Flynn effect

The Flynn effect is the continued year-on-year rise of IQ test scores, an effect seen in most parts of the world, although at greatly varying rates. It is named after New Zealand political scientist James R. Flynn, its discoverer. The average rate of rise seems to be around three IQ points per decade. Attempted explanations have included improved nutrition, a trend towards smaller families, better education, greater environmental complexity, and heterosis (Mingroni, 2004).

The Flynn effect is a perplexing phenomenon for those who believe that IQ tests represent a true measure of human intelligence, as it would suggest that people today are in general considerably more intelligent than those of previous generations. Flynn himself does not believe this to be the case. It is conceivable that something about modern society is responsible, e.g the greater need for abstract thinking, presence of computers, more visually-oriented culture.

Proposed explanations
Better nutrition has been proposed as a factor. However, there is evidence from Scandinavian countries that IQ scores rose even more, 20 points per generation, following the austerity of occupation during World War II. Another possible explanation is that people are maturing faster, so that, for example, a ten-year-old today may have the mental age that a twelve-year-old had sixty years ago, although this may also be ultimately due to nutrition.

In 2001, James R. Flynn and William T. Dickens, a Brookings Institution economist, presented a mechanism by which environmental effects on IQ may be magnified by feedback effects. The paper "Heritability Estimates Versus Large Environmental Effects: The IQ Paradox Resolved " was published in Psychological Review.

In 2005, Colom et al. (Colom, 2005) presented data supporting the nutrition hypothesis, which predicts that gains in IQ will predominantly occur at the low end of the distribution where nutritional deprivation is most severe. Two large samples of Spanish children were assessed with a 30-year gap. Comparison of the IQ distributions indicated that 1) the mean IQ had increased by 9.7 points (the Flynn effect), 2) the gains were concentrated in the lower half of the distribution and negligible in the top half, and 3) the gains gradually decreased from low to high IQ.

Possibly related to the Flynn effect is change in cranial vault size and shape during the last 150 years in the US. These changes must occur by early childhood because of the early development of the vault.

Studies that make use of multigroup confirmatory factor analysis test for "measurement invariance." Where tenable, invariance demonstrates that group differences exist in the latent constructs the tests contain and not, for example, as a result of measurement artifacts or cultural bias. Wicherts et al. (2004) found evidence from five data sets that IQ scores are not measurement invariant over time, and thus "the gains cannot be explained solely by increases at the level of the latent variables (common factors), which IQ tests purport to measure". In other words, according to this study, some of the inter-generational difference in IQ is attributable to bias or other artifacts, and not real gains in general intelligence or higher-order ability factors.

In the end, a number of varied phenomena may be contributing to the Flynn effect.

Contrary evidence
The Flynn effect may have ended in some places starting in the mid 1990s. Teasdale & Owen (in press) "report intelligence test results from over 500,000 young Danish men, tested between 1959 and 2004, showing that performance peaked in the late 1990s, and has since declined moderately to pre-1991 levels." They speculate that "a contributing factor in this recent fall could be a simultaneous decline in proportions of students entering 3-year advanced-level school programs for 16–18 year olds."

Another recent study done by Professor of Education Philip Adey and psychology professor Michael Shayer also show that the Flynn effect may have ended. According to Professor Adey, “The intelligence of 11-year-olds has fallen by three years’ worth in the past two decades.” The study compared results of IQ tests taken by 11 year old children in 2005, the mid 1990s, and 1976, showing a precipitous drop in average IQ.