Genetic and environmental etiologies of positive and negative affect, as measured by the Bradburn Affect Balance Scale, were studied to gain understanding of a two-factor theory of well-being. It was hypothesized that negative affect would demonstrate significant genetic and environmental variance and positive affect would be explained primarily by environmental influences. Data were combined from 105 pairs of twins (ages 18-72) and 220 multigenerational families (ages 16-98). Simultaneous model fitting indicated significant heritable effects for negative affect and a significant effect of common environment for twins. Significant effects for positive affect included common environment (for parents and offspring and for twin pairs) and assortative mating. These results, documenting differential genetic and environmental influences on positive and negative affect, provide further support for their being separate components of well-being.