Adaptations relating to family life index
Changing views on the family through history and across cultures is an interesting history in itself. Many authors, from Hobbes to Engels, have acknowledged the family as a central organizational unit of human life. In evolutionary terms, it is a hominid adaptation; our simian cousins do not organize in this way. The decisive innovation is monogamy, and the joint and sustained care of and investment in offspring by both the mother and the father. Similar arrangements are also found among birds, though they are uncommon among mammals.
This page contains some experimental data, some theorizing supported by experimental data, and mostly a list of predictions for emotional and behavioral proclivities based on the adaptationist program. Whether the claims are made on the basis of experimental data or on the basis of the logic of natural variation and differential reproductive outcomes ("evolution"), they do not indicate limits to human behavior, or statistical norms, much less ethical norms. Rather, they are intended to suggest that such behavior constitutes physiological and psychological attractors in our lives today, due to an evolutionary argument.
The evolutionary argument has several steps. Take menopause. It is not present in chimpanzees or gorillas, and presents an evolutionary puzzle: why would not women who remain fertile until they die leave a larger number of surviving children than menopausal women? To answer this question, we must situate ourselves in the environment of evolutionary adaptedness (the EEA) -- that is, in the historical setting in which our ancestors were subjected to a consistent pressure to evolve. We need to look into our evolutionary past and ask, "Under which conditions might it be more important to be a grandmother than a mother?" This is the adaptationist question; answering it requires an act of historical reconstruction common to all historiography. What comes in addition is the suggestion that the adaptations resulting from the problems posed in the EEA are still with us, even if those problems themselves are not. Thus, we still have menopause, even though many women do not care for their grandchildren, and instead undergo expensive fertility treatments to have children of their own. We would still expect, however, that menopausal women would be relatively motivated to devote time to their grandchildren, since the physiological adaptation of menopause would be useless without the psychological adaptation of being interested in your grandchildren.
It is important to bear in mind that since human beings have the capacity for running mental simulations, our evolved cognitive capacities and proclivities can be manipulated by our own thinking, by traditional cultural representations such as religion, and by novel cultural representations such as movies. This does not mean the proclivities are not present, but it does mean their expression cannot be predicted simply by external circumstances.
The following list, therefore, presents partially documented or theoretically hypothesized evolved proclivities rather than predictions of actual behavior. The page is under sporadic construction and unfortunately does not even begin to do justice to all the work done in this area. Suggestions for the inclusion of further work will be gratefully acknowledged.
On the paradox of parenting (joke).
Facial recognition: soon after birth, infants will smile at people's faces--and to fairly crude drawings that vaguely look like a face. Neuroanatomists have located the ability to recognize a face as a face deep in the midbrain. Only after several months does the normal infant clearly distinguish between faces, and becomes weary of strangers. The human ability to distinguish and remember thousands of faces is a good example of a cognitive skill we generally consider trivial, yet in reality is impressively complex.
Mark Johnson proposes and presents evidence for two distinct processes:
"...the first process consists of a system accessed via the subcortical visuomotor pathway (but possibly also involving some of the deeper, earlier developing, cortical layers) that is responsible for the preferential tracking of faces in newborns. However, the influence of this system over behavior declines (possibly due to inhibition by developing cortical circuits) during the second month of life. The second brain system depends upon cortical maturity, and exposure to faces over the first month or two, and begins to control infant orienting preferences from around two or three months of age" (113).
Johnson, Mark (1997). Developmental Cognitive Neuroscience. Blackwell.
The cross-cultural developmental trajectory of children's drawings also provides evidence for prepared cognitive equipment. At a certain age, children draw people as a huge face with stick legs and arms, and no trunk. The face has a privileged status in the child's cognition, and is a potent source of cues that trigger affect. (Citation missing.)
Facial expression imitation: human infants can reliably imitate adult facial expressions in the first days of life, even as young as 42 minutes (Meltzoff and Moore 1995, Bower 1977). If you look at a neonate and stick your tongue out, the infant will imitate your action. The same for eye fluttering, lip protrusion, mouth opening, and finger movement. This happens so early in life that it seems unlikely that infants have learned how to do this. Rather, such behavior must be innate; the genes somehow "encode" it.
Bower, T.G.R. (1997). A Primer of Infant Development. Freeman.
Meltzoff, Andrew N. and M. Keith Moore (1995). Infants' Understanding of People and Things: From Body Imitation to Folk Psychology. In Jose Luis Bermudex, Anthony Marcel, and Naomi Eilan, eds., The Body and the Self. MIT Press..
