Why Beautiful People Have More Daughters: From Dating, Shopping, and Praying to Going to War and Becoming a Billionaire–Two Evolutionary Psychologists Explain Why We Do What We Do (12 page)

Why Is Polygyny Related to Sex Differences in Body Size?

However, this begs the question: Why does the degree of sexual dimorphism in size correlate with the degree of polygyny? There are two possible explanations of this correlation. The first, more established theory posits that males have become larger throughout evolutionary history; the second, newer theory argues that females have become smaller.

Did Men Become Bigger…

The proponents of the first theory
¹⁷
point out that relative to monogamy, polygyny creates greater fitness variance (the distance between the “winners” and the “losers” in the reproductive game) among males than among females by allowing a few males to monopolize all females in the group. (See chapter 2, “Why Are Men and Women So Different?”) The greater fitness variance among males creates greater pressure for men to compete with each other for mates. Under such severe physical competition, only big and tall males can emerge victorious and get mating opportunities, while small and short males are left out of the reproductive opportunities altogether. At the same time, among pair-bonding species like humans, where males and females stay together to raise their children, females prefer to mate with big and tall males who can provide better physical protection for themselves and for their children against predators and other males. Thus, through both competition among men and preference by women, only big and tall males can reproduce and pass on their “big and tall” genes to their sons, while most or all females of all sizes reproduce and pass on their full range of sizes to their daughters. (Remember, the “fitness floor”—the worst one can do—is relatively high for women.) Over many generations, males will get bigger and taller, while females will retain the same distributions of height and weight in each generation.

Recent critics
¹⁸
point out that this theory assumes that body size (height and weight) is transmitted exclusively or largely along the sex lines, from fathers to sons, and from mothers to daughters. The theory assumes that tall men married to short women have tall sons but short daughters. The critics use Finnish data on twins
¹⁹
to demonstrate that this assumption is false. The data show that sons are just as likely to inherit their height from mothers as from fathers, and daughters are just as likely to inherit their height from fathers as from mothers. So a tall father will have both tall sons and tall daughters, and a short mother will have both short sons and short daughters. What gives?

…or Did Women Become Smaller?

The critics then point out that under polygyny, there is an evolutionary pressure for girls to mature earlier (see “Why Do Girls of Divorced Parents Experience Puberty Earlier Than Girls Whose Parents Remain Married?” in chapter 5). Under monogamy, most adult males are already married and cannot marry again, so there are no incentives for prepubescent girls to mature earlier; prepubescent boys in their age group are in no position to marry them. In contrast, under polygyny, married adult males can acquire additional wives. So girls who mature early can become a junior wife of a wealthy village chief while their prepubescent age mates cannot. Because girls who mature early attain smaller adult height than girls who mature late throughout the world (because girls essentially stop growing when they reach puberty),
²⁰
this suggests that height differences between the sexes should be greater in polygynous societies as a result of girls undergoing earlier puberty and becoming shorter. Cross-cultural data show that this is indeed the case. Girls in polygynous societies are shorter than girls in monogamous societies, whereas boys from both types of societies are about the same height.
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Whichever theory is correct, it appears to be the case that polygyny and sex differences in height are closely related. This is how we know that humans are naturally polygynous: because men are taller than women.

What Women Want

If humans are naturally polygynous, why, then, do many human societies in the world today practice monogamy (even though a large majority still practices polygyny)? One theory suggests that it is because that is what women want. In any species for which the female makes a greater investment in children than does the male (including humans), sex and mating is a female choice. Sexual intercourse occurs if and when the female wants it; the male has very little choice (outside of forcible rape).
²²
(See “What Do Bill Gates and Paul McCartney Have in Common with Criminals?” in chapter 6.) Humans are no exception. Monogamy emerges as the institution of marriage in the society when many or most women choose to marry monogamously, and polygyny similarly emerges as the institution of marriage when many or most women choose to marry polygynously.
²³

What, then, would lead women to choose to marry monogamously or polygynously? One important determinant of the institution of marriage is the degree of resource in equality among men (the difference between the richest men and the poorest men). In societies with a high degree of resource in equality, where rich men are very much richer than poor men, women (and their children) are better off sharing the few wealthy men, because one-half, one-quarter, or even one-tenth of a wealthy man is still better than a whole of a poor man when resource in equality is extreme. Or, as George Bernard Shaw puts it, “the maternal instinct leads a woman to prefer a tenth share in a first rate man to the exclusive possession of a third rate one.”
²⁴

In contrast, in societies with a low degree of resource in equality, where rich men are not much richer than poor men, women (and their children) are better off monopolizing a poor man than sharing a rich man, because one-half of a rich man will not be as good as a whole of a poor man.
²⁵
Thus, polygyny emerges as the institution of marriage in societies characterized by greater resource in equality among men, while monogamy emerges in societies characterized by lesser resource in equality. This theory is an extension to human society of what is known as the polygyny threshold model in biology, originally formulated to explain the mating systems of birds,
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thus once again illustrating the fundamental principle of evolutionary psychology that humans are no different from other species. (See “The Evolutionary Psychological Perspective” in chapter 1.)

