The following are examples of factors that have some evidence of an association with differences in normal cognitive ageing: smoking [16], physical fitness [17,18], personality [19,20], cardiovascular disease [20,21], social and intellectual engagement [15,22], diet [23,24], brain white matter hyperintensities [25] and brain white matter integrity [26]. Many of the effects are small, and not all are replicated. Some of these effects may in part be caused by shared genetic effects. Much of this type of information is obtained from studies which lack truly pre-morbid cognitive ability. Such studies are unable to test for reverse causality: the possibility that it is the lifelong trait of cognitive ability that brings about individual differences in the putative 'cause' of cognitive ageing.
