The Flynn effect is the increase in intelligence test scores that were measured in many parts of the world over the 20th century.
Intelligence quotient (IQ) tests are initially standardized using a sample of test-takers, by convention the average of the test results is set to 100 and their standard deviation is set to 15 or 16 IQ points.
When IQ tests are revised, they are standardized using a new sample of test-takers, usually born more recently than the first.
The average result is set to 100.
When the new test subjects take the older tests, in almost every case their average scores are significantly above 100.
Test score increases have been continuous and approximately linear from the earliest years of testing to the present.
IQ tests are updated periodically.
The average rate of increase seems to be about three IQ points per decade in the United States, as scaled by the Wechsler tests.
The increasing test performance over time appears to have occurred in every age range, at every ability level, and in every modern industrialized country.
A similar effect has been found with increases in attention and of semantic and episodic memory.
Some studies found the gains of the Flynn effect to be particularly concentrated at the lower end of the distribution.
Some studies have found a reverse Flynn effect with declining scores for those with high IQ.
Debates about whether the rise in IQ scores also corresponds to a rise in general intelligence, or only a rise in special skills related to taking IQ tests.
While test scores have improved over time, the improvement is not fully correlated with latent factors related to intelligence.
Flynn effect may be due in part to increasing intelligence, and in part to increases in test-specific skills.
Some suggest that the IQ gains reflect changes in modes of thinking that better reflected cognitive skills assessed by IQ tests rather than raw intelligence itself.
Children who do not attend school score drastically lower on the tests than their regularly attending peers.
There is an increased familiarity of the general population with tests and testing.
However the problem with this explanation and others related to schooling is that in the US, the groups with greater test familiarity show smaller IQ increases.
Some preschool (ages 3–4) intervention programs like “Head Start” do not produce lasting changes of IQ.
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.
The IQ gains in the experimental group compared to the control group was 4.4 points.
There is a high correlation between rising literacy rates and gains in IQ.
Changes in the human intellectual environment have come from the increase of exposure to many types of visual media, and individuals may have become more adept at visual analysis.
The Flynn effect can be explained by a generally more stimulating environment for all people, as those with a greater IQ tend to seek stimulating environments that further increase their IQ.
Environmental changes resulting from modernization: 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, as major portions of IQ tests deal with these abilities.
Improved nutrition is another possible explanation for the FLYNN effect.
Today’s average adult from an industrialized nation is taller than a comparable adult of a century ago, likely the result of general improvements in nutrition and health, has been at a rate of more than a centimeter per decade.
Similarly increases in head/brain size have occurred, however groups who tend to be of smaller overall body size (e.g. women, or people of Asian ancestry) do not have lower average IQs.
The nutrition hypothesis predicts that IQ gains will occur predominantly at the low end of the IQ distribution, where nutritional deprivation is probably most severe.
However, it appears that the effects of diet are gradual, taking effect over decades affecting mother as well as the child, rather than a few months.
In the United States, the average height before 1900 was about 4 inches shorter than it is today.
Possibly related to the Flynn effect is a similar change of skull size and shape during the last 150 years.
Height gains are strongly correlated with intelligence gains.
Both height and skull size increases are a result of a combination of phenotypic plasticity and genetic selection over this period.
Micronutrient deficiencies change the development of intelligence:
iodine deficiency causes a fall, on average, of 12 IQ points.
In the U.S. that the proliferation of iodized salt increased IQ by 15 points in some areas.
Infectious diseases-a developing human will have difficulty building a brain and fighting off infectious diseases at the same time, as both are very metabolically costly tasks: the Flynn effect may be caused in part by the decrease in the intensity of infectious diseases as nations develop.
It is suggested that improvements in gross domestic product (GDP), education, literacy, and nutrition may have an effect on IQ mainly through reducing the intensity of infectious diseases.
States with a higher prevalence of infectious diseases have lower average IQ.
A drop in blood lead levels in the United States has correlated with a 4-5 point increase in IQ.
Presently there is evidence that the Flynn effect has reversed, with a worldwide decline in intelligence, suggesting that both the original rise in mean IQ scores and their subsequent decline was caused by environmental factors.
The IQ gap between black and white people was gradually closing over the last decades of the 20th century, as black test-takers increased their average scores relative to white test-takers.
The Flynn effect suggests the observed differences in average IQ test performance between blacks and whites, is because of environmental factors.