalú maradt – ami részben a keletkezésének időszakában dominánsnak mutatkozó kognitív elméletek (így pl. Chomsky-féle innátista megközelítés, a Fodor képviselte modularizmus) feltételek nélküli elfogadására vezethető vissza. Az egyoldalúság kialakulásához az is hozzájárult, hogy az EP képviselői nem szenteltek kellő figyelmet a humángenetika, a fajok közötti összehasonlító pszichológia és a kulturális pszichológia terén zajló fejlődésnek. Az ebből adódó korlátokra és leegyszerűsítésekre az EP-vel szembeni kritikák már több alkalommal rámutattak. A kritika azonban önmagában nem jelent megújulást. Ebben a tanulmányban ezért az evolúciós pszichológiai megközelítés újraaktualizálásának lehetőségeit, a továbblépés irányait vesszük számba. A legújabb, a genetikai hatásokkal, az állati elme működésével vagy épp a pszichológiai különbségek kulturális hátterével kapcsolatos tudományos eredmények ugyanis lehetőséget kínálnak arra, hogy a Tinbergen négy kérdése által kijelölt területeken újragondoljuk és kiegészítsük az evolúciós pszichológia eredeti feltevéseit.
Chapter
Infant mortality rates (IMR) represent an important evolutionary selective pressure. Given that one must survive infancy (defined here as the first year of life) to pass on one’s genes, IMR were an important evolutionary filter. Historical and hunter-gatherer data reveal that IMR were universally high, about 26–27%, in the environments of evolutionary adaptation (EEA). These exceptionally high infant mortality rates persisted until quite recently and are likely responsible, at least in part, for a variety of human evolutionary adaptations. We discuss these adaptations alongside the common causes of EEA infant mortality and data from non-human primates, agriculturalists, and modern populations. Overall, our chapter reveals infant mortality as a potent force in the evolution of the human species.
Chapter
This chapter offers evolutionary developmental psychology models of caregiving and attachment as species-wide features of infant–maternal relationships. We explain that 3 years of breastfeeding were compelled by the leading causes of infant mortality in ancestral settings—infection and malnutrition-related disease—and discuss how it underpinned lactation-based caregiving and a biobehavioral bond, lactation-based cohesion, with fitness payoffs for infants: (1) protection against malnutrition and morbidity; (2) preservation of the inter-birth interval (IBI) as a haven against competition with a newborn sibling; and (3) psychological benefits of steady and enduring exposure to a profoundly satisfying manner of proximal contact with a caregiver. We theorize that lactation-based caregiving and cohesion satisfied infants’ physical and psychological needs, and in doing so laid the foundation of a psychological adaptation, child-to-mother attachment, an affectional bond able to withstand being untethered to lactation by infants’ third year. The timing of the transition from lactation-based cohesion to attachment coincided with attenuated dependence on breast milk due to maturation of infants’ digestive and immune systems, and with the eruption of infants’ molar teeth, which prompted mothers to bring breastfeeding to conclusion. At this juncture, mothers transitioned to caring for weanlings (rather than nurslings), which meant an end to maternal caregiving being upheld by biobehavioral features of lactation. We argue that absent such support, the costliness of caregiving of weanlings compelled an adapted psychological mechanism, mother-to-child attachment, defined as an affectional bond between ancestral mothers and their former nurslings that was anchored in 3 years of lactation-based caregiving and cohesion.
Chapter
In this chapter, nascent jealousy is theorized as a psychological adaptation to the threat of usurpation by a newborn sibling that ancestral infants could have encountered by the age of 9 months. This nature of threat is explained in terms of ancestral infants’ dependence on their mothers as their sole sources of breast milk, which was required for survival. We discuss how the need for exclusive access to mother for breast milk coevolved with expectations of exclusivity in the infant-maternal relationship and with exquisite sensitivity to violations of those expectations that, over deep time, shaped nascent jealousy and its expression through jealousy protest as a mechanism for protecting against usurpation. Next, jealousy protest is discussed in relation to separation protest. Commonality between the two patterns of protest and the overlap in timing are interpreted as the basis for conceptualizing infant-maternal attachment as an adaptation to usurpation. Distinctions are informed in terms of the adaptive advantages of exclusive (vs. nonexclusive) proximal contact with mother. Finally, we address milestones in socioemotional, social cognitive, and motor development that occur at 9 months. We explain how jealousy protest was enabled, not only by attachment formation for its role in establishing a valued relationship but also by skills in cognition and locomotion that facilitated infants’ abilities to recognize and manage usurpation. We propose that these milestones originated in tandem at 9 months to help prepare infants for potential challenges of usurpation as they entered toddlerhood.
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Declining mortality seems a natural explanation for the demographic transition. However, many economists have discarded improved infant survival as a causal trigger. Moreover, certain currents in Neo-Malthusian economics point to potentially beneficial side-effects of population shocks. Based on historical demography and evolutionary science, I challenge these views. The argument is that uncontrollable (“extrinsic”) mortality creates selective advantages for families with many “cheap” offspring, whereas stable environments favor child “quality”. Combining “life-history-theory” and a unified growth model, I demonstrate that declining mortality and medical progress facilitate the transition towards growth-promoting “low-fertility-high-quality” phenotypes. As it will turn out, this framework produces qualitatively and quantitatively closer predictions of the historical fertility decline than standard models of the Barro–Becker type. Moreover, evolutionary mechanisms provide a parsimonious explanation for diverse demographic transition patterns. Thus, evolved adaptations add a new and culture-free mechanism to older theories. Moreover, regarding sustainable growth, they suggest that natural selection eventually offsets the benefits from population shocks claimed by Malthusian theories.
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Filling a gap in a field with very few teaching books available, Childhood in World History provides a much-needed historical overview. Studying childhood historically greatly advances our understanding of what childhood is about, and a world history focus permits broad questions to be asked. Peter N. Stearns, an esteemed name in the field, focuses on childhood in several ways: Childhood across change - the shift from hunting and gathering to an agricultural society, the impact of civilization, and the emergence of major religions. New and old debates about the distinctive features of Western childhood, including child labour. The emergence of a modern, industrial pattern of childhood in the West, Japan and communist societies, focusing on education and economic independence. Globalization and the spread of child-centred consumerism. Highlighting the gains, the divisions, and the losses for children across the millennia, this fascinating book will appeal to students across the board, and will prove an excellent teaching resource.
Article
Because of its biological adaptive value, wariness of heights is widely believed to be innate or under maturational control. In this report, we present evidence contrary to this hypothesis, and show the importance of locomotor experience for emotional development. Four studies bearing on this conclusion have shown that (1) when age is held constant, locomotor experience accounts for wariness of heights; (2) “artificial” experience locomoting in a walker generates evidence of wariness of heights; (3) an orthopedically handicapped infant tested longitudinally did not show wariness of heights so long as he had no locomotor experience; and (4) regardless of the age when infants begin to crawl, it is the duration of locomotor experience and not age that predicts avoidance of heights. These findings suggest that when infants begin to crawl, experiences generated by locomotion make possible the development of wariness of heights.