7.4.3 Fertility and child-rearing
Since fertility and child-rearing are at the heart of reproduction, evolutionary theory explains parental behaviours as adaptations. Parents try to maximise their reproductive success by investing time, energy, care and resources that increase the chances of survival of their offspring. Since decisions about child rearing are essentially about how to allocate parental resources between children and parents, the payoffs depend on the relatedness of the child and the effect of the investment on the future reproductive value of the offspring and the effect of investment on the parent’s own reproductive value (Winterhalder and Smith 2000).
Increased fertility can reduce investment per child and reduce the overall reproductive success of the parents. Humans, like other mammals, typically follow a genetic strategy of producing relatively few offspring and caring for them until they can fend for themselves. People have evolved, not to produce as many children as possible, but to trade off quantity for quality, for example by achieving food security that will increase the chance that at least a few children will survive and prosper (Hrdy 2000).
Parental investment generates a conflict of interest between the parent and the child since their genetic interests only partly overlap. From a genetic perspective, parents are interested in all their children equally, because they each carry half a parent’s genes, but a child is concerned for its own welfare and discounts the welfare of its siblings, who share only half its genes (Dawkins 1989, Trivers 1985). Children get more from their parents than the parents are willing to give, but less than they would like to receive. ). Parent-child conflicts arise typically at weaning and adolescence when interests are most at odds.
Parental decisions about the allocation of resources among children are also driven by the imperative of reproductive success. To raise even a few children to adulthood, parents may face a choice between children. They thus typically wean an older child and prepare for a younger one when the benefits to the younger exceed the costs to the older. However, the older a child gets, the more likely it is to survive and reproduce, so when circumstances demand it, parents may sacrifice a younger child in favour of an older one (Daly and Wilson 1988).
The acquisition of wealth and status by parents can affect the reproductive success of their children. Wealth and status that is transmitted to children can permit them to find “better” mates (through assortative mating) than poorer, low status individuals. For example, rules of inheritance typically favour sons over daughters, particularly among the wealthy, and confer a genetic advantage, providing the son with the means not only to attract a high quality mate, but also to become a successful adulterer.
Children themselves have strategies to ensure the successful reproduction of their genes. Children have a psychological armoury to induce their parents to care for them, matched by the emotional responsiveness of parents. Babies and children manipulate their parents by being cute and responsive or by crying and temper tantrums. Parents who respond to these signals are likely to rear their children to adulthood than those who do not.
Children develop other strategies to improve their chances of survival within a family with other siblings who compete for resources and parental care and attention. They are sensitive to favouritism and try to ensure that they get their “fair share” of resources (Landsberg 1997). There is some evidence that personality is not wholly endogenous, but is affected by birth order since personality may be an adaptive strategy for competing with siblings for parental investment (Sulloway 1995).
The behaviour of men in relation to fertility and childrearing is also explained in terms of implicit strategies to increase their reproductive success. Because women have concealed ovulation (unlike other primates) and are not only sexually receptive during ovulation, men cannot be sure of the paternity of any children. According to evolutionary theory, uncertainty about paternity is a fundamental driver of male behaviour ranging from mate selection to sexual jealously and childcare. For example, since men cannot be sure of their paternity, they are predisposed to seek faithful women as wives, but also tend to seek liaisons with other women to increase their reproductive capacity (Wright 1994a).
The more confident men are of paternity, the more time, emotional involvement and resources they tend to invest in bringing up children (Gaulin and Schlegel 1980). Men have tried to increase the certainty of paternity by a variety of means, ranging from chastity belts to DNA testing. In societies where confidence in paternity is low, men invest not in their wives’ children, but in their sister’s (Alexander 1995). Certainty of paternity also affects the investment of grandparents—maternal grandparents invest more than parental grandparents, with the mother’s mother investing most and the father’s father investing least (Buss 1999).
The interests of kin in rearing children are also reflected in the preferences of mothers for entrusting childcare to relatives (notably grandmothers) rather than strangers, even professionals (Cabinet Office 1999). Human females have lived for decades past the time where they are fertile—in contrast, men remain fertile into old age. It is possible that humans have evolved to live long enough to get their last children to independence, and to become increasingly altruistic with age. The grandmother hypothesis suggests that postmenopausal women can increase their own genetic fitness by providing or caring for grandchildren, especially the offspring of daughters, since they have less to lose by assisting kin as their own reproductive capacity declines. This altruism could increase the survival of close kin, and increase the spread of genes for longevity (Hrdy 2000).
7.4.4 Infidelity
While humans, as discussed above, typically practise monogamy, both males and females are often unfaithful to their spouses. Evolutionary theory suggests that a predisposition towards infidelity is adaptive, enhancing reproductive success. Infidelity by both males and females within a monogamous marriage can confer advantages—males beget more young and females better young (Symons 1980).
There are obvious benefits for males. They father more children but do not incur the expense of rearing the offspring. In evolutionary terms, infidelity is rewarded, and a taste for sexual variety can thus be seen as an adaptation. Yet, although men may have taste for multiple sexual partners, adultery is not inevitable. Self-control, the availability of potential partners, the man’s attractiveness and the expected costs of infidelity can act as a brake on desire.
