Our work with ES twins has also contributed to two international collaborative efforts. In Hur et al. (2008), we combined forces with other twin registries to show that differences in the variances for height, weight, and body mass index distributions that exist between Caucasian and East Asian populations are attributable to genetic factors. These findings indicate that the genetic mechanisms underlying these variables, important in part to understanding obesity, are different in these two populations, and suggest that association studies can therefore expect to identify different genes in each population. In Haworth et al. (2009a), a collaboration pooling data from 11,000 twins, we showed that the heritability of general cognitive ability increases substantially from childhood to young adulthood. These findings support the hypothesis that as children grow up they select, modify, and create opportunities to accumulate knowledge and educational experience, reflecting the effects of gene-environment correlation. These results also suggest that adult samples, rather than those composed of children, are likely to be better for finding genes associated with cognitive ability.
