Many psychometricians and behavioral geneticists believe that high heritability of IQ test scores within racial groups coupled with environmental hypotheses failing to account for the differences between the mean scores for groups lends plausibility to explanations of mean differences in terms of genetic factors. This two-component argument cannot be sustained when viewed in the light of the conceptual and methodological themes introduced in Taylor . These themes concern the difficulties of moving from the statistical analysis of variance of observed traits (...) to investigation of measurable genetic factors and measurable environmental factors. One such theme is that quantities estimated by an AOV of observed traits cannot be equated with measurable genetic or environmental factors involved in the development of those traits. Once this distinction is clear, the argument that environmental factors have failed to explain the differences lacks weight because it does not consider whether genetic factors have been more successful. This article exposes additional flaws in the lines of thinking associated with the two-component argument, with the distinction between passive, reactive, and active associations between genetic and environmental factors, and with the reciprocal causation models Dickens and Flynn propose in order to reconcile high estimates of heritability and large IQ test score gains between generations. Human heritability estimates are irrelevant in developing explanations of differences across groups or across generations. My critique is directed at opening up more conceptual space for deriving empirically validated models of developmental pathways whose components are heterogeneous and differ among individuals at any one time and over generations. (shrink)