(Contributed by Bill Benzon)
Sibling rivalry:Conflict between siblings was first proposed by Hamilton (1964a, b) as a consequence of his kin selection theory; this was further extended by Trivers (1974) by means of a cost-benefit approach. To optimize the parental investment, siblings can pursue a variety of strategies, not all of which entail brotherly love. Full siblings share half of each other's genes, and there are numerous adaptations that accord with and encourage brotherly love. However, siblings also compete for access to their parents' limited resources, and this may lead to a range of adaptative strategies. Such evolved strategies function like a library of action and disposition patterns that the child can bring on-line when the situation calls for it. The extreme expression of rivalry would be siblicide, anecdotally often comtemplated but rarely performed in practice. It occurs predictably in some other species, however; Sulloway (1996:63) cites the case of the blue-footed boobie, a small bird. The oldest offspring, if its weight drops below 80% of normal, "pecks the younger to death or excludes it from the nest" with the ruthless connivance of the parents. In this case, the parents are adapted to accept the computation of the killer that chances are low for raising more than one offspring successfully. Letting one die is their way of protecting their investment.
Westermarck effect: "In 1891, the anthropologist Edward Westermarck proposed that early childhood association inhibits sexual attraction and that this aversion was manifested in custom and law as the basis of the universal incest taboo. His argument was accepted as 'the solution of the problem' by such eminent scholars as Edward Burnett Tylor and Alfred Russel Wallace. Then, in 1910, in the essays later published as Totem and Taboo, Sigmund Freud challenged the 'Westermarck hypothesis' on the ground that 'the earliest sexual excitations of youthful human beings are invariably of an incestuous character.' The incest taboo only existed, Freud argued, because of this natural propensity." From the jacket of Wolf (1995).
To account for Freud's counterintuitive conviction, it may be worth noting he was raised by a wetnurse. Since the Westermarck effect is cue-based, the infant's psychology may have been left in partial doubt as to who his mother was. Freud's hypothesis shows traces of this ambiguity: having sex with your mother is supposed to be fun but still forbidden. It is unclear why there are strictures against something so universally desired. As Wolf notes, however, "Freud's challenge carried the day and became the standard view throughout the social and biological sciences."
In parts of South China people commonly adopted and reared their sons' wives, many women nursing their future daughters-in-law. Wolf compares these marriages with those in which the bridal pair did not meet until their wedding day, and uses a wide range of research to demonstrate that in fact Westermarck was right and Freud mistaken. There is a critical period in human development -- approximately the first thirty months of life -- during which association permanently inhibits sexual attraction. In contrast to Freud's predictions, encouraging early association between father and daughter is probably the best way of preventing sexual abuse.
Wolf, Arthur P. Sexual attraction and childhood association: A Chinese brief for Edward Westermarck. Stanford, CA: Stanford University Press, 1995.
The Psychology of Love
Birkhead, Tim (2000). Promiscuity: An Evolutionary History of Sperm Competition and Sexual Conflict. London: Faber. Reviewed by James Meek, Sex is best when you lose your head (London Review of Books 22. 22 (16 Nov 2000), external).
Geary, David (1998). Male, Female: The Evolution of Human Sex Differences (APA, 1998). Sample chapters: Developmental sex differences and sex differences in brain and cognition (full external text).
Gangestad, S and JA Simpson (1999). The Evolution of Human Mating: Trade-Offs and Strategic Pluralism (full external text).
Hegstrom, Carol (1999). Sexual dimorphisms of the brain and the dangers of discrimination.
Is there a neurobiology of love? Psychoneuroendocrinology 23, 8 (November 1998): (special issue).
Kimura, Doreen (1999). Sex and Cognition. Cambridge, MA: MIT. Abstract.
Lewis, Thomas, Fari Amini, and Richard Lannon (2000). A General Theory of Love. New York: Random House. First chapter and review (external).
Low, Bobbi S. Why Sex Matters. A Darwinian Look at Human Behavior. Princeton, NJ: Princeton UP, 2000. First chapter and linked review.
Miller, Geoffrey F. (April 18, 2000). The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature. New York: Doubleday. Reviewed for CogWeb by Timothy Horvath, "Our Tales are Our Tails." Also reviewed by Anjana Ahuja (external).
Tennov, Dorothy (1999 ). Love and Limerence: The Experience of Being
in Love. New York: Scarborough House. Reviewed
by Danny Yee (external). Author's
web site (external).
