Cohen considers and rejects that low average pre-Neolithic growth rates could be due to periods of negative growth. He also considers cultural restraints, but points out that the speed with which the Americas were colonized far exceeds average growth rates for hunter-gatherers, suggesting that in periods of super-abundance no cultural constraints apply and therefore are not likely to exist.
Cohen suggests that the growth rate of early peoples was constrained by birth spacing--how frequently women had children--and that birth spacing is affected by physiological factors such as mother's body fat composition. He cites studies of modern hunter-gatherer peoples to demonstrate that female body fat has an effect on birth spacing.
In his model, female body fat is affected by population density. Presumably, as populations became more dense, available calories diminish in a given area and births decline until some part of the population dies or moves away, at which time available calories increase and births speed up. Beyond body fat, he suggests that the ratio of calories to overall nutritional availability might also act as a signaling mechanism.
As this article is from 1980, there may be more up to date research and thinking on this topic. Without knowing what that might be however...
As Cohen points out, while the modern !Kung fit his model, other populations with marginal caloric availability do not (i.e., they do not respond to decreased availability with slower birth rates). His explanations for this deviation from his theory are two: (1) perhaps, body fat is only a "signal" to women; (2) the mechanism is short-circuited in modern times by unnatural diets. Presumably by (1) he means that there could be cultural factors that override physiological ones (although it seems to me that, if the physiological mechanism were so weak, pre-Neolithic cultural factors could have just as easily overpowered them).
His greater interest seems to be in (2). Why do third world populations, he asks, continue with high fertility rates in the absence of the protein they need to be successful at these rates? Because, he says, excessive starch in Neolithic cultures fools the body into thinking that there is a greater caloric abundance in the environment than what there really is.
If this explanation is correct, it implies that, regardless of overall caloric intake, starches are the mechanism the body uses to determine whether or not to store fats. If body fat = environmental abundance & environmental abundance is measured by starches, then body fat = starches.
For our own purposes, the model of eating this suggests for us is to pick a level of adequate protein intake, eat that consistently, and modulate starch intake based on our activity levels/BMI. It strikes me that this is what Dr. Kurt Harris found by experimenting on himself (see his posts on Rice Crispies.) Perhaps if you are very fat, then as Dr. Atkins suggests you may need a period of VLC induction if you can fix your regulatory mechanisms at all.
- Cohen, M.N. (1980) "Speculations on the Evolution of Density Measurement and Population Regulation in Homo sapiens" in Cohen et al (Eds) Biosocial Mechanisms of Population Regulation. Yale University Press: New Haven.