Data were obtained from participants in the Vietnam Era Twin Study of Aging (VETSA), a longitudinal study of cognition and aging of male twins (Kremen et al. 2013; Kremen et al. 2013; Kremen et al. 2006). All participants in the VETSA were drawn from the Vietnam Era Twin (VET) Registry, a nationally distributed sample of twin pairs who served in the United States military between 1965 and 1975 (Goldberg et al. 2002). Detailed descriptions of the VET Registry's composition and method of ascertainment have been previously reported (Eisen et al. 1987; Henderson et al. 1990). This community-based sample has been found to be representative of middle-aged males in the United States with respect to their distributions of socioeconomic and health characteristics. (Schoneneborn & Heyman, 2009---KREMEN 2012).
