This work was supported by a Sir Henry Wellcome Postdoctoral Fellowship, awarded to R.M.F. (Wellcome Trust grant: 085541/Z/08/Z). B.S. is funded by the Peninsula National Institute for Health Research Clinical Research Facility. M.N.W. is a Vandervell Foundation Research Fellow. A.T.H. is a Wellcome Trust Research Leave Fellow. The UK Medical Research Council, the Wellcome Trust and the University of Bristol provide core support for ALSPAC. Funding to pay the Open Access charge was provided by the Wellcome Trust.