The relation between individual trait differences, social mobility and social structure is central to social biology. Because genetic variance underlies phenotypic variance in some of these traits, for example IQ, several mechanisms determine the population variance. Polygenic inheritance is the basic mechanism. Social mobility and assortative partner choice distribute the trait variance within generations. This feedback circle is constrained by sociological conditions at several levels of analysis. Fundamental to this theory of social assortment is the relation between social–biological traits and social class on the one hand, and these traits and social mobility on the other hand. The focus here is on the relation between social class, social mobility and cognitive ability. The National Child Development Study is drawn upon, including the last follow-up (1999–2000). By approaching this relationship through various methods, both social–biological and sociological aspects of this research question can be assessed.