Average 1815 Americans would score below IQ 70 on today’s scale.

Evidence

Now, if we were to begin moving back in time, century by century, picking adults at random from all living humans and seeing what culturally acquired mental tools they had in their heads, we’d find that the ones now in your head would gradually disappear. Often, we’d find other very cool mental tools and abilities, such as the mental abacus, improved underwater vision, tons of subtle farming heuristics, mental maps for spotting animal tracks, and an acute sense of smell. As I’ve noted, many of these mental tools come with some costs. For example, acquiring eleven basic color terms improves our ability to distinguish between colors with different verbal labels, although it also degrades our ability to distinguish shades with the same label. And although biological categories can highlight important relationships (penguins are a type of bird, so they lay eggs), these categories can also conceal other relationships (penguins are aquatic, so they have solid bones, unlike most other birds). Nevertheless, it’s still the case that past peoples would be less good at doing many of the things we now think are “smart.” For example, if we measured IQ scores and scaled them using today’s scale, we’d find that at our second stop, in in 1815, the average American would have an IQ below 70. — Joseph Henrich, The Secret of Our Success, 1035

Categorization

• QUESTION: Worth chasing: is the below-70 figure a real finding, and what were the methodology and historical estimates?