We thank all the families and professionals who participated in the study, Caroline Durrant and Jean-Baptiste Cazier for statistical advice, members of the Monaco lab for support, and Leila Jannoun, Jane Addison, Clare Craven, Deborah Jones, Tilly Storr, Til Utting-Brown, Margaret Main, Jane Steele, and Alan MacLean for assistance with data collection and management. We are extremely grateful to all the families who took part in the ALSPAC study, to the midwives for their help in recruiting them, and to the whole ALSPAC team, which includes interviewers, computer and laboratory technicians, clerical workers, research scientists, volunteers, managers, receptionists, and nurses. The UK Medical Research Council, the Wellcome Trust, and the University of Bristol provide core support for ALSPAC. This publication is the work of the authors, and D.F. Newbury and A.P. Monaco will serve as guarantors for this paper's contents. The Wellcome Trust specifically funded this research. All laboratory work and the collection of data from families ascertained by Guy's Hospital and the University of Manchester were funded by The Wellcome Trust. CLASP was funded by The Wellcome Trust, British Telecom, Isaac Newton Trust, National Health Service (NHS) Anglia & Oxford Regional R&D Strategic Investment Award, and an NHS Eastern Region R&D Training Fellowship Award. The Edinburgh group was supported by the Chief Scientist's Office, Scotland. The Aberdeen group was supported by Grampian Healthcare Trust and Grampian Primary Care NHS Trust. D.V.M. Bishop is a Wellcome Trust Principal Research Fellow, and S.E. Fisher is a Royal Society Research Fellow.