For example, a study published in the year 2009 found that British children's average scores on the Raven's Progressive Matrices test rose by 14 IQ points from 1942 to 2008.

[13][14][15] Flynn stated that, if asked, he would have named the effect after Read D. Tuddenham[16] who "was the first to present convincing evidence of massive gains on mental tests using a nationwide sample"[17] in a 1948 article.

[22] In contrast, Pietschnig and Voracek (2015) reported, in their meta-analysis of studies involving nearly 4 million participants, that the Flynn effect had decreased in recent decades.

[23] In 1987, Flynn took the position that the very large increase indicates that IQ tests do not measure intelligence but only a minor sort of "abstract problem-solving ability" with little practical significance.

He argued that if IQ gains did reflect intelligence increases, there would have been consequent changes of our society that have not been observed (a presumed non-occurrence of a "cultural renaissance").

Historian Daniel C. Calhoun cited earlier psychology literature on IQ score trends in his book The Intelligence of a People (1973).

[28] Robert L. Thorndike – not to be confused with his famous father Edward – drew attention to rises in Stanford-Binet scores in a 1975 review of the history of intelligence testing.

On average, the scores of African-American children who received no formal education during that period decreased at a rate of about six IQ points per year.

One problem with this explanation and others related to schooling is that in the US, the groups with greater test familiarity show smaller IQ increases.

The "Abecedarian Early Intervention Project", an all-day program that provided various forms of environmental enrichment to children from infancy onward, showed IQ gains that did not diminish over time.

[20][38] This argument had been thought to suffer the difficulty that groups who tend to be of smaller overall body size (e.g. women, or people of Asian ancestry) do not have lower average IQs.

They were either in the womb or were recently born, during the great Dutch famine of 1944—when German troops monopolized food and 18,000 people died of starvation.

[citation needed] In support of the nutritional hypothesis, it is known that, in the United States, the average height before 1900 was about 10 cm (~4 inches) shorter than it is today.

[44] Both height and skull size increases probably result from a combination of phenotypic plasticity and genetic selection over this period.

[45] With only five or six human generations in 150 years, time for natural selection has been very limited, suggesting that increased skeletal size resulting from changes in population phenotypes is more likely than recent genetic evolution.

[46] Scientists James Feyrer, Dimitra Politi, and David N. Weil have found in the U.S. that the proliferation of iodized salt increased IQ by 15 points in some areas.

"[47] Daley et al. (2003) found a significant Flynn effect among children in rural Kenya, and concluded that nutrition was one of the hypothesized explanations that best explained their results (the others were parental literacy and family structure).

From pictures on the wall to movies to television to video games to computers, each successive generation has been exposed to richer optical displays than the one before and may have become more adept at visual analysis.

Environmental changes resulting from modernization—such as more intellectually demanding work, greater use of technology, and smaller families—have meant that a much larger proportion of people are more accustomed to manipulating abstract concepts such as hypotheses and categories than a century ago.

Malaria prophylaxis was shown to improve cognitive function and school performance in clinical trials when compared to placebo groups.

[55] Heterosis, or hybrid vigor, associated with historical reductions of the levels of inbreeding, has been proposed by Michael Mingroni as an alternative explanation of the Flynn effect.

[56][57] However, James Flynn has pointed out that even if everyone mated with a sibling in 1900, subsequent increases in heterosis would not be a sufficient explanation of the observed IQ gains.

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.

[62] In the United Kingdom, a study by Flynn (2009) himself found that tests carried out in 1980 and again in 2008 show that the IQ score of an average 14-year-old dropped by more than two points over the period.

Flynn argues that the abnormal drop in British teenage IQ could be due to youth culture having "stagnated" or even dumbed down.

[63] Bratsberg & Rogeberg (2018) present evidence that the Flynn effect in Norway has reversed between the years 1962–1991, and that both the original rise in mean IQ scores and their subsequent decline within this period can be observed within families consisting of native-born parents and their children, indicating that environmental factors were the likely cause for these changes.

The authors identified various novel factors including social media dependency and the COVID-19 pandemic which may have contributed to a reduced Flynn effect.

[61] Also, if the Flynn effect has ended for the majority in developed nations, it may still continue for minorities, especially for groups like immigrants where many may have received poor nutrition during early childhood or have had other disadvantages.

[69] Reviews by Flynn and Dickens,[70] Nicholas Mackintosh,[71][page needed] and Nisbett et al.[72][73] all concluded that the gradual closing of the gap was a real phenomenon.

[75] Flynn also argued that his findings undermine the so-called Spearman's hypothesis, which hypothesized that differences in g factor are the major driver of the blacks-whites IQ gap.