Plomin & Deary (2015)
"Although there are many types of cognitive ability tests of individual differences, they almost all correlate substantially and positively; people with higher ability on one cognitive task tend to have higher ability on all of the others. Intelligence (more precisely, general cognitive ability or g, as discovered and defined by Spearman in 190417) indexes this covariance, which accounts for about 40 per cent of the total variance when a battery of diverse cognitive tests is administered to a sample with a good range of cognitive ability.18,19 As long as a battery of cognitive tests is diverse and reliable, a general ‘factor’ (often represented by the first unrotated principal component, which is not strictly a factor, but that is the terminology that is often used) indexing intelligence differences will emerge and correlate highly with such factors derived from other batteries using wholly different cognitive tests.20 The general intelligence component (factor) is a universally found statistical regularity, which means that some have tried to provide an epithet for what it might capture. According to one view, the core of this general intelligence factor is ‘the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly, and learn from experience’ (Gottfredson et al.21 p.13; see also Deary22). Intelligence is at the pinnacle of the hierarchical model of cognitive abilities that includes a middle level of group factors, such as the cognitive domains of verbal and spatial abilities and memory, and a third level of specific tests and their associated narrow cognitive skills.18,23
Intelligence is important scientifically and socially. Because intelligence represents individual differences in brain processes working in concert to solve problems, it is central to systems approaches to brain structure and function,24, 25, 26 and to the conceptualisation of how diverse cognitive abilities decline with age.27 It is also one of the most stable behavioural traits, yielding a correlation of 0.63 in a study of people tested at age 11 and then again at age 79.28 Socially, intelligence is one of the best predictors of key outcomes such as education and occupational status.29 People with higher intelligence tend to have better mental and physical health and fewer illnesses throughout the life course, and longer lives.22,30
...It would be reasonable to assume that as we go through life, experiences—Shakespeare’s ‘whips and scorns of time’—have a cumulative effect on intelligence, perhaps overwhelming early genetic predispositions. However, for intelligence, heritability increases linearly, from (approximately) 20% in infancy to 40% in adolescence, and to 60% in adulthood. Some evidence suggests that heritability might increase to as much as 80% in later adulthood47 but then decline to about 60% after age 80.48"