Franz CE, Lyons MJ, O'Brien R, Panizzon MS, Kim K, Bhat R, Grant MD, Toomey R, Eisen S, Xian H, Kremen WS. A 35-year longitudinal assessment of cognition and midlife depression symptoms: the Vietnam Era Twin Study of Aging. Am J Geriatr Psychiatry. 2011 Jun; 19(6):559-70.