Participants in the current study were drawn from the MacArthur Foundation Survey of Midlife Development in the United States (MIDUS), a nationally representative survey of persons age 25–74 years in the noninstitutionalized civilian population of the continental United States. The MIDUS study used telephone and mail questionnaires to assess variables related to physical health, psychological well-being, and social responsibilities. A subset of the larger MIDUS sample consists of 998 twin pairs (Kessler, Gilman, Thornton, & Kendler, 2004). To collect this twin sample, approximately 50,000 households were screened through telephone surveys to determine if any members of the household, or a relative, was a member of a twin pair. Respondents who agreed and met eligibility criteria were referred to the MIDUS recruitment process (see Kendler, Thornton, Gilman, & Kessler, 2000). All participants: 1) were administered a computer-assisted telephone interview that lasted approximately 45 minutes; and 2) completed two questionnaire booklets that they received in the mail which required an average of 1.5 hours to complete. Data collection procedures began in 1994 and lasted approximately 13 months, concluding in 1995.
