Since the twentieth century, IQ scores have increased at an average rate of around three IQ points per decade in most parts of the world. This phenomenon has been named the Flynn effect (aka the "Lynn-Flynn effect") named after Richard Lynn and James R. Flynn. Attempted explanations have included improved nutrition, a trend towards smaller families, better education, greater environmental complexity, and heterosis. Some factions believe that modern education has become more geared toward IQ tests, thus rendering higher scores, but not necessarily higher intelligence. Tests are therefore renormalized occasionally to obtain mean scores of 100, for example WISC-R (1974), WISC-III (1991) and WISC-IV (2003). This adjustment specifically addresses the variation over time, allowing us to compare scores from different times.

Some researchers argue that the Flynn effect may have ended in some developed nations starting in the mid 1990s, namely in Denmark and in Norway.