The basic challenge: The reproductive challenges of human females stems from two central facts--that she is the one to grow the baby inside herself (a destiny she shares with all her mammalian sisters), and that her baby will benefit greatly if the father invests in it after that quick and easy moment of conception. Babies are extremely resource demanding -- the sheer fact of pregnancy places severe stresses on the woman. A pregant mother shouldn't have to do heavy labor; see for instance a major study in the US by Ellen Mozurkewich (2000), showing that heavy work during pregnancy was associated with premature delivery, high blood pressure and underweight babies. In brief, the prediction is that the human female should have a set of cognitive adaptations for avoiding conception until she has found a mate that is worthy and willing to invest in their baby--not an easy task.
Why is this the female's challenge, rather than the male's? In humans, as in many species of nesting birds, males are critically important in provisioning for their offspring (this is not true for other primates). This has been established through painstaking research on hunter-gatherers, by Hillard Kaplan and others. Since it is their common baby, and since the father can contribute very significantly to the child's health, we do in fact find massive paternal investment in the human animal. Yet theoretical considerations suggest that the father can be predicted to be less motivated than the mother under a range of circumstances. To claim that some of this difference may have roots in evolved cognitive structures is not to provide deadbeat dads with an excuse. If the evolutionary logic has any validity, it is a dimension worth paying attention to; possibly, it can help us act more rather than less wisely.
Female beauty: Why do human females often look and typically aim to look beautiful, while their avian and animal sisters are so uniformly dour? In most species, it is the male that is visually striking.
First, human females need to be choosey, since a human baby requires an enormous amount of parental investment. This places a premium on males that not only has good genes, but one that will be just hers, that will be devoted to her, stick with her, and not go off with someone else one sunny day, leaving her with the children. For certain major purposes, she needs a husband, not just a lover.
Secondly, consider the adaptive effect of female mate demands. If all a male needs to do is inseminate, there will be no shortage of males, since a single male can inseminate a very large number of females. Males will be motivated to do this, since the metabolic investment is negligible and the genetic gain maximal. In such a situation, there is no adaptive pressure on females to be visually attractive. In many avian species, females are typically brown and dour when they don't need to compete for males. When the female requires fidelity and investment, she changes the market for the males, and makes them choosey too. This poses a novel problem: how to attract a male?
Contraceptives may appear to dispose of the need to be choosey in sexual relations. If humans have psychological adaptations for mate selection, however, these are unlikely to respond to the evolutionarily unanticipated presence of contraceptives. Research has found that young females tend to tire of one-night stands, to find them emotionally unsatisfying in the long run, and to learn to avoid them. While women's actions in this area are in no sense directly determined by their evolutionary history or genetic makeup, the evidence suggests that women have subjective preferences that make sense in an adaptationist framework. These preferences are not iron-clad laws; they are like a wind or a snowstorm -- you can walk against it, it's just harder. Besides, it's unpleasant -- the snow gets in your eyes.
Pre-menstrual syndrome: females who do not conceive may feel negative towards the current partner or an urge to find a partner if she does not have one; symptoms will typically be relieved by pregnancy.
Mate selection: The criteria for what is a good choice will vary between the sexes; for a treatment, see the work of David Buss. Females are found to value kindness, intelligence, status, and a sense of humor in potential mates--and this is just the beginning. A female also needs to be sensitive to cues for a willingness to invest--an attentive suitor brings gifts, perhaps, and doesn't double-time. Finally, a willingness to invest isn't worth much unless it is accompanied by the ability to do so--health, wealth, and connections should matter--that is, in the logic of evolution.
Buss, David M. (1994). The Evolution of Desire: Strategies of Human Mating. New York : Basic Books.
Dugatkin (2000) ran a survey suggesting women use other women's choices as an important cue for mate value; see news report (external). See also a news report on Robin Dunbar and Susan Kelly's (2000) study suggesting a female mate preference for bravery over altruism.
Cross-species comparisons: All animals that reproduce through mating have to find each other somehow. A useful tool is the ability to call up a mental "search image" that can help track down and recognize a potential mate. Kirkpatrick (1987) points out this is one of the phylogenetically oldest tasks of cerebral organization.
Kirkpatrick, M. (1987). Sexual selection by female choice in polygynous animals.
Annual Review of Ecology and Systematics 18:43-70.
Female-to-female competition. A recent literature examines the ways
in which young women are nasty to each other, suggesting that females
commonly use a variety of strategies to attempt to lower the
attractiveness of rivals in the eyes of potential mates.
