A 35-year longitudinal assessment of cognition and midlife depression symptoms: the Vietnam Era Twin Study of Aging.

OBJECTIVE: To determine whether early adult cognitive ability is a risk factor for depressive symptoms in midlife and how genetic and environmental influences explain the association and to examine cross-sectional relationships between depressive symptoms and specific cognitive abilities at midlife.
DESIGN: A 35-year longitudinal and cross-sectional twin study of cognitive aging.
SETTING: Large multicenter study in the United States. PARTICIPANTS: One thousand two hundred thirty-seven male twins aged 51 to 60 years. MEASUREMENTS: At the age of 20 years and midlife, participants completed the same version of a general cognitive ability test (Armed Forces Qualification Test [AFQT]). Midlife testing included an extensive neurocognitive protocol assessing processing speed, verbal memory, visual-spatial memory, working memory, executive function, and visual-spatial ability. Participants completed the Center for Epidemiologic Studies Depression Scale before cognitive testing and provided health and life style information during a medical history interview.
RESULTS: Lower age 20 AFQT scores predicted higher levels of depressive symptoms at age 55 years (r = -0.16,p <0.001). In bivariate twin modeling, 77% of the correlation between early cognitive ability and midlife depressive symptoms was due to shared genetic influences. Controlling for current age, age 20 AFQT, and nonindependence ofobservations, depressive symptoms were associated with worse midlife AFQT scores and poorer performance in all cognitive domains except verbal memory.
CONCLUSION: Results suggest that low cognitive ability is a risk factor for depressive symptoms; this association is partly due to shared genetic influences. Crosssectional analyses indicate that the association between depressive symptoms and performance is not linked to specific cognitive domains.