Are schools failing us? Are we getting dumber?
What if I told you that the population mean was 100 throughout the entirety of the series in this image? Well, it really is.
The only thing that changed was that education increased. As education increased, some of the more intelligent people who would have attained lower levels of educational attainment moved up to the higher ones, bringing both groups down in the process. This is the “Will Rogers Phenomenon,” best encapsulated by the whimsical line “When the Okies left Oklahoma and moved to California, they raised the intelligence level in both states!”
The Will Rogers phenomenon is a real thing that shows up all the time. For example, during the Great Migration, African American migrants who left the South had more European admixture than those who stayed behind, but less European admixture than those already living in the North. You might have seen this recently: