Any visitor to North Carolina and California will know that the two states have their differences. The former is a typically “red state”; California is staunchly “blue.” Each has certain geographic, ethnic and cultural peculiarities, different demographic makeup, family income levels, and more. Yet perhaps the most surprising divide, one many wouldn’t expect, is that North Carolina appears to be a hotbed for attention deficit hyperactivity disorder, or ADHD—especially when compared to California. A child who lived in North Carolina instead of California in 2007, according to U.S. academics Stephen Hinshaw and Richard Scheffler, was 2½ times more likely to...