Participants in the second sample were mothers of twins in the Environmental Risk (E-Risk) Longitudinal Twin Study. The E-Risk sample was formed in 1999–2000, when 1116 mothers with 5-year-old twins (93% of eligible families) underwent home-visit assessments (Moffitt, 2002). The women were, on average, 33 years old (SD=5.8; range 20–48) at the initial assessment. Three additional assessments were undertaken when they were, on average, 35, 38, and 40 years old. Attrition was minimal and the last assessment in 2007–2008 included 96% of the women. Genetic data were available for 94% participants. To minimize population stratification, only 930 Caucasian women were included. Women gave written informed consent before participating. The Maudsley Hospital Ethics Committee approved each phase of the study.
