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... Those small gains added up, meaning that someone's genius-level great-grandparents might only be considered a bit smart by their own generation.

This increase was called the "Flynn Effect" after the educator James Flynn, who published about it in 1984. ...

What caused it?

Hypotheses ranged from population-wide changes like better nutrition, fewer siblings, earlier education, more complex environments, and genetic effects (unlikely, since genes take longer than a few generations to change).

Increasing IQ was seen as a positive thing, since it's correlated with higher educational attainment, better jobs, and even a longer, healthier life. ...

It was expected that this increase would keep going, even if at a slower pace, as humanity advanced.

And then people started getting 'dumber'.

Or at least, their scores on an IQ test dropped. In Europe, several countries reported declining intelligence as determined by standardized tests.

To find out whether the same pattern is emerging in the US, social scientist Elizabeth Dworak made that question the topic of her master's thesis at Northwestern University. She and her colleagues analyzed cognitive test scores from nearly 400,000 American adults collected between 2006 and 2018. ...