Whether the twentieth century has seen intelligence gains is controversial. Whether there have been massive IQ gains over time is not. I will: (1) describe the range and pattern of IQ gains; (2) discuss their historical and social significance; (3) argue that they suggest a new theory of intelligence; and (4) urge that understanding them is more important than classifying them (as either intelligence or non-intelligence gains).

The evidence and its peculiarities

Reed Tuddenham (1948) was the first to present convincing evidence of massive gains on mental tests using a nationwide sample. He showed that US soldiers had made about a 14-point gain on Armed Forces tests between World War I and World War II or almost a full standard deviation (SD = 15 throughout). The tests in question had a high loading on the kind of material taught in the classroom, and he thought the gains were primarily a measure of improved schooling. Therefore, they seemed to have no theoretical implications, and because the tests were not among those most often used by clinical psychologists the practical implications were ignored.

When Flynn (1984, 1987) showed that massive gains had occurred in America on Wechsler and Stanford–Binet IQ tests, and that they had occurred throughout the industrialized world even on tests thought to be the purest measures of intelligence, IQ gains took center stage. Within a decade, Herrnstein and Murray (1994), the authors of The Bell Curve, called the phenomenon the “Flynn effect.”