Footnotes

• Funding: this study would not have been undertaken without the financial support of the Medical Research Council, the Wellcome Trust, the UK Department of Health, the Department of the Environment, the DfEE, the National Institutes of Health, and a variety of medical research charities and commercial companies. ALSPAC is part of the World Health Organisation-initiated European longitudinal study of parents and children.

• Competing interests: none declared.