 One might react to this with confusion: why is a body of theory needed to make these claims? Of course we acquire traits from others by learning. And of course those others from whom we learn can include peers as well as parents. In part, we can respond to this bewilderment by pointing to the virtues of clarifying the conditions required for cultural inheritance to overcome natural selection. Cavalli-Sforza and Feldman argue that if women simply acquired whichever preference for family size was the most widely adopted in their local cultural environment, then cultural inheritance would not have enough of an effect to overcome natural selection. Women must be disposed to acquire the preference for small family size even when it is present in only a small proportion of their cultural circle, if small family size is to replace large family size in the population as a whole. This is an illuminating claim, and it takes a quantitative model to show it.