Evolutionary theory also suggests that infidelity benefits females. Evidence from studies of birds suggests that by marrying a mediocre male (who invests in her children) but mating with a better male, females can produce better offspring—when women have an affair they tend to choose a man of higher status than their husbands (Ridley 1993). Women can also obtain gifts in return for sex. Hunting of large animals is generally a specialty of men, and is typically communally shared. While the meat provides nutritional benefits, it also credibly enhances the status of the hunter and provides access to women (Hawkes and Bird 2002).
Buss (2000a) has found that male sexual jealously is universal. Evolutionary theory suggests that it may be an adaptive defence against cuckoldry. A cuckolded male cannot be certain of his paternity of his wife’s children and may invest in rearing children that are not his own, so female infidelity is a threat. Male infidelity is less of a threat to the genetic interest of a woman, since someone else will rear any child, although it might divert the male’s resources. Men are therefore jealous of the infidelity of their wives, whereas wives are jealous of their husbands giving time, resources and attention to another woman (Buss 1994). As Pinker (1998: 488) notes “men may be upset about affection because it could lead to sex; women might be upset about sex because it could lead to affection”.
In contemporary society, a predisposition towards male sexual jealously may help to explain domestic violence where women are hurt by men. Evolutionary theory explains the tendency of men to seek to control the reproductive capacity of their daughters, wives and girlfriends in a variety of ways as adaptive. Jealous men may confine women in their homes, segregate them from contact with men (purdah), cover them up (veils, chador), incapacitate them (foot-binding) or follow or guard them to prevent infidelity. Language can also help absent men monitor the behaviour of their wives through gossip (Ridley 1993).
The differences in the genetic consequences of infidelity are mirrored in the “double standard” by which adultery by a married woman is punished more severely than adultery by a married man. This double standard has historically been reflected in the law—adultery was a tort and rape an offence against the husband (Daly and Wilson 1988).
As noted above, human ovulation is hidden, and is an underlying driver of many male behaviours. Yet the evolutionary advantage of cryptic ovulation itself is not clear (Ridley 1993). It may induce faithfulness by husbands who must remain close to a woman and have sex often to be sure of fathering her children, or it may allow a woman to be unfaithful, allowing her to mate with a superior man by stealth.
7.5 Family dissolution and reformation
Evolutionary theory has particularly powerful explanations of the behaviour of step-parents in re-formed families—the “Cinderella” hypothesis. This suggests that step-parents, who have no genetic investment in step-children differentiate between them and their own children. This explanation of the behaviour of step-parents can assist in understanding the biological drivers underlying child abuse in re-formed families (discussed in Section 7.6.1), and perhaps in devising policies that prevent or ameliorate it.
7.5.1 Divorce
The evolutionary explanation of divorce is closely linked to that of infidelity in terms of providing husbands with increased opportunities to pass on their genes. The longer he is married, the greater the temptation for a husband to desert his wife, since his reproductive success can be enhanced by finding another, younger woman. Older men, who can command status and resources, can start new families, typically with a younger wife.
In some societies rich men have many wives and families simultaneously (polygyny). In monogamous societies rich men start new families and abandon former ones. Male discontent is a strong predictor of divorce, and men are more likely than women to remarry after divorce (Wright 1994b). Men do not prefer to marry higher-status women. In the US, the divorce rate among couples where the wife earns more than the husband are 50% higher than among couples where the husband earns more (Cronin and Curry 2000: 3).
In contrast, seeking another husband cannot increase the wife’s inherently limited reproductive capacity. In any case, she becomes less attractive as a potential mate as she ages and her fertility declines. Her children (who would impose costs on a new husband) may also make her unattractive. But a woman married to a low-status man who cannot provide adequately for the family may be better off on her own or with another man, so a husband’s failure to provide for his family is a major cause of divorce (Buss 1994). However, an absent father may have negative consequences for offspring (see Belsky 1997, Chisholm 1999, Draper and Harpending 1988, Moffitt, Crespi, Belsky and Silva 1992, Waynforth 2002).
7.5.2 Step-families
The evolutionary explanation of parental solicitude is that it is designed to allocate investment towards the parent’s own young in order to promote reproductive success. However, step-parents are not, and know they are not, related to the children of their spouse, and so are predisposed to be less solicitous to their step-children than to their own children (Daly and Wilson 1998). In evolutionary terms it makes sense that the welfare of one’s own child is more important the welfare of a step-child. At the same time, investing in a predecessor’s offspring can be the price paid for future opportunities to mate with the genetic parent. Step-parents are primarily replacement mates and only secondarily replacement parents.
There is a large folklore about cruel step-parents that suggests that the tensions in step-families and concern about the effect on the well-being of children are universal. However, although most step-families function reasonably well, a number of studies show that step-parents invest less emotionally and materially than genetic parents. Daly and Wilson (1998) suggest that the presence of a step-parent is the single most powerful risk-factor for child abuse. Moreover, martial happiness is less in step-families and step-parents and step-children both view the relationship as less loving and dependable than relationships between genetic parents and children. Less is expected of step-children academically, and they leave home at a younger age than genetic children.
