 ‘Population thinking’ means many things to many people. For Boyd and Richerson it denotes any effort to abstract from a characterisation of individual psychological profiles, in a way that allows an exploration of the consequences of these individual-level dispositions for population-level properties. We have already seen an example of this sort of population thinking in action. It is far from obvious that conformist bias among individuals can enable population-level inheritance in spite of individual-level errors in copying. To show that these properties of individual psychology (conformist bias and error-prone learning) combine to yield population-level inheritance requires some abstract mathematical modelling. And the establishment of this population-level consequence is important, for it enables the investigator to revise the constraints one might naively think must bear on cultural inheritance if cumulative cultural evolution is to occur.