Differential Fertility and the Distribution of Traits: The Case of IQ

A recurrent fear during the past century is that the mean IQ level of populations will decline because persons with lower IQ scores have above-average fertility. Most microlevel data demonstrate such fertility differentials, but population IQ levels have risen rather than fallen. In this article, a simple two-sex model shows that negative fertility differentials are consistent with falling, rising, or constant IQ distributions. Under a wide variety of conditions, a constant pattern of fertility differentials will produce an unchanging, equilibrium distribution of IQ scores in the population. What matters for IQ trends is how the IQ distribution in one generation relates to the equilibrium distribution implied by that generation's fertility differentials. Intuition fails in this important area because it does not account for the macro structure within which micro results must be interpreted.