In the United States between 1932 and 1978, mean IQ scores rose 13.8 points, or approximately 0.33 points each year (Flynn, 1984), and IQ scores continued to increase at least into the mid 1990s (Rowe & Rodgers, 2002). Even more striking increases in IQ scores were reported in other countries; for example, IQ scores in Great Britain surged 27 points between 1942 and 1992 (Flynn, 1999). Smaller increases were reported in numerous other countries (e.g., France, the Netherlands, and Norway) during shorter time periods (Flynn, 1987). The Flynn effect, as it is referred to by researchers, is supported by a growing body of research that indicates that even within relatively short timeframes, mean IQ scores tend to increase (Dickens & Flynn, 2001). Furthermore, the Flynn effect is not limited to developed countries (Daley, Whaley, Sigman, Espinosa, & Neumann, 2003). Meanwhile, research has identified numerous practical problems created by the Flynn effect. For example, rising scores require intelligence tests to be restandardized, which alters the scoring of tests such as the WAIS, and changes in test norms create difficulties in assessing the mental capacity of the mentally retarded (Tomoe & Ceci, 2003) and the elderly (Verhaeghen, 2003). Moreover, the Flynn effect may undermine the current theoretical concept of intelligence or the validity of intelligence tests (Flynn, 1984). Although numerous explanations for the Flynn effect were proposed (e.g., Dickens & Flynn, 2001; Flynn, 1987), debate on the origins of the Flynn effect continues (e.g., Rowe & Rodgers, 2002).
Before beginning an investigation of the causes of the Flynn effect, it is important to understand the nature of the increase in mean IQ scores over the past decades. Although some researchers have challenged the existence of the Flynn effect, they failed to explain the dramatic rise in mean IQ scores observed by a growing body of research (Rodgers, 1998). Because the Flynn