Such a description of female-to-female rivalry fails to explain
community events that involve large numbers of young females expressing
their preference for and appreciation of the same young males. Sorority
events, cheerleading, and rock concerts frequently exhibit large
numbers of females cheering for the same males without any suggestion
of rivalry. A possible biological function of such community events is
to coordinate one's mate-value assessment. A male that is viewed
by one female as highly attractive, but by everyone else as
unattractive, may under a range of circumstances be a poor choice. Only
by coordinating mate choice criteria would a female know if she is
making an optimal choice in her particular cultural environment.
Even if a female should decide to utilize mate choice criteria that
differ from the norm (which clearly has its own potential rewards), an
intimate familiarity with the norm would be an asset. We would
therefore expect such coordinating mate choice assessment events to be
popular primarily with girls who are entering the market and are
orienting themselves in it, rather than making their final choice.
Older women may be more inclined to look for bargains -- that is to
say, to search for mates using personalized criteria that deviate from
the local mate quality assessment norm.
Pregnancy sickness: A common condition during the early months of pregnancy for a majority of women around the world (Tierson, Olson, & Hook, 1986); symptoms include nausea, vomiting, and food aversions. Profet (1992) has proposed that pregnancy sickness ("morning sickness") can be understood as an adaptation to protect the developing fetus from teratogens. She points out that women acquire aversions to food that are highest in toxins and tend not to develop aversions to foods that are more apt to be toxin-free. Pregnancy sickness, including food aversions, coincides with the period when the fetus is most at risk for the effects of teratogens. In support of the theory, Weigel & Weigel (1989) have shown that women who experience pregnancy sickness have lower levels of spontaneous abortions than women who do not.
Profet, M. (1992). Pregnancy sickness as adaptation: A deterrent to maternal ingestion of teratogens. In Barkow et al. (1992): 327-365.
Tierson, F. D., Olsen, C. L., & Hook, E. B. (1986). Nausea and vomiting of pregnancy and association with pregnancy outcome. American Journal of Obstetrics ad Gynecology, 155, 1017-1022.
Weigel, R. M., & Weigel, M. M. (1989). Nausea and vomiting of early pregnancy and pregnancy outcome: A meta-analytic review. British Journal of Obstetrics and Gynecology, 96, 1304-1318.
Postpartum depression: Some mothers become depressed after giving birth (O'Hara, 1995). Hagen (1999, 2000) argues that this depression may be adaptive, facilitating paternal and kin investment in the baby.
Hagen, Edward H. (1999). The functions of postpartum depression. Evolution and Human Behavior 20: 325-359. Full text of expanded version.
Hagen, Edward H. (2000). Depression as bargaining: the case postpartum (external PDF file).
O'Hara, M. W. (1995). Postpartum depression: causes and consequences. New York: Springer-Verlag.
Monogamy and infidelity:Falling in love and staying true are adaptations that arose to handle the problems of raising a smart and large-brained child. Sometimes it still pays to cheat. See Robin Dunbar, Your cheatin' heart (New Scientist November 1998, external) and James Meek, Sex is best when you lose your head (London Review of Books 16 Nov 2000, external).
Lactation: A study suggests that women's libido is stimulated by seeing other women breastfeed; see "Sexual desire boosted by breastfeeding odours" (New Scientist) and news report (BBC 24 April 2002). Scandinavian countries, which permit nursing in public, may thus be encouraging a higher birthrate than countries which forbid this practice.
Menopause: Hawkes et al (1998) present a hypothesis that the long postmenopausal lifespans that distinguish humans from all other primates may have evolved with mother-child food sharing, a practice that allowed aging females to enhance the fertility of their daughters, the practice thereby increasing selection against senescence. The authors suggest their hypothesis also accounts for human late maturity, small size at weaning, and high fertility, and that the hypothesis has implications for past human habitat choice and social organization, and for ideas about the importance of extended learning and paternal provisioning in human evolution. Contact E.L. Charnov, Univ. of Utah, Dept. of Biology 801-581-5636. Proceedings of the National Academy of Sciences, US, 3 Feb 98.
The formal definition of a male is the sex with the smaller gametes; human males produce millions of them on the cheap, and pay to give them away. However, the small initial investment in copulation, encouraging philandering, must be weighed against the benefit of long-term investment in offspring. It is a fact that human infants benefit from extensive investment, and males have cognitive adaptations for providing such investment, in the right circumstances. Since a male can provide a significant and continued investment in no more than a limited number of children, he can be predicted to reluctant to commit, but also a devoted father.
What follows is very sketchy, just to create pegs for the ideas.
