Participants were members of the Environmental Risk (E-Risk) study, which tracks the development of a birth cohort of 2232 British children. The sample was drawn from a larger birth register of twins born in England and Wales in 1994-95.11 Full details about the sample are reported elsewhere.12 Briefly, the E-Risk sample was constructed in 1999-2000, when 1116 (93% of those eligible) families with same sex 5 year old twins participated in home visit assessments. Families were recruited to represent the UK population of families with newborns in the 1990s, on the basis of residential location throughout England and Wales and mother’s age (older mothers having twins via assisted reproduction were under-selected, and teenage mothers with twins were over-selected). We used this sampling to replace high risk families who were selectively lost to the register through non-response and to ensure sufficient numbers of children growing up in high risk environments. The sample includes 55% monozygotic and 45% dizygotic twin pairs. Sex is evenly distributed within zygosity (49% male). Follow-up home visits took place when the children were aged 7 (98% participation), 10 (96% participation), and, most recently, 12 years (96% participation).
