Abstract
Intelligence is an important human trait on which people differ. Few studies have examined the stability of intelligence differences from childhood or youth to older age using the same test. The longest such studies are those that have followed up on some of the participants of the Scottish Mental Surveys of 1932 and 1947. Their results suggest that around half of the individual differences in intelligence are stable across most of the human life course. This is valuable information because it can be used as a guide to how much of people’s cognitive-aging differences might be amenable to alleviation.