Perhaps because of the aggregate nature of genetic influence as studied in twin designs, findings from twin studies have a far better record of replication than findings from specific candidate gene association studies. Heritabilities for clinical disorders have been remarkably consistent across populations, and gene-environment interaction effects have also yielded consistent results (e.g., (Legrand et al. 2007, Rose et al. 2001). This is not the case with gene finding efforts, where replications have been notoriously difficult, both for main effects (e.g., Lind et al. 2009, Wang et al. 2004) and gene-environment interactions (Caspi et al. 2003, Risch et al. 2009).
