Can a heritability value tell us something about the weight of genetic versus environmental causes that have acted in the development of a particular individual? Two possible questions arise. Q1: what portion of the phenotype of X is due to its genes and what portion to its environment? Q2: what portion of X’s phenotypic deviation from the mean is a result of its genetic deviation and what portion a result of its environmental deviation? An answer to Q1 provides the full (...) information about X’s development, while an answer to Q2 leaves out a large portion unexplained—that portion which corresponds to the phenotypic mean. Q1 is unanswerable, but I show it is nevertheless legitimate under certain quantitative genetics models. With regard to Q2, opinions in the philosophical and biological literature differ as to its legitimacy. I argue that not only is it legitimate, but in particular, under a few simplifying assumptions, it allows for a quantitative probabilistic answer: for normally distributed quantitative traits with no G-E correlation or statistical G × E interaction, we can assess the probability that X’s genes had a greater effect than its environment on its deviation from the mean population value. This probability is expressed as a function the heritability and the individual’s phenotypic value; we can also provide a quantitative probabilistic answer to Q2 for an arbitrary individual where the probability is a function only of heritability. (shrink)

Despite a long history of debates about the heritability of human traits by researchers and other critical commentators, the possible heterogeneity of genetic and environmental factors that underlie patterns in observed traits has not been recognized as a significant conceptual and methodological issue. This article is structured to stimulate a wide range of readers to pursue diverse implications of underlying heterogeneity and of its absence from previous debates. Section 1, a condensed critique of previous conceptualizations and interpretations of heritability studies, (...) consists of three core propositions centered on heterogeneity and six supplementary propositions. Reference is made to agricultural evaluation trials in which a number of different genetically replicable varieties are raised in multiple replicates in one or more locations. In such analyses, the best case for illuminating genetic and environmental factors can be achieved; analyses in human genetics, in contrast, fall far short of the ideal. Section 2 identifies a wide range of questions that invite philosophical, historical, sociological, and scientific inquiry. These are organized under four headings: debate over the conceptual implications of heterogeneity; history of translation of methods from agriculture and laboratory breeding into human genetic analysis; racialized imaginaries in the analysis of differences among groups; and areas of scientific inquiry that may allow more attention to underlying heterogeneity. (shrink)