In smaller, prior follow-up samples from the Scottish Mental Surveys of 1932 and 1947 we reported significant contributions to variance in normal cognitive ageing from variation in the following genes: APOE [12], COMT [36], PRNP [37], DISC1 [38], and BDNF [39]. APOE provided a clear example of a genetic polymorphism that, in the same sample was related to cognitive ability in old age but not in youth [12]. All of these analyses used cognitive ability measures in old age adjusted for mental ability at age 11 in the Scottish Mental Survey data. Because we had data on both childhood IQ and IQ in old age, we found some genes to be related to both, such as NCSTN [40], and KL [41]. Overall, there are, as yet, few replicated genetic associations with normal cognitive ageing.
