There is strong evidence that genes determine individual differences in physical weight and weight gain (Fox et al. 2007; Haberstick et al. 2010; Yang et al. 2007). There is also a great deal of variability in the estimated influence of genotype on BMI; with an average of roughly 60 %, heritability estimates for BMI range from as little as 5 % to as high as 90 % (Loos and Bouchard 2003). This variation is in line with the GxE perspective, which anticipates differential associations between genotype and phenotype across different environments (Shanahan and Hofer 2005), and some work has demonstrated that genetic factors linked to obesity-related phenotypes are socially moderated (Boardman et al. 2012; Lee et al. 2011b). Additional evidence for the importance of the social environment with respect to the genetic influences on obesity-related phenotypes comes from a recent report by Rokholm and colleagues (2011). These researchers used measured height and weight from nearly 4,000 twin pairs from the Swedish Twin Register born between 1951 and 1985. They showed a steady increase in the contribution of genetic factors to
