People are getting dumber. So concludes a new study in the Proceedings of the National Academy of Sciences (PNAS): Using military conscription data from Norwegian males born from 1962 and 1991, the authors find that the secular increase in population intelligence observed throughout the 20th century has peaked and has now gone into reverse.

Average IQs, as measured on standardized intelligence tests, increased for most of the 20th century. This astonishing fact was first reported in 1984 by the New Zealand political scientist James Flynn, when he analyzed the trend in U.S. test scores. This upward trend has since been dubbed the "Flynn effect." As a 2014 review article in the Psychological Bulletin notes, "The Flynn effect implies that an individual will likely attain a higher IQ score on an earlier version of a test than on the current version." Flynn's study revealed a 14-point increase in IQ scores between 1932 and 1978, amounting to a 0.3-point increase per year, or approximately 3 points per decade. Subsequent research found similar upward trends across most of the rest of the world.

The new PNAS study finds that the average IQ score for conscripts rose from 99.5 for the 1962 birth cohort to 102.3 for the 1975 cohort. Following 1975, the average score trended down, reaching 99.4 for the 1989 cohort (then rising slightly to 99.7 for the 1991 cohort). In other words, average IQ rose at about the same decadal rate identified by Flynn in the first sets of conscripts and then fell at about the same rate in the second sets. The researchers restricted their analysis to individuals born in Norway to two Norwegian-born parents.

About a decade ago various researchers reported that the Flynn effect had begun to reverse in some countries, with average IQ scores starting to decline again. More recently, some observers have suggested that average IQs are coming down because of dysgenic fertility—that is, because less intelligent people are having more children than smarter folks—or because of lower-IQ immigrants and their children. These trends, they argue, are now beginning to swamp the IQ-boosting effects that improvements in nutrition, education, and falling pathogen stress had during the 20th century.

In trying to figure out what could be going on, the Norwegian researchers took advantage of another IQ trend. First-born children tend to have higher IQs than do later-borns. The Flynn effect tends to narrow the gap between first- and later-borns within families. The researchers found that this was in fact occurring in the pre-1975 cohorts. After 1975, the gap between first- and later-born brothers began to grow. Since siblings share genetics and environments, this "within family" decrease tends to rule out dysgenic fertility or immigration as significant explanations for falling average IQs.

For the 1962–1975 Flynn increase period, the researchers estimate a .2 average annual IQ point increase within families and a .18 increase across families. For the 1975–1991 decrease period, they estimate a .33 annual IQ point decline within families and a .34 decline across families.

"The results show that large positive and negative trends in cohort IQ operate within as well as across families," note the researchers. "This implies that the trends are not due to a changing composition of families, and that there is at most a minor role for explanations involving genes (e.g., immigration and dysgenic fertility) and environmental factors largely fixed within families (e.g., parental education, socialization effects of low-ability parents, and family size). While such factors may be present, their influence is negligible compared with other environmental factors."

If falling average IQ scores cannot be attributed to dysgenic or immigration effects, they must be the result of some environmental effects. But what? The researchers conclude that "our results remain consistent with a number of proposed hypotheses of IQ decline: changes in educational exposure or quality, changing media exposure, worsening nutrition or health, and social spill-overs from increased immigration."

As George Mason University economist Tyler Cowan pithily puts it, "We have started building a more stupidity-inducing environment. Or at least the Norwegians have."

On the bright side, a 2018 review article by Flynn and his University of Otago colleague Michael Shayer reports that America continued to show a steady rate of average IQ gain from 1989 to 2014 at about its historic rate of .3 IQ points per year.