in terms of socio-economic status and ethnicity [27,28]. Zygosity was assigned in an earlier wave of the study [23] using a parent-rated twin similarity questionnaire [29,30]. Non-responders were initially sent reminder postcards and were then re-mailed the questionnaire. Of 1755 families contacted, at least one individual from 1214 families returned questionnaires (a response rate of 69%; the individual response rate was 61%). Families received gift vouchers as a token of appreciation.
