Evidence is presented for multiple components in the distribution of human fatness across several large twin samples, after removing age effects and allowing for residual skewness in component distributions. The upper component distributions corresponded to overweight or obesity in samples of middle-aged or older individuals. A bivariate analysis demonstrated that, while monozygotic co-twins appeared to be drawn from the same component distributions (normal or overweight), the twin correlations varied across components, with the lowest correlation in the overweight group. While these analyses cannot provide a definitive test of competing genetic and environmental hypotheses, this approach is useful for generating hypotheses about the causes of obesity. When combined with other published literature, our results suggest that the genetic background largely determines the propensity to become obese. Whether a predisposed person becomes obese and the extent of obesity depend on environmental exposures that are largely independent of early family experience. Both genes and environment appear to be important in obesity, but it appears that some genotypes may be much more sensitive to the environment than are others.