The heritability of life events: An adolescent twin and adoption study

Although life events are often conceptualized as reflecting exogenous risk factors for psychopathology, twin studies have suggested they are heritable. We undertook a mixed twin/adoption study to further explore genetic and environmental contributions to individual differences in the experience of life events. Specifically, a sample of 618 pairs of like-sex adolescent twins, 244 pairs of like-sex adopted adolescent and young adult siblings, and 128 pairs of like-sex biological siblings completed a life events interview. Events were classified as independent (not likely to have been influenced by respondent's behavior), dependent (likely to have been influenced by respondent's behavior), or familial (experienced by a family member), and then summed to form three life event scales. Variance on the scales was assumed to be a function of four factors: additive genetic effects (a²), shared environmental effects (c²), twin-specific effects (t²), and nonshared environmental effects (e²). Data were analyzed using standard biometrical models. Shared environmental effects were found to be the largest contributor to variance in familial events (c² = .71; 95% confidence interval of .65, .76); additive genetic effects were the largest contributor to dependent events (a² = .45; Cl = .31, .58); and nonshared environmental effects were found to be the largest contributor independent events (e² = .57; Cl = .51, .64). A significant twin-specific effect was also found for independent life events, indicating that twins are more likely to be exposed to such events than non-twin biological siblings. Findings are discussed in terms of their implication for understanding the nature of psychosocial risk.