People are getting dumber. That's not a judgment; it's a global fact. In a host of leading nations, IQ scores have started to decline.

Though there are legitimate questions about the relationship between IQ and intelligence, and broad recognition that success depends as much on other virtues like grit, IQ tests in use throughout the world today really do seem to capture something meaningful and durable. Decades of research have shown that individual IQ scores predict things such as educational achievement and longevity. More broadly, the average IQ score of a country is linked to economic growth and scientific innovation.

For a while, rising IQ scores seemed like clear evidence of social progress, palpable proof that humanity was getting steadily smarter — and might even be able to boost brainpower indefinitely. Scholars called it the "Flynn effect," in homage to J.R. Flynn, the researcher who recognized its full sweep and import.

These days, however, Flynn himself concedes that "the IQ gains of the 20th century have faltered." A range of studies using a variety of well-established IQ tests and metrics have found declining scores across Scandinavia, Britain, Germany, France and Australia.