This work was supported by a UK Medical Research Council special training fellowship, awarded to L.Z. [G0501864/76656], by the Wellcome Trust [083506] and by the European Union 6th Framework Coordination Action [518148]. The UK Medical Research Council, the Wellcome Trust and the University of Bristol provide core support for ALSPAC. Funding to pay the Open Access publication charges for this article was provided by the UK Medical Research Council.
