Surprisingly, until the very recent past almost nothing had been known about genetic influences on human obesity. The powerful genetic effects described with such assurance in the textbooks were based almost entirely on extrapolation from animal studies. The first strong evidence of genetic influence on human obesity was obtained from an adoption study in Denmark that showed a high correlation of the body mass index of adoptees with that of their biological parents and no correlation with that of their adoptive parents. The body mass index of the adoptees was also highly correlated with that of their siblings and showed evidence of recessive transmission. These findings have been extended by a twin study that revealed very high heritabilities at both age 20 and age 45. A bivariate analysis of the identical twins of this population revealed high intrapair correlations among the normal weight twins and low correlations among the obese twins distributions, suggesting a strong environmental influence on the genetically vulnerable obese twins. The best estimate of heritability, the correlation coefficient of 93 identical twin pairs reared apart, from the Swedish Adoption Study of Aging, revealed high levels of heritability, indicating that traditional twin studies have overestimated the heritability of body mass index only slightly, if at all.