Researchers tracked down the four-year-olds who participated in the legendary "Stanford Marshmallow Delayed Gratification Experiment" of 1972 (recently recreated above) to gauge whether they're still the type to go for the single marshmallow right away, or whether they've learned to hold out for that second marshmallow. Since "marshmallows aren't as irresistible to adults," though, the participants — now in their 40s — were instead shown pictures of happy and sad faces, but how that part works is actually still a little confusing. ("[T]he researchers asked participants to react to a series of emotional pictures, primarily happy and sad faces. 'The happy face took the place of the marshmallow,' said ... [the] lead author of the study. 'The positive social cue interfered with the low delayers' ability to suppress his or her actions.'" Wait, what?)
But if the science of happy/sad faces is to be trusted, the updated experiment apparently shows that people who can't delay gratification when they're four can't do it when they're adults, either. So, give your kid a marshmallow, see what happens.