differentiation–dedifferentiation hypothesis
two complementary proposals about the development of intelligence. In one, U.S. psychologist Henry E. Garrett (1894–1973) proposed in 1938 that intelligence begins as a fairly unified, general ability that then differentiates into separate, albeit related, abilities as children develop. This proposal has come to be called the age differentiation hypothesis. According to the other proposal, put forth by U.S. psychologist Benjamin Balinsky in 1941, a process occurs after age 65 during which the separate abilities developed in childhood become increasingly intercorrelated as a result of decreasing fluid intelligence (see Cattell–Horn theory of intelligence); at this time, intelligence dedifferentiates and returns to a single factor of general ability. This proposal has become known as the age dedifferentiation hypothesis. Both proposals have garnered much debate over the years, but the evidence for each remains limited.