The reason most Western industrial societies are monogamous, despite the fact that humans are naturally polygynous, is that men in such societies tend to be more or less equal in their resources, compared to their ancestors in medieval times. The degree of in equality tends to increase as societies become more complex, from hunter-gatherer and pastoral societies, to horticultural and agrarian societies, and typically reaches its maximum in advanced agrarian societies.
²⁷
Industrialization tends to decrease the level of in equality in society.

Individual decisions of women to marry monogamously rather than polygynously combine to produce social institution and norms.
²⁸
If many or most women choose to marry monogamously, then the society becomes monogamous. However, the true polygynous nature of humans is never too far beneath the surface, even in nominally monogamous societies such as ours.

All Human Societies Are Polygynous, Simultaneously or Serially

Wealthy and powerful men throughout history, even while monogamously married, have always mated polygynously by having mistresses, concubines, and other extramarital affairs.
²⁹
(See “What Do Bill Gates and Paul McCartney Have in Common with Criminals?” in chapter 6.) And it is true even today. Whether married or not, wealthier men in the United States and Canada have more sex partners and have sex more frequently than less wealthy men.
³⁰
This is not because wealthy men can afford the ser vices of prostitutes; wealthy men are no more likely to have sex with a prostitute than are poorer men. They do not have to. Wealthy men have more sex partners and have sex more frequently because women seek them out.

Most nominally monogamous societies also allow people to get a divorce, and in many societies, such as the United States, divorce is both very easy and very common. Liberal divorce laws allow men in these societies to practice
serial polygyny
(a man having multiple wives, not simultaneously but sequentially, through a series of divorce and remarriage). In the United States, the strongest predictor of remarriage after divorce is sex (male vs. female): men typically remarry, women typically do not. As we discuss in chapter 3, this is because men become more desirable with age to potential mates (thanks to the greater income and higher status that typically accompany age), while women become less desirable with age due to declining reproductive value and fertility. While some women do remarry after divorce and thus practice serial polyandry, a far greater number of men practice serial polygyny through divorce and remarriage. Contemporary Westerners who live in nominally monogamous societies that nonetheless permit divorce are therefore in effect polygynous; they practice serial polygyny.

Most Women Benefit from Polygyny, Most Men Benefit from Monogamy

When there is resource in equality among men (which there always is in every human society), most women benefit from polygyny. This is because under polygyny, women can share a wealthy man, whereas under monogamy, they are stuck with marrying a poorer man. If the resource in equality is large enough, then a fraction of a wealthy man is bigger and thus better than a whole of a poor man.
³¹

The only exceptions are extremely desirable women. These women can marry the most desirable, wealthiest men under any circumstance (polygyny or monogamy). Under monogamy, they can monopolize the wealthiest men, whereas under polygyny, they must share them with other, less desirable women. So the most desirable women benefit from monogamy, but all other women benefit from polygyny.

The situation is exactly opposite for men. Most men benefit from monogamy, because it guarantees that every man can find a wife. True, less desirable men can only marry less desirable women, but marrying a less desirable woman is much better than not marrying anyone at all.

Once again, extremely desirable men are the exceptions. Such men can have multiple wives under polygyny, whereas they are limited to only one wife (albeit an extremely desirable one) under monogamy. So extremely desirable men benefit from polygyny, but all other men benefit from monogamy.

When men in monogamous societies imagine what their life might be like under polygyny, they imagine themselves with multiple wives. So they may think they would be better off under polygyny. What they don't realize is that for most men, who are not extremely desirable, polygyny means no wife at all, or, if they are lucky, one wife who is much less desirable than one they could get under monogamy (because under polygyny, more desirable wives are taken by men who are more desirable than them). If they do the math, they will come to the right conclusion that most of them are better off under monogamy than under polygyny.

Q. Why Does Having Sons Reduce the Likelihood of Divorce?

Sociologists and demographers have discovered that the presence of sons decreases the probability of divorce.
³¹
Couples who have at least one son face a significantly lower risk of divorce than couples who have only daughters. Why is this?

Remember from chapter 3 that a man's mate value is largely determined by his wealth, status, and power, whereas a woman's mate value is largely determined by her youth and physical attractiveness. This means that the father has to make sure that his son will inherit his wealth, status, and power, regardless of how much or how little of these resources he has. A working-class father still has to make sure that his son will inherit what little wealth he has, because the more the son inherits, the greater his expected reproductive success. In sharp contrast, there is relatively little that a father (or mother) can do to affect the daughter's expected reproductive success; once she is born, there is very little parents can do to keep her youthful or make her more physically attractive.

The evolutionary psychological logic therefore predicts that the continued presence of (and investment by) the father is important for the son, but not as much for the daughter. Strictly in reproductive terms, there is very little fathers (or anyone else) can do for daughters beyond keeping them alive and healthy. The presence of sons therefore deters divorce and departure of the father from the family more than the presence of daughters, and this effect should be stronger among wealthy families.