A longitudinal twin study of 1-year prevalence of major depression in women

OBJECTIVES: This study seeks to clarify the etiologic importance and temporal stability of the genetic and environmental risk factors for 1-year prevalence of major depression (1YP-MD) in women. DESIGN: One-year prevalence of major depression was personally assessed, using DSM-III-R criteria, at two time points a minimum of 1 year apart. PARTICIPANTS: Both members of 938 adult female-female twin pairs ascertained from the population-based Virginia Twin Registry. RESULTS: The correlation in liability to 1YP-MD was much greater in monozygotic (MZ) than in dizygotic (DZ) twins at time 1 alone, time 2 alone, or at either time 1 or time 2. Model fitting suggested that the liability to 1YP-MD was due to additive genes and individual specific environment with a heritability of 41% to 46% and was not biased by violations of the equal environment assumption. Jointly analyzing both times of assessment using a longitudinal twin model suggested that, over a 1-year period, genetic effects on the liability to 1YP-MD were entirely stable, while environmental effects were entirely occasion specific. CONCLUSIONS: These results suggest that: (1) genetic factors play a moderate etiologic role in the 1YP-MD, (2) the temporal stability of the liability to major depression in adult women is largely or entirely genetic in origin, and (3) environmental factors play a significant role in the etiology of major depression, but their effects are generally transitory and do not result in enduring changes in the liability to illness. (November 1993)