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A study of 730,000 Norwegian conscripts found the IQ gains of the 20th century reversed after 1975, falling by roughly seven points a generation, and ruled out genetics as the cause by comparing brothers raised in the same family
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ScienceBlog.com
For most of the 20th century, one of the most reliable trends in psychology was that intelligence test scores kept rising.
Bratsberg and Rogeberg drew on 730,000 of those test results, covering men born between 1962 and 1991 and tested between 1970 and 2009.
It is worth being specific about what the test measures, since “intelligence test” can suggest more than the data actually supports.
Men born later scored lower, on average, than the men born just before them, a decline the researchers put at roughly seven points across a generation.
The Flynn effect and its reversal are a reminder that the number the tests produce, whatever it is measuring, is not fixed.