People differ in their cognitive abilities, and the origins and impacts of these differences are sought after and much debated. The quantitative trait of general intelligence reflects the fact that diverse cognitive abilities show universally positive covariation; that is, no matter the cognitive task being undertaken, much of the human variation in any cognitive task will be caused by people’s differences in general intelligence4. The latent trait of general intelligence ranks people very similarly, irrespective of the group of cognitive tests used to extract it5. Human intelligence is highly stable through the lifecourse and the resemblance between relatives suggests it is highly heritable throughout adulthood3. Intelligence is strongly associated with many important outcomes in life. People with higher intelligence tend to have more education, more professional occupations, higher incomes, and longer lives1,2. The foundations of intelligence differences in brain structure and function are becoming increasingly clear6-8. Especially in later adulthood, an important distinction is made between general fluid and crystallized intelligences, with the former showing earlier and more rapid age-related decline9,10.
