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In poor societies richer couples have more kids, but richer societies have fewer kids. This may be because female kid desires are biased low, relative to genetic interests, and in rich societies women have more relative power. Ted Bergstrom explains in the May American Economic Review:
The demographic transition … presents a challenge to … evolutionary theories of reproductive behavior. In Western Europe, starting in about 1870, real wages began to rise about 2 percent per year. Net reproductive rates fell from an average of three children per woman in 1860 to fewer than two in the modern era. … Evolutionary biologists find it puzzling that a species reproduces less rapidly when individuals have access to more material resources. … Why was there a positive correlation between wealth and fertility before the demographic transition, but not after? …
Because of a genetic conflict of interest between mates, evolution could have shaped preferences so that "human females would fail to bear the optimal number of children in the absence of pressure of mates and kin." … Thus men would desire more children and women fewer children than their own genetic interest dictates. Differences in birth rates across time and between cultures would occur as one side or the other gains increased leverage in this tug-of-war. In modern economics, women have increased influence in household decisions and, together with improved contraceptive technology, have gained greater control of their own fertility. …
Malaysian husbands want more children than their wives and, when measurable household bargaining power favors the wife, a couple tends to have fewer children. In a survey of Brazilian households, … as the ratio of the wife’s nonlabor income increases, couples tend to have fewer children.
I’ve long been puzzled by the demographic transition, and so am excited to hear of a plausible theory that roughly fits people I know. If it is true, and if we now have too many or just enough kids, relative to a social or moral optimum, then empowering women has helped. But if, as I suspect, we now have too few kids, then empowering women may be largely to blame.
Added: Bergstrom credits Barkow & Burley. Ethology and Sociobiology, 1980. If you want to play "find a better theory," at least try to explain all the related data, including fertility of the rich in poor societies, and the robustness of the demographic transition to cultures and contraception technology.
Biased Birth Rates
Maybe the meek will truly inherit the earth?Are you saying that the groups that reproduce the fastest have the lowest rates of assault? I think that might be the case for Mormons, Hassidics and Mennonites, but not lots of other groups.
Doug, that's interesting. One of the (secret) fears of the wealthy minority, is of the fertility of the poor, especially people of color. I chuckle in discussions, where after a couple of drinks, folks are honest enough to speak of being "overrun."
I live in a neighborhood where the fertility rate among immigrant groups of color is very high. Birth control and all that aside, you need to take into account cultural dynamics, and/or religion, which can stifle behavioral evolution. For many people of color, lots of kids is still seen as evidence of a man's virility (hawkishness, Stuart?). And no, poor women secretly don't want to have all these kids, that wear you and your reproductive system out. But what else do you have to show for your life, and how do you keep your man? Children are seen as a necessary blessing. This may seem simplistic to you all, but it is a reality for a vast majority of us.
It should not be puzzling that having more material resources, means less kids. We're not just economics, or genetics. For wealthier couples, less kids mean more discretionary income for themselves, and for the few kids they do have; consumerism, enjoyment, and quality of life, are the key words here. I have to explain this concept to my neighbors, who view this as selfish. As I have no kids of my own (partially due to economic factors), I am viewed as the epitome of selfishness. My behavioral evolution is viewed through lenses, varying from sympathy to hostility. I also have to explain to them that our history, economic and political policies have never favored large families, from the Irish Catholics to the present Mexicans, residing around me.
There is a huge cultural and ideological gap in many of the theories I read and discuss; it's a shame too, as the people who many academics render invisible, are some of the fastest reproducing folks on the planet. Maybe the meek will truly inherit the earth?