n 1987, political scientist James Flynn of the University of Otago in New Zealand documented a curious phenomenon: broad intelligence gains in multiple human populations over time. Across 14 countries where decades’ worth of average IQ scores of large swaths of the population were available, all had upward swings—some of them dramatic. Children in Japan, for example, gained an average of 20 points on a test known as the Wechsler Intelligence Scale for Children between 1951 and 1975. In France, the average 18-year-old man performed 25 points better on a reasoning test in 1974 than did his 1949 counterpart.1
Flynn initially suspected the trend reflected faulty tests. Yet in the ensuing years, more data and analyses supported the idea that human intelligence was increasing over time. Proposed explanations for the phenomenon, now known as the Flynn effect, include increasing education, better nutrition, greater use of technology, and reduced lead exposure, to name but four. Beginning with people born in the 1970s, the trend has reversed in some Western European countries, deepening the mystery of what’s behind the generational fluctuations. But no consensus has emerged on the underlying cause of these trends.