I know that I shouldn’t feed my two daughters, who are eight and twelve, dinner at 7:30. It’s too late. But my wife and I are overscheduled and sometimes it just happens. And so, a few weeks ago, faced with yet another late meal, my younger daughter fell into one of those anger vortices. Annoyed at a perceived inequity in chore distribution, she slammed my glass of soda onto the counter, somewhat inadvertently splattering the liquid onto the floor and me. She told me, “You’re not being nice! You’re being a stupid old parent who knows nothing!” Maybe so: when these evening outbursts occur, I feel responsible. After all, as a parent I’m supposed to provide them with timely calories.
Shortly after this particular blowup, I came across a study in which men and women stuck pins—sometimes many pins—into voodoo dolls as a measure of their resentment and annoyance with their spouses. The researchers found that the subjects were more likely to stick in lots of pins when their glucose level was low. I was intrigued—the findings seemed to illuminate my experience with my kids (and people in general), and offered a clear solution: eat regularly, enhance self-control. My response was common, I think, because the study got a lot of attention—stories on the “Today” show, ABC News, NPR, and lots of newspapers around the world. Part of the study’s appeal is its obviousness: anyone with kids, or a partner or friends or any level of self-awareness, probably knows on some level that hunger begets irritability. It’s somehow comforting when science confirms what we already know, as if the researchers are just now catching up to our common sense.
“Self-control is a limited resource,” says the Ohio State psychologist Brad Bushman, who led the study. “With less glucose the brake on self-control is weakened.” He notes that his research has found that diabetics, who have trouble keeping their blood sugar levels stable, tend to have higher-than-average levels of aggression—in other words, a paucity of self-control. He recommends eating regularly to insure that self-control doesn’t flag. The glucose theory had its origins with a well-known (at least in the field) 2007 study in which subjects were given lemonade flavored with sugar or Splenda, which tastes sweet but doesn’t increase blood glucose. In an experiment that tested self-control by asking people to keep their focus during a difficult mental task, the people who drank sugary lemonade did better than those who drank the lemonade with Splenda.
But do we really know what we think we know about blood sugar and the brain? For years, the University of Pennsylvania psychology researcher Robert Kurzban has been watching the glucose theory garner positive press. He finds this deeply frustrating. “It’s a very entertaining idea,” he says, “but as a scientist I feel like we should think about whether it is correct.” His views on that question are clear: the glucose theory is, he says, “simplistic and implausible.” The idea, Kurzban and other respected scientists argue, doesn’t reflect the reality of how the brain works or how humans have evolved.
He and other doubters don’t deny that hunger affects mood. This is obvious, they say. But it doesn’t prove that glucose fuels self-control, or that low glucose causes a malfunction of self-control. For one, Kurzban told me, the brain rarely runs short of glucose. Although it uses more calories than most other organs, the brain doesn’t consume that much energy—a little more than ten calories an hour. In a study published in 2012, the Northwestern University social psychologist Daniel Molden found that mental activity doesn’t consume extra glucose. This indicates that self-control probably doesn’t depend on calorie levels. Molden and others that I talked to also say that self-control is a complex behavior that involves many brain areas working in concert. “The idea of a simple one-stop explanation for self-control is flawed on so many levels,” says Chris Beedie, a psychology researcher at Aberystwyth University, in Wales. “The brain is not a simple machine.”
And because the brain is indispensable to survival, we have evolved so that gray matter continues to receive a steady supply of energy even when we run short of calories. Studies have found that in people who are starving, the brain is among the last organs to lose mass. “The brain is rarely in a situation where it doesn’t have enough glucose,” says Beedie. “If the brain doesn’t have enough, that’s a fairly serious situation.”
Beedie, who studies the psychology of how athletes reach maximum performance, pointed to the London Marathon as a good example of how the brain protects itself. “At the end of the race, all of these runners have low levels of glucose. That’s almost forty thousand people,” he says. “But you don’t see a mass loss of self-control.”
Marathons haven’t been the only situations in which humans get really low on energy. Michael McCullough, a psychologist at the University of Miami, says that for most of the two hundred thousand or so years that humans have been on earth, lack of food has been a regular part of their experience. Not occasional dinner-is-an-hour-late hunger, but weeks or months of profound, gnawing deprivation. Nearly every traditional society ever studied endures this. It is very unlikely, he argues, that evolution would have allowed for the survival of a species that easily loses control when it gets hungry.
“We have three weeks of food in our kitchens,” McCullough says. “But that’s not how we evolved. It doesn’t make sense that cognition is so fragile that two hours after your last meal, thinking goes haywire. I don’t think natural selection would have been kind to humans whose brains shut down whenever they got hungry.”
I asked him how he would explain my kids’ tendency to insurrection when they’re hungry. “I am perfectly willing to believe that hunger is linked to angry outbursts in your kids,” he said. “If a child or animal is running low on blood glucose, it will act differently.” But this, he points out, does not prove that glucose regulates self-control. The change in behavior could be a direct, perhaps unconscious, way to signal hunger. (If that’s true, the strategy works, at least in our house. Outbursts usually lead to a pre-dinner snack.) Or it may be a vestigial sign that the organism is sick of being hungry, and is getting ready go find some food on its own.
Kurzban notes that many animals get more aggressive when they’re hungry, but not because they have lost self-control. Their behavior is a rational choice to more pursue food in a more zealous way, he says: “I think the link between hunger and aggression is an artifact from our evolutionary past.”
The Princeton neuroscientist Matthew Botvinick sees lack of glucose as just one reason among many that we become unglued. He says that we should think of hunger not as a lack of fuel, but as an unpleasant state, no different from other such states: having a headache, doing a tedious chore, having to stay late at work. Such experiences tax us, and thus make us less willing to devote energy to regulating our moods and responses. Molden says that the distinction is crucial. It’s not that we are unable to control ourselves because we’ve run out of fuel; it’s that we choose not to control ourselves because we’re less willing to do so. So when his eight-year-old son starts acting out, Molden focusses not on calories but on persuasion. “I try to engage him,” he says. “I acknowledge his lack of motivation to behave. I say ‘I know you don’t feel like doing this. But if you do what you have to, you can do what you want.’”
To be fair, even the proponents of the glucose idea agree that self-control is multifaceted and not solely regulated by glucose. But to Molden, giving so much weight to glucose can lead to flawed strategies for activating self-control in real life. It minimizes the fact that humans, with their big, complex brains, have a unique ability to direct their own behavior and override their own impulses. “You can’t throw fuel at self-control problems,” he says. “Talking about motivation, helping people focus, that’s not easy, but it can work. It is much more effective than giving people snacks.”
Illustration by Laurie Rosenwald.
