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Ever wonder why your willpower fails you just when you need it most? The results of a new long-term study, which first began more than 40 years ago with the now-famous marshmallow test in preschoolers, may offer some clues.

In the late 1960s, researchers submitted hundreds of four-year-olds to an ingenious little test of willpower: the kids were placed in a small room with a marshmallow or other tempting food and told they could either eat the treat now, or, if they could hold out for another 15 minutes until the researcher returned, they could have two.

Most children said they would wait. But some failed to resist the pull of temptation for even a minute. Many others struggled a little longer before eventually giving in. The most successful participants figured out how to distract themselves from the treat’s seduction — by turning around, covering their eyes or kicking the desk, for instance — and delayed gratification for the full 15 minutes.

Follow-up studies on these preschoolers found that those who were able to wait the 15 minutes were significantly less likely to have problems with behavior, drug addiction or obesity by the time they were in high school, compared with kids who gobbled the snack in less than a minute. The gratification-delayers also scored an average of 210 points higher on the SAT.

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Scientists have now continued this research with a subgroup of 60 participants, now in their 40s, whose level of self-control remained stable from childhood into adulthood. (The study does not include those who struggled to delay gratification early on, but learned to master temptation as they matured.)

The authors, led by B.J. Casey, professor of developmental psychobiology at Cornell University, wanted to know whether the kids who couldn’t wait to get the treat, the “low delayers,” would show an underlying inability to exert self-control in adulthood, or whether their self-discipline failed only in certain, emotionally charged cases. “Is it the case that these individuals have difficulty controlling their impulses or is it also due to sensitivity to just how alluring the cues are?” Casey says.

The researchers also wanted to see if low-delaying adults showed differences in brain activity compared with those who had greater self-control.

Participants were asked to perform two versions of a computer task. In one version, the volunteers had to press a button when they saw an image of either a male or a female face bearing a neutral expression. Those who were first asked to identify male faces would eventually be switched to identifying female faces (and vice versa), which required participants to suppress their impulse to respond to the previously correct answer. This test measured people’s ability to exert self-control in the absence of emotional material.

In the second, emotionally “hot” version of the test, people were asked to press a button in response to a happy or fearful face. Previous research has shown that people find seeing happy faces rewarding — so the researchers expected low delayers to have trouble preventing their responses to them when required. Indeed, the high delayers were significantly better at not pressing the button when a smiling face flashed on the screen.

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Researchers also scanned the brains of 26 of the participants while they completed both tests. They found that high delayers showed more activity than low delayers in a region of the prefrontal cortex associated with impulse and behavior control, particularly while completing the task involving emotionally charged expressions. Meanwhile, low delayers showed more activation of a deeper region of the brain associated with pleasure, desire and addiction, especially in response to the happy faces.

You might say that high delayers have better mental brakes, while low delayers are driven by a stronger engine. “The low delayers don’t tend to activate the prefrontal cortex as much as the high delayers do. The high delayers are very effective at being able to regulate their behavior and not activating this deep system,” Casey says. “There’s not as much of a push-and-pull for the high delayers.”

But the differences emerged primarily when people were faced with “hot,” emotional cues, not when they had to choose between neutral faces. That suggests that low delayers aren’t globally unable to control impulses, like people with attention deficit/hyperactivity disorder (ADHD); rather, they have trouble stopping themselves in the context of particular temptations.

“What the [researchers] found is that their 11 participants who had lifelong low self-control did not have a problem with controlling their brains’ actions in ordinary circumstances,” notes Terrie Moffitt, professor of psychology and neuroscience at Duke University, who was not associated with the study. “Instead, their distinctive brain activity started to emerge only when something alluring and rewarding was shown to them.”

Moffitt says she has had similar results in her own study of 1,000 people whom she followed from age 3 to 38.

Both Moffitt and Casey stress that low delayers don’t lack general intelligence and that they have qualities that are extremely important to society. In uncertain times, delaying gratification can be the wrong choice, and people who follow their emotional impulses can become great explorers or entrepreneurs. (Apple’s Steve Jobs is a classic example).

But there’s no doubt that low delayers can also get into serious trouble. Moffitt describes one such participant who told her, “Believe me, I understand all about saving for retirement, but I haven’t saved any money because when I see a hot motorcycle, I buy it!”

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But there’s nothing that says low delayers can’t change. Moffitt’s research has found that in fact, childhood self-control isn’t very strongly correlated with adult self-mastery. For example, the “correlation between participants’ rank on IQ tests from childhood to adulthood is .70 on a scale from -1 (total change) to +1 (total stability). That’s stable!” Moffitt says. “But the same correlation for self-control skills is only .30, indicating that many people change as they grow up, and fewer people remain stable.”

Previous work with the preschoolers and the marshmallow test showed that high-delaying kids often found success not only by distracting themselves from tempting situations but also by reframing them to lower their emotional temperature. For example, they would imagine that the marshmallow was a cloud or just a picture of a marshmallow instead of an actual edible treat.

Perhaps these same techniques can be employed in other situations. “What this study may suggest is that we should inform people about ways in which we can ‘cool down’ the ‘hot’ cues for those individuals who have the most difficulty suppressing actions toward them,” Casey says.

The research was published in the Proceedings of the National Academy of Sciences.

Maia Szalavitz is a health writer for TIME.com. Find her on Twitter at @maiasz. You can also continue the discussion on TIME Healthland‘s Facebook page and on Twitter at @TIMEHealthland.