Reply from M. Borgerhoff Mulder.

Penn thinks that a classification of hypotheses that distinguishes maladaptive from culturally transmitted behaviour is confusing because cultural evolutionary models often generate non-optimal outcomes. In trying to present coherent alternative hypotheses, I too wrestled with this issue. I decided that since the evolved learning biases on which cultural evolutionary processes depend produce adaptive as well as non-adaptive outcomes1xCulture and the Evolutionary Process. Boyd, R. and Richerson, P.J. See all References1, it was wisest not to conflate the two hypotheses.It is premature for Penn to assume that Lack’s hypothesis will fail in groups provided with social welfare, especially since welfare rarely fully compensates the needy. Thus, I maintain that empirical investigation of long-term fitness outcomes2xKaplan, H.S. et al. : 96–136See all References2 is an essential first step in determining whether fertility limitation is adaptive. Nevertheless, I agree with Penn that such studies are best conducted outside of the developed world (and am currently conducting such research), I also agree that Lack’s reasoning in combination with cultural evolutionary processes may provide the most fertile area of future enquiry.Penn and Nentwig are right to point to the complex web of confounded variables (e.g. education, female autonomy and economic opportunities) implicated in demographic transition, perhaps via intersexual conflict. Critical demographers who are aware of this problem3xMason, K.O. Genus. 1992; 48: 1–15PubMedSee all References3 lament the fact that while national (or regional) level statistics rarely allow these factors to be analytically separated, micro-level studies suffer from insufficient statistical power. Thus any speculations as to the true correlates of demographic transition remain just that—speculations.Nentwig’s misunderstanding of my article allows me to clarify the thrust of Box 2. An evolutionary explanation of the well-studied patterns of demographic transition isn’t an alternative to conventional theories of fertility decline, just as ultimate explanations are not alternatives to accounts of proximate motivation or mechanism. The question that intrigues evolutionists is this: What kinds of optimizing rules, be these psychological or physiological, might account for the patterned responses of fertility to changing environmental constraints as witnessed in the demographic transition? Or, since I didn’t deal with physiology in the article, what kind of evolved psychology might generate the fertility choices we see in Europe in the last century, and more recently in much of the developing world?Nentwig is right to clarify that most demographers (sensibly, in my opinion) view the number of children an individual produces as an outcome of the intersection of supply and demand. The ‘supply’ of children is affected by diseases such as malaria (which precipitates pregnancy loss), the availability of maternal and pediatric health services affecting child survival, and broader environmental factors (including the sterility associated with environmental toxins). A lowering in the ‘demand’ for children is attributed by many observers to increased costs of children (typically their education) and social security pension schemes that lessen elderly parents’ dependence on their children. This supply/demand framework has been widely adopted by modern demographers for years4xSee all References4, and is central to evolutionary accounts5xKaplan, H.S. Yearb. Phys. Anthropol. 1996; 39: 91–135CrossRefSee all References5.Nentwig’s final point is mistaken. We are of course hopelessly overpopulated. I write this from a village in western Tanzania where children are severely undernourished and yet the modal completed fertility is still seven. However, to suggest that fertility decline is a density-dependent response flies in the face of evidence that population decline has been slowest in some of the most densely populated regions of the world. Furthermore, as stressed in the article, the decline usually starts amongst those who have most.