Mean scores on IQ tests rose generally in the U.S. beginning in the 1930s, apparently reflecting better nutrition and improved schooling. But that trend has now been reversed. A paper published in an upcoming journal finds IQ scores now declining:
Labeled the Flynn effect (Herrnstein and Murray, 2010), intelligence quotient (IQ) scores substantially increased since 1932 and through the twentieth century, with differences ranging from 3.0 to 5.0 IQ points (0.20 to 0.33 SD) per decade (Flynn, 1984, Flynn, 1987, Flynn, 2007). These findings imply younger generations are expected to have higher IQ scores than the previous cohort. For example, if we tested a sample of Baby Boomers (born between 1946 and 1964) when they were 20 years old and compared their scores on the same test to a sample of Millennials (born between 1981 and 1996) tested at age 20, we would expect the latter group’s IQ scores to be between 0.66 and 1.1 SD higher.
Those who have experience with Millenials might not expect any such thing.
Not all aspects of intelligence have been rising equally:
[T]he magnitude for the differences in scores often discussed for the Flynn effect over the last century are more related to problem solving skills and abstract reasoning.
The linked article is very long and quite technical. It finds that average IQ scores have recently been falling rather than rising:
Regardless of education, gender, and age, lower annual scores were observed for composite cognitive ability measured by 35 items, and the matrix reasoning and letter and number series scores measured across the 13 years of assessment. These differences were replicated across the 60-item composite ability scores from 2011 to 2018, however, three-dimensional rotation scores measured during this 8-year period showed evidence of a Flynn effect of varying magnitudes across 18- to 60-year-olds. The largest differences in mean ability scores were often observed for participants between the ages of 18 to 22. Beyond age, a reverse Flynn effect was also present across all levels of educational attainment, with the rate of decreasing scores being steeper for those with less than a 4-year college degree.
Some speculate that declining IQ scores may reflect worse schools. Certainly our education system has gone badly downhill, and that may account for the trend. I suspect that other aspects of our culture may be more to blame, however.