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April 1992

A Population-Based Twin Study of Major Depression in Women: The Impact of Varying Definitions of Illness

Author Affiliations

From the Departments of Psychiatry (Drs Kendler and Eaves) and Human Genetics (Drs Kendler, Neale, Heath, and Eaves), Medical College of Virginia/Virginia Commonwealth University, Richmond; and the Institute for Social Research, University of Michigan, Ann Arbor (Dr Kessler). Dr Heath is now with the Department of Psychiatry, Washington University School of Medicine, St Louis, Mo.

Arch Gen Psychiatry. 1992;49(4):257-266. doi:10.1001/archpsyc.1992.01820040009001

• Although depression aggregates in families, the degree to which this aggregation results from genetic vs environmental factors remains uncertain. We examined this question in 1033 female-female twin pairs from a population-based registry. Both members of each twin pair were "blindly" assessed by structured psychiatric interview. Nine commonly used definitions of major depression, which produced lifetime prevalence rates ranging from 12% to 33%, were examined. For all definitions, the results of model fitting to twin correlations suggested that the liability to depression results from genetic factors and environmental experiences unique to the individual. For seven of the definitions, the estimated heritability of liability was similar, ranging from 33% to 45%. For the two definitions that included only primary cases of depression, the heritability was lower (21% to 24%). The results document that in women (1) genetic factors play a substantial, but not overwhelming, role in the cause of depression; (2) the tendency for depression to aggregate in families results largely from shared genetic and not from shared environmental factors, (3) except for definitions that exclude secondary cases, the magnitude of genetic influence is similar in broadly and narrowly defined forms of major depression, and (4) most environmental experiences of causative importance for depression are those not shared by members of an adult twin pair.

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