Give a four-year-old a marshmallow, and she’ll eat it, no hesitation. Unless she’s promised a second if she waits 15 minutes before eating the first. Stanford psychologist Walter Mischel tested the ability of kids to delay gratification in this way in the 1960’s. Behind the one-way mirror, Mischel noticed that some kids—I’ll call them the planners—might squirm and sniff and squeeze the prize but ultimately managed to resist temptation. Others could not. They gobbled right away.