I was just reading about that today but don't know how to think about
this material yet.
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 Kanazawa
http://www.amazon.com/Beautiful-People-Have-More-Daughters/dp/0399533656
Q. Boy or Girl? What Influences the Sex of Your Child?
It is commonly believed that whether parents conceive a boy or a girl
is entirely up to chance. Close enough, but not quite; it is largely
up to chance, 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 Billionair
