Despite the long history of scientific, philosophical, and political debate around heritability studies, certain fundamental conceptual issues have not been recognized or well appreciated. The starting point is that heritability does not measure the degree of influence that genes have on a trait or provide a reliable basis for choosing which traits to investigate further with molecular genetic research. The short argument on this point revolves around two issues: the disconnect between analyzing measurements of a trait and exposing the measurable (...) genetic and environmental factors underlying the trait’s development; and the possibility of heterogeneity in these underlying factors, that is, different factors may lead to the same trait value. The idea of underlying heterogeneity is elaborated through schematic diagrams and distinguished from other senses of heterogeneity. Five conceptually distinct approaches for addressing underlying heterogeneity are identified, corresponding to distinct ways of managing the reciprocal relationship between the degree of knowledge of the dynamics through which the trait develops and the actions that can be reliably be based on what is known . This framework, which extends the interventionist notion of causality, allows the scope and limitations of heritability studies to be clarified in greater detail. It can also inform critical appreciation of newer methods of analysis of genetic and environmental factors. The issues discussed in this article do not centre on empirical data or technical detail and should be accessible to non-specialists as well as challenging active researchers. (shrink)