Differential rates of secular increase of five major primary abilities.

A number of studies have shown that mean intelligence levels have been increasing at a rate of around 3 IQ points per decade during the last half-century. This paper presents data for cohorts of 9-11 year olds in Northern Ireland tested in 1978 and 1988 and shows that the rates of increase differ markedly for different primary abilities. Verbal comprehension has shown virtually no increase, while there have been large increases in spatial relations and perceptual speed. Moderate increases have taken place in the numerical and reasoning primaries. The results are interpreted as supporting a nutrition theory of the secular increases in intelligence.