Paternal investment: In a situation that broadly parallels that of the female, the problem of mate selection in the male is focused on the prospect of a commitment of future resources. If a man is going to devote all his reproductive potential on one woman, he must choose well. Much of the popular evolutionary debate argues that men seek to avoid investing in their offspring. This is a peculiar argument in the case of humans, who rely massively on paternal investment. For a parallel in the world of mice, see Gubernick and Teferi (2000).
Canadian researchers have found that hormone levels fluctuate widely in expectant fathers; see article and news story on Fatherly Love, New Scientist, 8 January 2000 (external).
It has been suggested that after giving birth, a woman and her relatives show a bias towards claiming the child resembles the domestic father. The hypothesis is confirmed in McLain, D. Kelly et al. (2000). Ascription of resemblance of newborns by parents and nonrelatives. Evolution and Human Behavior 21.1: 11-24. The point would be to reassure him he is the biological father.
Nipples. Erasmus Darwin speculated that male nipples were a relic of a time when mammals were hermaphrodite -- male and female in one individual. In a sense, he was on the right track: male and female mammalian embryos start out along a common developmental pathway before sex-specific hormones channel them into male or female.
Lawrence, Eleanor (1999). Why do men have nipples? Nature Science Update (5 Aug 1999). Full text (external).
Penis and testes. The comparative size of the male penis and testes is generally seen as an index of sperm competition. Sperm competition in turn is expected to correlate with the frequency of multiple matings among females. If a female has several partners, a male increases his chances of fertizing her if his ejecta contains a large number of sperm, which requires larger testes to produce. The selective benefits inherent in the ability to remove sperm from a previous partner may have played a role in the evolution of the size and shape of the penis.
For a discussion of sperm competition with further references, see James Meek, Sex is best when you lose your head (London Review of Books 22. 22 (16 Nov 2000), external).
The correlations between comparative penis and testes size and multiple matings appear to bear out well among the apes. Gorillas, where dominant silverback males maintain exclusive mating access to a group of females (the harem model), have a comparatively very small penis and testes. Bonobos and chimpanzees, who live in promiscuous groups, have a comparatively very large penis and testes. Since human penis and testes size fall between those of the gorilla and the two species of Pan, but lie closer to that of the chimpanzees, the theory inversely predicts that human females should be relatively promiscuous. There is unfortunately no data on this prediction (joke).
A number of other factors may have played a role in the evolution of human penis and testes size, such as the complex psychology of pair bonding, the role of sex in maintaining stable bonds, the reproductive benefits to both sexes of occasional infidelity, and the possibility that our recent ancestors the australopithecines were promiscuous.
Cremasteric reflex. This reflex is elicited by stroking or gently pinching the skin of the upper inner thigh while observing the scrotum. A normal response is contraction of the cremasteric muscles on the ipsilateral side with unilateral elevation of the testis. When the muscle relaxes, the testicle returns to the scrotum (cf. illustration, external). One study found that the cremasteric reflex was intact in 100 percent of boys 30 months to 12 years of age, though not always fully functional.
The cremasteric reflex appears designed to protect the testes from injury. However, it is unclear if the reflex is triggered by danger. Comparative data from apes and monkeys would be of interest.
Caesar RE, Kaplan GW. (1994). The incidence of the cremasteric reflex in normal boys. J Urol 152 (2 Pt 2): 779-80.
Territoriality: males may use territoriality and other asset or status advertizing to attract females
Mate selection: There is some data suggesting that males value symmetry, a .7 waist to hip ratio, youth, and other cues to reproductive potential. It is unclear how these criteria vary cross-culturally.
Monogamy and infidelity: A significant paternal investment would often have been required to raise children successfully, and human males have evolved strong emotional adaptations for bonding. Since the initial investment in a mating is low, however, males have a general incentive to obtain additional mating partners. Males with ample resources may attract several females, either at once (polygamy) or (in today's society) in a sequence (so-called 'serial monogamy').While mate desertion would have tended to lower the reproductive fitness of the children, the father's investment would have tended to commit a smaller portion of his entire reproductive potential than the mother's. This may have resulted in a psychology where males may be more prone to desert their spouse and children than women. See Robin Dunbar, Your cheatin' heart (New Scientist November 1998) (external full text).
There is also an interesting discussion somewhere - perhaps not yet
written - of jealousy, sexual virtue, the value of chastity, and similar
topics. Just don't put yourself in the situation of this dog (joke).
