>Perhaps the trait came about from periods of boom and bust in the
>climate. Families with fewer children wound up fatter in boom times,
>and tended to survive the next drought better than large families of
>skinny kids.
The problem is that the same trade-off applies to both poor and rich
families. More children, given the same health, gives a greater probablity
of surviving offspring. Better health, given the same number of children,
also gives a greater probability of surving offspring. Choosing between
quantity and quality of offspring is thus a classic economic choice between
two goods with a limited budget.
The normal outcome if you have to choose amounts of two good things when your
budget (here wealth) increases is to get more of *both*. Hence you would
expect wealthier families to choose to have more kids *and* invest more in
the kids they have. The number of kids will go up more slowly than the
wealth, as each kids is getting more resources, but it will still increase.
This is indeed how animals respond to increased resources, insofar as they
have control over the quantity and quality of offspring, and it was also the
situation in early modern (pre-industrial) Europe. It's conventional wisdom
that this situation applies to most other pre-industrial societies.
The puzzle is why this outcome does not apply to industrial societies.
Incidentally, I think there is some complexity to the relation between
industrial-era wealth and population growth. England was by far the most
advanced country from the 1600's almost up to WWI by a plethora of economic
measures (per capita income, per capita industrial production, date of first
freedom from famine, etc.) But the first country to undergo the demographic
transition (smaller families for the wealthy and voluntarily falling
birthrates) was France.
I believe the cause of falling birthrates is genetic, but memetic. A
fundamental characteristic of modern society that is not characteristic of
any prior society is "too much interesting to do". I think memes are
evolving to take advantage of us by being so interesting and fun that we want
to spend our time enjoying (and spreading) them rather than having children.