The mean IQ of Americans: Massive gains 1932 to 1978.

This study shows that every Stanford-Binet and Wechsler standardization sample from 1932 to 1978 established norms of a higher standard than its predecessor. The obvious interpretation of this pattern is that representative samples of Americans did better and better on IQ tests over a period of 46 years, the total gain amounting to a rise in mean IQ of 13.8 points. The implications of this finding are developed: The combination of IQ gains and the decline in Scholastic Aptitude Test scores seems almost inexplicable; obsolete norms have acted as an unrecognized confounding variable in hundreds of studies; and IQ gains of this magnitude pose a serious problem of causal explanation.