Between 1962 and 1991, more than 700,000 Norwegian men sat the same IQ test before military service. Their results are now one of the clearest records of a reversal that researchers still cannot explain.
Scores climbed with each cohort until the men born around 1975. After that they fell. Economists Bernt Bratsberg and Ole Rogeberg reported the pattern in the Proceedings of the National Academy of Sciences in 2018, and they put the later drop at roughly 0.2 points a year.
Genes ruled out
The two economists compared brothers raised in the same families. The rise, the peak and the decline all showed up inside families, which means the shift cannot be pinned on immigration or a change in who became a parent. Their conclusion was that environment drove the gains and is behind the losses too.
That fits the broader picture. Nearly every researcher agrees that a swing this large across one or two generations cannot be genetic.
The Flynn effect and its limits
For most of the 20th century, each generation scored a bit higher than the last. The pattern is named after the political scientist James Flynn of the University of Otago, who died in 2020. A 2015 meta-analysis by Jakob Pietschnig and Martin Voracek of the University of Vienna covered 271 samples and nearly 4 million participants from 31 countries. It found gains from 1909 to 2013.
Flynn never claimed people had become innately smarter. His 1987 paper argued that IQ tests have only a weak causal link to intelligence itself.
Not just Norway
Denmark shows a similar arc. Thomas Teasdale and David Owen, writing in Intelligence, examined over 500,000 Danish men tested between 1959 and 2004. Performance peaked in the late 1990s and slid afterward. The authors ruled out nutrition and pointed to how schooling emphasized abstract reasoning.
In Finland, Edward Dutton and Richard Lynn studied roughly 25,000 conscripts. Scores rose about 4.0 points a decade from 1988 to 1997, then fell about 2.0 points a decade to 2009. The study found strong evidence of reversal mainly in Finland.
The United States is harder to read. A 2023 study led by Elizabeth Dworak of Northwestern University’s Feinberg School of Medicine looked at 394,378 Americans surveyed from 2006 to 2018. Verbal and matrix reasoning scores dropped, while spatial reasoning rose. Dworak said: “It doesn’t mean their mental ability is lower or higher; it’s just a difference in scores.”
What is left on the table
No single cause has been confirmed. Education changes and declining reading habits are candidates. Psychologist Jean Twenge of San Diego State University found that the share of 12th graders reporting no pleasure reading nearly tripled between 1976 and 2016, to one in three.
Attention is a weaker suspect. A 2024 meta-analysis in Personality and Individual Differences, by Andrzejewski, Zeilinger and Pietschnig, found moderate gains in adult concentration performance.
The reversal does not mean people are suddenly less intelligent. The numbers are not in dispute. The reason they are moving is.
Sources
- After Decades Of Rising IQ Scores, A Puzzling Decline Is Emerging
- IQ Scores Began Falling in The 1990s as The Flynn Effect Reversed, Latest Study Confirms – North American Community Hub
