Heritability of depressive symptoms: a case study using a multilevel approach

We present a case study using a multilevel modeling approach to determine whether depressive symptoms are affected by genetic factors. Existing studies examining this question have focused on twins. The present study built on the literature by conducting a preliminary study of the heritability of depressive symptoms within extended families. At the same time, this study assessed the need for adjustment of a heritability measure in a family study using a multigenerational sample. The sample consisted of 230 community‐dwelling extended families that included 431 adult offspring, comprising full siblings, half siblings and cousins that participated in the University of Southern California Longitudinal Study of Generations. All participants filled out the Center for Epidemiologic Studies Depression (CES‐D) scale. The multilevel analysis allowed us to model the natural hierarchy of the extended family. Results indicate that the proportion of the phenotypic variance for CES‐D that occurs due to genetic differences is not significantly larger than zero among these participants [h 2 = 8.6%, 95% confidence interval (CI) = 0–57%, p = 0.71]. Our findings suggest that future studies examining depressive symptoms in this sample can focus on non‐genetic explanatory factors without the necessity to control for genetic variation. However, our study may be limited by measurement of prevalent depressive symptoms, which may not generalize to lifetime depressive symptoms. Copyright © 2009 John Wiley & Sons, Ltd.