What Informs Variable Human Fecundity?

David Friedman ponders whether the demographic transition will reverse itself due to genetic variation in the desire to reproduce: http://daviddfriedman.blogspot.com/2006/11/darwin-reproduction-and-religion.html

I don’t reckon that evolution has endowed humankind with something as complex as a hard wired desire to reproduce. I don’t think there is a family size gene. It seems more plausible that simpler drives and tendencies combine to have the effect of facilitating reproduction. For example, the sex drive combined with the “don’t eat the resulting babies” module might suffice to assure sufficient reproduction. It is conceivable that our hominid precursors and even some humans had no idea about the connection between copulation and procreation so as to be able to make and act upon decisions about completed family size.

Optimal family size depends on social and environmental conditions. “Clutch size”, as one of my physical anthropology professors puts it, varies according to circumstances, and this flexibility has permitted humans to flourish on an unprecedented scale. Foragers, which humans have been for most of their existence, have fewer children than farmers. Inter-birth intervals are significantly longer among foragers than farmers. For example, !Kung Bushmen’s intervals have been observed to exceed 48 months. This is accomplished in large measure through extended lactational infecundity. This works out well for the mothers, since they are not burdened with more than one infant at a time that must be carried and surveilled closely. This also means that even the most fecund foraging mothers will be limited to 4 or 5 births over their reproductive careers. Child mortality, combined with such small families, leads to a rate of reproduction that scarcely exceeds replacement. For a variety of reasons, a higher birth rate would be suboptimal under the conditions of foraging.

One might even argue that the foragers’ family size and extended lactation are “natural” for humans insofar as they may provide clues as to the mechanisms that regulate birth rates. I don’t think it would make sense to argue that foragers don’t like kids as much as farmers do as an explanation for their smaller family size, although it is possible that sedentism permitted any genetic variation in affection for children to express itself for the first time in human evolution. If so, it has been expressing itself for a scant 10,000 years at the most and probably has not had time to work any evolutionary mojo.

Farmers have shorter inter-birth intervals than foragers and tend to wean their children earlier, perhaps thanks to an increased availability of weaning foods. The Mennonites whom I studied had inter-birth intervals of about 18-24 months on average and average completed family size of about 8 children over two centuries. Census records from the farming families in my ancestry and their neighbors show a similar pattern. It appears that these families were making no attempt to control family size and were having as many children as they could. My ancestry is full of big families right up to my parents’ generation, so one might expect that my relatives and I would be endowed with the posited big family module. Yet, my parents and their siblings, having given up farming and entered the world of wage slavery or entrepreneurship, epitomized the demographic transition by having much smaller families. My mother and her siblings had an average of 2 children, as did my father and his siblings. My cousins have even smaller families. I am content to be childfree. If the big family module existed, it seems to have become inactive in my family and just about every family I know.

Some religious folks see it as their duty to have as many children as they can so as to boost their numbers. Is this because they have a genetic predisposition to value children more than other folks? Is there any reason to believe that their descendants will overrun the planet thanks to their enhanced love of children? Not if the past experience of such people is any indication. For example, if Antibaptist fecundity were genetic, there would be billions of Antibaptists by now, so many that it would be necessary for them to colonize space, quite a feat with 18th century technology. The same goes for Hasidic Jews and to a lesser extent Mormons (they have only been around a few generations). Why aren’t they ubiquitous? It seems to me that such folks experience a good bit of ideological attrition. It’s hard to follow the big family rule under social and economic conditions that render small families optimal for most people.

What’s the optimum American family size? The consensus seems to be that 2 children, a boy and a girl, are ideal. You might reluctantly have a third if you get stuck with two sons or two daughters in your first two attempts or if you think you might need an emergency back up child.