>> I imagine many will simply say it is random . . . which is another way
>> of saying "I don't know". :-)
> but there are factors that very subtly influence the sex
> of an offspring. It is also commonly believed that exactly half the
> babies born are boys and the other half are girls. Close enough, but
> not quite; the normal sex ratio at birth is 0.5122-that is, 105 boys
> born for every 100 girls. But the sex ratio varies slightly in
> different circumstances and for different families. So what factors
> affect the sex of the child?
>
> The Genius of Robert L Trivers
>
> Any discussion of sex ratio at birth must begin with the work of
> Robert L. Trivers, who is one of the greatest evolutionary biologists
> of the last century. In 1973, Trivers teamed up with a mathematician,
> Dan E. Willard, to formulate one of the most celebrated principles in
> evolutionary biology, called the Trivers-Willard hypothesis.3 The
> hypothesis states that wealthy parents of high status have more sons,
> while poor parents of low status have more daughters. This is because
> children generally inherit the wealth and social status of their
> parents. Sons from wealthy families, who themselves become wealthy,
> have, throughout most of evolutionary history, been able to expect to
> have a large number of wives, mistresses, and concubines, and produce
> dozens or hundreds of children,4 whereas their equally wealthy sisters
> can have only so many children. So wealthy parents should "bet" on
> sons rather than daughters.
>
> Conversely, poor sons can expect to be completely excluded from the
> reproductive game, because no women would choose them as their mates.
> But their equally poor sisters can still expect to have some children
> if they are young and beautiful. (Recall from chapter 2 that the
> "fitness ceiling"—the best one can do—is much higher for men than for
> women, while the "fitness floor"-the worst one can do-is much higher
> for women than for men.) So natural selection designs parents to have
> a biased sex ratio at birth depending upon their economic
> circumstances-more boys if they are wealthy, more girls if they are
> poor.
>
> There is evidence for this hypothesis throughout human societies.
> American Presidents, Vice Presidents, and cabinet secretaries have
> more sons than daughters.5 Poor Mukogodo herders in East Africa have
> more daughters than sons, both at birth and in the zero to four age
> group.6 Church parish records from the seventeenth and eighteenth
> centuries in Germany show that wealthy landowners in Leezen, Schleswig-
> Holstein, had more sons than daughters, while farm laborers and
> tradesmen without property had more daughters than sons.7 Among the
> Cheyenne Indians on the American Plains, prestigious, high-status
> "peace chiefs" have more sons than daughters, while poor and marginal
> "war chiefs" have more daughters than sons in the zero to four age
> group.8 In the contemporary United States and Germany, the elite-
> judged by the listing in their respective country's Who's Who-have a
> greater proportion of sons among their offspring than does the
> population in general.9 In an international survey of a large number
> of respondents from forty-six different nations, more wealthy
> individuals are more likely to indicate a preference for sons if they
> could only have one child, whereas less wealthy individuals are more
> likely to indicate a preference for daughters.10 While there is some
> counterevidence,11 most evidence is in support of the Trivers-Willard
> hypothesis.12
>
> Extending Trivers's Genius
>
> Recently, there has been a theoretical extension of the original
> Trivers-Willard hypothesis, called the generalized Trivers-Willard
> hypothesis.13 The idea behind the new hypothesis is the same as that
> behind the old one, but it extends the idea to many other factors
> besides the family's wealth and status. The new hypothesis suggests
> that if parents have any trait they can pass on to their children that
> is better for sons than for daughters, then they will have more boys.
> Conversely, if parents have any trait they can pass on to their
> children that is better for daughters than for sons, then they will
> have more girls. Parental wealth and status are just two of the traits
> they can pass on to their children that are more beneficial for sons
> than for daughters, but there are many other factors.
