Genetic and environmental contributions to depression symptomatology: evidence from Danish twins 75 years of age and older.

The heritability symptoms of depression were investigated in a sample of 406 same-sex Danish twin pairs 75 years of age and older. Twins completed an interview assessment that included symptoms of depression, which were scored on the following 3 scales: Somatic, Affect, and Total. Heritability estimates (h2) for the Total (h2 = .34), Somatic (h2 = .31), and Affect (h2 = .27) scales were all moderate and statistically significant. For not one of the scales did h2 vary significantly over the age range sampled, and although the observed twin correlations were substantially smaller among men as compared with women, none of the sex differences in heritability were statistically significant. Multivariate analyses indicated that all of the heritable effects on the Affect and Somatic subscales could be attributed to a single genetic factor. Depression symptoms in older adults may thus be more heritable than indicated in previous studies, although nonshared environmental factors clearly account for a majority of the variance. The implications of these findings for understanding the nature of late-life depression symptomatology are discussed.