Nature via nurture: what to with cases of gene-environment covariance

Heritability studies have traditionally sought to partition phenotypic variation into genetic and environmental sources. In simple cases, a high heritability estimate (H²) is meant to correspond to phenotypic differences which have been, in some sense, genetically caused. Gene-environment (G-E) covariance, which occurs when different genotypes assort nonrandomly among different environments, can lead to a H² that does not accord with common-sense ascriptions of genetic causation. Some have argued that this phenomenon undermines the effectiveness of H² as a means for causal inquiry. Others believe that the resulting variance can be ascribed to existing variables in the heritability model, however,which variables is subject to further debate. The aims of this thesis are twofold. The first is to draw attention to the gene-environment (G-E) covariance in both animal and human research, as an underrepresented source of phenotypic variance, and potential contributor to 'missing heritability'. The second is to examine the controversy surrounding the interpretation of G-E covariance. Although G-E covariance does not necessitate a causal relation, it is often interpreted causally, and causal motivations appear to shape different interpretations. I use concepts from the interventionist account of causation to demonstrate: That 1) heritability represents a (limited and specific) causal relationship and 2) different types of G-E covariance vary in their underlying causal structures. While identifying causal differences between cases provides some clues regarding interpretation differences, I show that these structures are not sufficient to explain the discord between common-sense ascriptions of genetic causation and the results of some G-E covariant heritability results. Other considerations including the environmental variables specified, the role that agency and blame play in causal attributions, and the concepts embedded in the phenotype under study, are also built into the interpretation of these cases. Taken together these factors account for the dispute regarding interpretation, and shed light on how identified cases of G-E covariance should be treated.