It has long been observed that men and women often have very different attitudes about sex, with resulting differences in their behavior. Women tend to be much more selective about whom they mate with; men are typically less choosy. Men are frequently eager to engage in casual sex; women are generally reluctant to do so, and often insist on a long-term commitment before engaging in sex. To the extent that men are selective, they tend to be especially attracted to women with youthful good looks. Women place relatively more importance on a man's wealth, income, and influence, and on his apparent sincerity.1
There are some extreme cultural determinists who claim that the male-female differences in attitude and behavior are due entirely to upbringing and social pressures; and they assert that if males and females were reared in the same environment those differences would not exist. This, however, seems implausible. Not only are those differences found in all known human societies, but similar behavioral patterns are commonplace among other mammals.2 This suggests that the differences are genetic in origin, although they may be magnified by upbringing and social pressures.
There is a well-known theory ("parental investment theory") that explains in a straightforward fashion how these differences could arise through natural selection.3 Parental investment theory begins with the hard facts of human reproduction: The female — who must carry the fetus in her body for nine months — of necessity has a large investment in each child. (A male might invest a lot of time in a child, but it is not a biological necessity.) In addition, a male's sperm cells are very tiny and extremely numerous. A man produces many billions of sperm cells during his lifetime, and they can be rapidly replaced after being expended. In contrast, egg cells are relatively large, and a female releases only a few hundred of them during her lifetime.
Because eggs are in relatively short supply, they are expensive compared to sperm. As a result, males compete with each other, sometimes violently, for reproductive access to the scarce, valuable eggs; and the losers in this competition, fail to pass on their genes. The most effective reproductive strategies for males and females are there fore quite different.4 The "best" strategy for a male is to:
• Obtain exclusive access to one or more fertile females; and
• Copulate with other females whenever possible.
Notice that the word "best" in the preceding paragraph is not used in the moral sense of the word. The behavior outlined is best only in the Darwinian sense of reproductive success. Males who follow this strategy will pass down more copies of their genes to succeeding generations than males who follow alternative strategies.
For a female, a very different reproductive strategy is called for. A woman has a limited supply of eggs, and she cannot voluntary release more than one per month. In addition, once she is fertilized she is tied up reproductively for at least nine months (perhaps a few years, if she nurses the resulting child). Consequently, she is limited to a relatively small number of successful pregnancies, and to succeed reproductively she must make the most of those opportunities. Furthermore, she is vulnerable during the late stages of her pregnancy (as is the fetus), and for a considerable time after she gives birth her child is weak and helpless, and needs to be supported.
That being the case, the best reproductive strategy for a woman (i.e., the one which maximizes the number of her descendants) is to:
• Select a mate with good economic prospects, thus making it likely that he will be able to adequately support their children; and
• Obtain in advance a commitment from him to protect her during her pregnancy, and to protect and support their offspring.
Throughout the Paleolithic Era — and, indeed, all past ages — adopting a policy of promiscuous copulation was likely to harm a woman's prospects of propagating her genes. Such a policy might result in a higher number of pregnancies than if she confined her sexual activities to just one man, but only slightly higher. However, failing to obtain a commitment from a man to support her children — or choosing for a mate a man who is unable to support them — would probably result in fewer of her children surviving to maturity.
Note that there is no requirement that a female engage in careful, cold-blooded calculations before adopting the "best" strategy. Females of other species — who are not even aware of the connection between copulation and childbearing — follow similar strategies without engaging in any analysis; they merely follow their inherited inclinations. However, those females who inherited the "wrong" inclinations passed down fewer copies of their genes than those who inherited the "right" inclinations.
Similarly, in deciding whom to mate with, most males do not carefully calculate which female is likely to bear him the greatest number of children. But those of our male forebears who were attracted to females with full, firm breasts had (on average)
more children than those men who were attracted to women with sagging breasts, or with flat chests. This is because full, firm breasts are most commonly possessed by young, fertile women near the beginning of their childbearing years.5
Human behavior, of course, is also strongly influenced by socialization. Male promiscuity (including the "hit and run" strategy often pursued by young males) is bad for the group as a whole, and is therefore disapproved of by most moral s. This has had some effect on male behavior; nevertheless, males who broke the moral code often passed on more genes than those males who followed it. On the other hand, the tendencies that females inherit with regard to mating strategies are generally good for society as a whole, and females are therefore more likely to conform to the moral code than men are.
It should be remembered that these behavioral tendencies evolved in the Paleolithic world, not the modern world of the West where there are social welfare programs and easily available birth control. But the Paleolithic world was not uniform; it included a variety of different environments. All of those environments favored the reproductive strategies for males and females described above, but not necessarily to the same degree. We should therefore expect differences in sexual attitudes and behaviors between those that evolved in strikingly different Paleolithic environments.
FOOTNOTES - CHAPTER 7
1) (a) Symons, Donald (1979), The Evolution of Human Sexuality.
(b) Wilson, Glenn (1992), The Great Sex Divide, especially chapter 1.
(c) Pinker, Steven (1997), How the Mind Works, especially pp. 469-476.
(d) Pinker, Steven (1997), chapter 7, especially pp. 463-471.
(b) Wilson, Glenn (1992), chapter 1, especially pp. 20-22.
5) Pinker, Steven (1997), chapter 7, especially pp. 483-487.
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