Dr Mortimer and Theresa Sackler Foundation. PAT, DJP, IJD and AMM are members of The University of Edinburgh Centre for Cognitive Ageing and Cognitive Epidemiology, part of the cross council Lifelong Health and Wellbeing Initiative (MR/K026992/1). Funding from the Biotechnology and Biological Sciences Research Council (BBSRC) and Medical Research Council (MRC) is gratefully acknowledged, supported in part by National Institutes of Health grants RC2 DA028909, R01 DA12690, R01 DA12849, R01 DA18432, R01 AA11330, R01 AA017535, P50 AA12870, MSTP T32GM07205 and CTSA 8UL1TR000142. This work is supported by the Wellcome Trust through a Strategic Award, reference 104036/Z/14/Z.