A total of 1214 families returned questionnaires (69% response rate). Participants were, on average, 16.11 years old (SD = 1.96). In the United Kingdom it is illegal to purchase cigarettes under the age of 16 and alcohol under the age of 18. Information was available for cigarette use on 1165 twins under 16 years of age and 861 twins aged 16 or over. For alcohol use, information was available on 1612 individuals under 18 years of age and 420 individuals aged 18 or over.
