The study population is an urban / rural mix with little outward migration located in Southwest England (Golding et al, 2001). 14,541 pregnant women resident in the county of Avon with a due date between April 1991 and December 1992 were enrolled in the study, representing approximately 85% of all eligible participants. ALSPAC is therefore a community-representative sample. Only Caucasian children (95% of the sample) were included in the current study in order to minimize the effects of population stratification. All children were unrelated. A total of 4034 boys and 3976 girls had data on hyperactivity at 4 years which reduced to 3182 boys and 2947 girls who also had MAOA-LPR genotypes. Approval for the study was obtained from the ALSPAC Law and Ethics Committee and Local Research Ethics Committees.
