The average American in the year 1900 had an I.Q. that by today’s standards would measure about 67. Since the traditional definition of mental retardation was an I.Q. of less than 70, that leads to the remarkable conclusion that a majority of Americans a century ago would count today as intellectually disabled.

The trend of rising intelligence is known as the “Flynn Effect,” named for James R. Flynn, the New Zealand scholar who pioneered this area of research. Countless other scholars worldwide have replicated his findings, and it is now accepted science — although there is still disagreement about its causes and significance.

The average American I.Q. has been rising steadily by 3 points a decade. Spaniards gained 19 points over 28 years, and the Dutch 20 points over 30 years. Kenyan children gained nearly 1 point a year.