Kin-selection Effects index
Parents and their offsping have overlapping but not coinciding interests. The underlying logic of this fact was perceived by Hamilton (1964) as an aspect of kin selection theory, and extended by Trivers (1972) by means of a cost-benefit approach.
For a recent discussion of altruism in a selectionist perspective, see Nicholas Humphrey (1997). Varieties of Altruism - and the Common Ground Between Them. Social Research 64: 199-209. Full text (external).
The most striking kin-selection effect is reflected in parents' eagerness to take care of their offspring. As discussed above, the particular circumstances of human evolutionary history have made paternal investment an important characteristic.
This area in human relations has traditionally been the tender seat of loving-kindness, as shown for instance in the Virgin and Child iconography. Even according to the rather ruthless algorithms of evolutionary logic, mother and child have strong common interests, and the warm feelings mothers usually have towards their babies shall not be denied. However, there are situations when the mother is stressed and feels unable to take on a child, or when she needs to reduce her investment. It is also well known that if a female does not have enough body fat--the requirement is a percentage of body weight--she will not be fertile.
These are not proposed to be conscious decisions, and in fact may not even cross over into conscious awareness at all, but organic processes designed to propagate the mother's genes that worked in the evolutionary past. Since this past, for many adaptions, stretches back tens of millions of years, they form part of a heritage we have in common with many other kinds of creatures.
Among humans, many twins die in the womb. The mother's body receives signals it would risk losing both, and absorbs one of the fetuses into the uterine lining. Moreover, David Haig's work suggests that even a single fetus will attempt to draw more resources from the mother than the mother optimally would like to provide. Enzymes in the placenta (which has the genotype of the fetus), for instance, may have evolved to manipulate the mother's level of blood insulin in order to increase access to glucose. This is proposed to explain pregnancy diabetes.
Among many mammals, breast-feeding suppresses ovulation. As long as the mother is nursing, the chances of her getting pregnant stay low. This patterns is used consciously as a means of spacing births by many primitive cultures (primitive in the anthropological sense of not having social units larger than those based on kinship). It decreases the chances of committing the mother to a child she will not be able to care for adequately.
This grim aspect of parent-offspring conflict is unfortunately more common than one would expect. Many species commit infanticide (Sulloway, 1996). Nutritional neglect, or the failure to feed an offspring adequately, is also a form of infanticide.
Since the mother shares her genes equally with each child, while the child is, as it were, more closely related to itself than to its future siblings, the child is designed to be reluctant to give up nursing. When the mother feels she has given enough and begins to wean the child, she is preserving some of her strength and resources to a future pregnancy, thus diverting the stream of goods from the current baby. The baby has a general interest in the mother's well-being, as well as in its potential future siblings, but nevertheless does not quite agree with the mother that the cutoff point should come so soon. There is a gray area of conflicting interests in which the mother and the child struggle.
Since an oldest child has already received a good chunk of parental investment by the time the second and third come along, he or she acquires a higher value in the parents' eyes--there is more to lose if something goes wrong. To avoid the danger of diluting their resources so that none of their children will do well, it is a common cultural practice in many societies to bestow the bulk of one's accumulated possessions or properties on the first-born.
It is not clear that this can literally be considered an adaptation, since the accumulation of resources would have been fairly insignificant in our ancestral environment, the stone-age Pleistocene. However, parents may have an evolved disposition preferentially to help their first-born throughout life, as an early and hopeful investment in their grandchildren. We must assume that life was difficult at times and resources scarce, so that alleles coding for such first-born favoritism would tend to increase in frequency in the population.
The biological definition of a male in sexual species is the organism with the smaller gametes. Sperm is generally orders of magnitude smaller than eggs, and in placentals and mammals the mother devotes very significant additional resources to the offspring. One consequence of this is that males can father several orders of magnitude more offspring than females--all they need is mating opportunities.
This logic might make it seem that all parents should be designed to give birth to more males, but this would not be a stable evolutionary strategy (SES), for fairly obvious reasons. The more males there were, the more valuable would females be. In addition, since males can have offspring with many females, males that have desirable traits are frequently preferred by several females, thus effectively excluding many males from mating altogether.
One result of these forces in human societies
is that families with ample resources--and by resources is not intended
primarily money or goods, but social capital in the form of connections
and status--tend to prefer males, who are able to afford both a wife and
a mistress or two. Low status families, according to this equation, would
on the contrary be expected to prefer females, since a female will rarely
fail to attract a mate, and may succeed in attracting one with higher status
than herself. Low status males cannot count on such luck.
|Index of Adaptations|