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Willpower is out of fashion these days.

In fact, as we learn more and more about the subtle and powerful ways that our bodies and brains conspire to control our behaviour, it seems downright impolite to mention self-control in the context of exercise or eating habits. That's blaming the victim for forces beyond his or her control.

But the latest update from a classic behavioural psychology experiment that started more than four decades ago offers a surprising twist. It turns out that your ability to delay gratification as a four-year-old can partly predict your weight decades later, adding to growing evidence that helping kids improve their "cognitive control" could have powerful impacts on their later lives.

The "Marshmallow Test" was pioneered by Stanford University psychologist Walter Mischel, who tested 653 four-year-olds between 1968 and 1974. There were several variations, but the standard version involved offering the preschoolers a choice: They could have one marshmallow (or similar treat) immediately, or they could wait 15 minutes and then receive two marshmallows.

The subjects showed a wide variation in the ability to delay gratification: one-quarter of them managed to wait the full 15 minutes, while nearly half lasted less than three minutes. The average time was six minutes and 21 seconds.

The most interesting results of the experiment began to emerge in subsequent decades. Follow-up surveys found clear correlations between the marshmallow scores and outcomes like academic performance. Similar studies linked childhood self-control to financial stability, and even the likelihood of being convicted of a crime.

Most recently, a group of researchers including Dr. Mischel surveyed 164 of the original participants to find out their body-mass index (BMI), publishing the results in the Journal of Pediatrics. The results: Every additional minute a subject lasted on the Marshmallow Test predicted a lower BMI 30 years later by 0.2, corresponding to about half a kilogram of weight.

So does this confirm the well-worn claim that obesity results from a failure of willpower? Hardly. The researchers estimate that differences in delayed gratification account for just 4 per cent of the overall variation in BMI among subjects. The brain does play a role, but it's clearly not a dominant one.

Still, the results raise the question of whether these differences are hard-wired, or whether efforts to enhance delay of gratification and other forms of cognitive control can help you lose weight (or stay out of jail).

"There is a new body of research that shows cognitive control can indeed be improved, both in children and in adults," says Ozlem Ayduk, a professor of psychology at the University of California, Berkeley, and the corresponding author of the new study.

Much of the initial research focuses on computer games designed to improve working memory, a form of cognitive control that's related to but distinct from delay of gratification. For young children, games like Red Light, Green Light and Simon Says have been shown to improve self-control over the course of a school year. Other successful interventions include aerobic exercise, which appears to affect the physical structure of the brain itself, and traditional martial arts like tae kwon do, which combine physical activity with meditative activities that enhance self-control and mindfulness.

Ultimately, even if you're able to successfully boost your impulse control using these techniques, it's not a miracle fix. There's plenty of evidence that obesity is multifactorial; you need to make choices that you can live comfortably with about what you eat, how you spend your leisure time, how you handle stress, and so on.

That's why initiatives like New York's proposed ban on mega-sized sodas make sense, says University of Wisconsin psychologist Tanya Schlam, the study's lead author. "The best self-control is setting up a situation where you don't need to exercise self-control," she says.

Nonetheless, the new study is a powerful retort to the overly fatalistic view of body weight that's currently popular – the idea that we're mere slaves to our appetites, incapable of overcoming hardwired biological imperatives. It's not easy, but sometimes it's possible, and worthwhile, to resist the marshmallow.

Alex Hutchinson blogs about research on exercise at His latest book is Which Comes First, Cardio or Weights?

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