Since 2000, Northwestern Medicine has tracked 290 volunteers over age 80 using a 15-word memory test, and after 25 years the ones who still score like adults in their fifties and sixties turn out to share one habit above the rest: close, frequent contact with other people.

After tracking cognitive superstars in their eighties for 25 years, researchers found the one thing they all have in common—and it isn’t what you’d expect.