The Flynn effect is the year-on-year rise of IQ test scores, an effect seen in most parts of the world, although at greatly varying rates. It was named by Richard Herrnstein and Charles Murray in The Bell Curve after the New Zealand based political scientist James R. Flynn, who did much to document it and promote awareness of its implications (Flynn, 1984, 1987). 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).
IQ scores are re-normalized periodically, such that the average score is reset to 100.
Starting with Thorndike's (1975) discussion of the Binet increases, many possible explanations have been offered but few of these hold up to detailed examination (Neisser, 1998). One possible contributor has to do with nutrition ... but it has proved just as difficult to identify what these may be as in the case of height increases. For example, 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. But the quest for a single-factor explanation of increases in intellectual functioning may be just as unrealistic as a quest for a single factor explanation of the doubling of life expectancy which has occurred over the same period.
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.
Some studies focusing on the distribution of scores have found the Flynn effect to be primarily a phenomenon in the lower end of the distribution. Teasdale and Owen 1987, for example, found the effect primarily reduced low-end scores, resulting in a pile up of moderately high scores, with no increase in very high scores. Colom et al. 2005 found similar results, and 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.
There is, however, very substantial evidence to the contrary - and it comes from an unlikely source. Data re-published in Raven (2000) show that, as Flynn suggested, data reported by many previous researchers that had previously been interpreted as showing a decrease in many abilities with increasing age must be re-interpreted as showing that there has been a dramatic increase in these abilities with date of birth. On many tests this occcurs at all levels of ability. To take an analogy: tall people have got still taller: the whole distribution has moved up.
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.
Some researchers, such as Pioneer fund grantee Arthur Jensen, argue the Flynn effect largely has not changed the general intelligence factor (g), which would mean practical significance of the effect would be limited (Jensen 1987; Rushton 1999). More recent studies have found that g has improved substantially.
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, as with human functioning more generally, a number of varied phenomena may be contributing to the Flynn effect.
The Flynn effect may have ended in some places starting in the mid 1990s. Teasdale & Owen (2005) "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 shows that the Flynn effect may have ended in the United Kingdom. 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.
Some have claimed that the Flynn effect was masking a dysgenic decline in human reproduction and that in developed countries the only direction that IQ scores will now move is downwards. However, if the Flynn effect has ended for the majority, it may still continue for minorities, especially for groups like immigrants where many may have received poor nutrition during early childhood.
- American Psychological Association
- "Changes in vault dimensions must occur by early childhood because of the early development of the vault." Secular change in craniofacial morphology "During the 125 years under consideration, cranial vaults have become markedly higher, somewhat narrower, with narrower faces. The changes in cranial morphology are probably in large part due to changes in growth at the cranial base due to improved environmental conditions. The changes are likely a combination of phenotypic plasticity and genetic changes over this period." Cranial change in Americans: 1850-1975.
- Failing to teach them how to handle real life, Children are less able than they used to be, Intelligence in UK declining?
- For another thoughtful analysis on the topic, this article discusses rising g effectively.
- The Flynn Effect by Indiana University.
- Marguerite Holloway, Flynn's effect, Scientific American, January 1999; online edition
- Increasing intelligence: the Flynn effect
- Flynn biography
- "An Explanation for the Flynn Effect"
- "Heritability Estimates Versus Large Environmental Effects: The IQ Paradox Resolved" - article by Dickens and Flynn
- Heredity, Environment, and Cranial Form: A Reanalysis of Boas’s Immigrant Data
- Did Boas get it right or wrong?