>
> Brain types are another example of such heritable traits. Strong "male
> brains," which are good at systematizing (figuring things out), are
> more beneficial for sons than for daughters, while strong "female
> brains," which are good at empathizing (relating to people), are more
> beneficial for daughters than for sons.14 Since brain types are
> heritable, the generalized Trivers-Willard hypothesis would predict
> that parents with strong male brains, such as engineers,
> mathematicians, and scientists, are more likely to have sons, while
> those with strong female brains, such as nurses, social workers, and
> school teachers, are more likely to have daughters. This is indeed the
> case.15 While the sex ratio at birth among the general population is
> 0.5122-that is, 105 boys for every 100 girls-the study shows that the
> sex ratio among engineers and other systemizers is 0.5833-that is, 140
> boys for every 100 girls. The comparable sex ratio among nurses and
> other empathizers is 0.4255-that is, 140 girls for ' every 100 boys.
> 1161
>
> By the same token, tall and big parents have more sons and pro-! duce
> more male fetuses (because size was a distinct advantage in, male
> competition for mates in the ancestral environment, while body size
> has no particular advantage for women), and short and small parents
> have more daughters and produce more female fetuses.17 Because
> violence was probably a routine means in the male competi-' tion for
> mates in the ancestral environment18 (as it is among our primate
> cousins),19 tendency toward violence was adaptive for ancestral men
> but not for ancestral women. Accordingly, violent men have more sons,
> both in the United States and the United Kingdom.20
>
> Why Beautiful People Have More Daughters...
>
> Physical attractiveness can also bias the sex of your children. Now,
> unlike being big and tall or having a tendency toward violence, which
> increases the reproductive success of only men and not women, being
> beautiful is good for both men and women. Beautiful women have greater
> mating success than less attractive women, and handsome men do better
> than less attractive men. But beautiful men and beautiful women tend
> to do "better" in slightly different ways.
>
> Physically attractive women tend to do well both in long-term and
> short-term mating; men prefer beautiful women for both. In contrast,
> handsome men tend to do well mostly in short-term mating. Women seek
> out handsome men for short-term mating (possibly to get good genes for
> their children by being impregnated by them but then passing the
> resulting offspring off as that of their unsuspecting husband) but not
> necessarily for long-term mating, for which other qualities like the
> man's resources and status become more important. In fact, as we
> suggest in chapter 4 (see "Why Might Handsome Men Make Bad
> Husbands?"), physically attractive men may not make desirable long-
> term mates for many reasons.
>
> So physical attractiveness, while a universally positive quality,
> contributes even more to women's reproductive success than to men's.
> The new hypothesis would therefore predict that physically attractive
> parents should have more daughters than sons. Once again, this is
> indeed the case. Young Americans who are rated "very attractive" have
> a 44 percent chance of having a son for their first child (and thus a
> 56 percent chance of having a daughter). In contrast, everyone else
> has a 52 percent chance of having a son (and thus a 48 percent chance
> of having a daughter) for their first child.21 Being "very attractive"
> increases the odds of having a daughter by 36 percent!
>
> ... and Women Are More Beautiful Than Men
>
> If you look around and rate the men and women around you on their
> physical attractiveness, you should notice that, whether you are a man
> or a woman, gay or straight, women on average are objectively more
> attractive than men. Why might this be the case?
>
> Think about it. "physical attractiveness is heritable, such that
> beautiful parents beget beautiful children (and less attractive
> parents beget similarly less attractive children), and z/beautiful
> people are more likely to have daughters than sons, then it logically
> follows that over time, women will become more attractive on average
> than men. Once again, studies confirm this implication of the new
> hypothesis.22 The average level of physical attractiveness among women
> is significantly higher than the average level of physical
> attractiveness among men. Women are more beautiful than men because
> beautiful parents have more daughters than sons.
>
> Far from being random chance, there are a large number of factors that
> appear to influence, even if only very slightly, whether a couple will
> have a son or a daughter. The generalized Trivers-Willard hypothesis
> can explain the evolutionary reasons why these factors affect the sex
> of the child. Evolution helps parents pass on their genes in the most
> efficient way possible.
>
> 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 - by Alan S. Miller,
> Satoshi
Kanazawahttp://www.amazon.com/Beautiful-People-Have-More-Daughters/dp/0399533656
