The Flynn effect
Worldwide, IQ scores appear to be slowly rising, a trend known as the Flynn effect. Flynn, 1999. The Flynn effect has been observed to be so severe that it is difficult to explain. However, tests are only renormalized occasionally to obtain mean scores of 100, for example WISC-R (1974), WISC-III (1991) and WISC-IV (2003). Hence it is difficult to compare IQ scores measured years apart.