[Affluence—not willpower—seems to be what’s behind some kids’ capacity to delay gratification.] The marshmallow test is one of the most famous pieces of social-science research: Put a marshmallow in front of a child, tell her that she can have a second one if she can go 15 minutes without eating the first one, and then leave the room. Whether she’s patient enough to double her payout is supposedly indicative of a willpower that will pay dividends down the line, at school and eventually at work. Passing the test is, to many, a promising signal of future success. But a new...
