SOURCES OF INDIVIDUAL-DIFFERENCES IN DEPRESSIVE SYMPTOMS - ANALYSIS OF 2 SAMPLES OF TWINS AND THEIR FAMILIES

Objective: Self-reported symptoms of depression are commonly used in m ental health research to assess current psychiatric state, yet wide va riation in these symptoms among individuals has been found in both cli nical and epidemiologic populations. The authors sought to understand from a genetic-epidemiologic perspective, the sources of individual di fferences in depressive symptoms. Method: Self-reported symptoms of de pression were assessed in two samples of twins and their spouses, pare nts, siblings, and offspring: one sample contained volunteer twins rec ruited through the American Association of Retired Persons and their r elatives (N=19,203 individuals) and the other contained twins from a p opulation-based twin registry in Virginia and their relatives (N=11,24 2 individuals). Model fitting by an iterative, diagonal, weighted leas t squares method was applied to the 80 different family relationships in the extended twin-family design. Results: Independent analyses of t he two samples revealed that the level of depressive symptoms was mode stly familial, and familial resemblance could be explained solely by g enetic factors and spousal resemblance. The estimated heritability of depressive symptoms was between 30% and 37%. There was no evidence tha t the liability to depressive symptoms was environmentally transmitted from parents to offspring or was influenced by environmental factors shared either generally among siblings or specifically between twins. With correction for unreliability of measurement, genetic factors acco unted for half of the stable variance in depressive symptoms. Conclusi ons: Depressive symptoms in adulthood partly reflect enduring characte ristics of temperament that are substantially influenced by hereditary factors but little, or not at all, by shared environmental experience s in the family of origin