suggest that these concerns may be more important when extended to the genome-wide scale. Our results do not challenge the relevance of GWAS studies, nor do they challenge GxE studies in general. Rather, we believe that the SNP-by-SNP GxE approach (GWGEI) is not likely to provide results that can be used to test existing GxE theory. In sum, we believe that SNPs indicate something important about an individual's risk of an unhealthy BMI, but taken individually, they do not hold much information. This is somewhat obvious: people do not inherit a single SNP. Rather, they inherit their entire genome from two sources—hence the importance of genome-wide data as a cumulative indicator of biological proximity among related and unrelated persons. Disentangling the genomic component of family risk from the social and behavioral component has been the work of behavior geneticists for decades, but new evidence for genetic influences on most health behaviors, new statistical methods, and new genetic data sources make this type of approach important and timely for demographic researchers.
