In a new New York Times Magazine piece, John Tierney pulls together the results of several studies that suggest willpower is finite and decisionmaking exhausting. While these findings are important in many ways (Tierney leads off with an example from the criminal justice system), I was especially interested in the implications for dieting.

The whole article is well worth a read, but in a nutshell, researchers have found that subjects’ willpower can be depleted by resisting temptation and be restored by glucose — but not by artificial sweetners that provide less energy. When subjects’ willpower is compromised, the quality of their decisions suffers. Tierney explains what this means for those who are trying to diet:

The discoveries about glucose help explain why dieting is a uniquely difficult test of self-control — and why even people with phenomenally strong willpower in the rest of their lives can have such a hard time losing weight. They start out the day with virtuous intentions, resisting croissants at breakfast and dessert at lunch, but each act of resistance further lowers their willpower. As their willpower weakens late in the day, they need to replenish it. But to resupply that energy, they need to give the body glucose. They’re trapped in a nutritional catch-22:

1. In order not to eat, a dieter needs willpower.

2. In order to have willpower, a dieter needs to eat.

As the body uses up glucose, it looks for a quick way to replenish the fuel, leading to a craving for sugar. After performing a lab task requiring self-control, people tend to eat more candy but not other kinds of snacks, like salty, fatty potato chips. The mere expectation of having to exert self-control makes people hunger for sweets. A similar effect helps explain why many women yearn for chocolate and other sugary treats just before menstruation: their bodies are seeking a quick replacement as glucose levels fluctuate. A sugar-filled snack or drink will provide a quick improvement in self-control (that’s why it’s convenient to use in experiments), but it’s just a temporary solution. The problem is that what we identify as sugar doesn’t help as much over the course of the day as the steadier supply of glucose we would get from eating proteins and other more nutritious foods.

Psychologist Roy F. Baumeister is one of the leading figures in the research Tierney describes, and his studies also suggest that there are ways to conserve willpower:

“Good decision making is not a trait of the person, in the sense that it’s always there,” Baumeister says. “It’s a state that fluctuates.” His studies show that people with the best self-control are the ones who structure their lives so as to conserve willpower. They don’t schedule endless back-to-back meetings. They avoid temptations like all-you-can-eat buffets, and they establish habits that eliminate the mental effort of making choices. Instead of deciding every morning whether or not to force themselves to exercise, they set up regular appointments to work out with a friend. Instead of counting on willpower to remain robust all day, they conserve it so that it’s available for emergencies and important decisions.

There’s a limit to how many scheduling tactics you can employ to limit your willpower-sapping choices, though — and that’s especially true if you’re poor. Jamie Holmes wrote about this in The New Republic back in June:

Taking this model of willpower into the real world, psychologists and economists have been exploring one particular source of stress on the mind: finances. The level at which the poor have to exert financial self-control, they have suggested, is far lower than the level at which the well-off have to do so. Purchasing decisions that the wealthy can base entirely on preference, like buying dinner, require rigorous tradeoff calculations for the poor. As Princeton psychologist Eldar Shafir formulated the point in a recent talk, for the poor, “almost everything they do requires tradeoff thinking. It’s distracting, it’s depleting … and it leads to error.” The poor have to make financial tradeoff decisions, as Shafir put it, “on anything above a muffin.”

Last December, Princeton economist Dean Spears published a series of experiments that each revealed how “poverty appears to have made economic decision-making more consuming of cognitive control for poorer people than for richer people.” In one experiment, poor participants in India performed far less well on a self-control task after simply having to first decide whether to purchase body soap. As Spears found, “Choosing first was depleting only for the poorer participants.” Again, if you have enough money, deciding whether to buy the soap only requires considering whether you want it, not what you might have to give up to get it. Many of the tradeoff decisions that the poor have to make every day are onerous and depressing: whether to pay rent or buy food; to buy medicine or winter clothes; to pay for school materials or loan money to a relative. These choices are weighty, and just thinking about them seems to exact a mental cost.

Tierney gives one example of how financial constraints come into play in food purchasing decisions. Poor shoppers face “more mental tradeoffs” during a trip to the store, so “by the time they reach the cash register, they’ll have less willpower left to resist the Mars bars and Skittles.”

I can only imagine how hard it must be to make tough decisions about allocating limited income and diet at the same time. I’m fortunate enough not to have to agonize over routine financial decisions, and I *still* have trouble resisting the call of the many pastry purveyors within a three-block radius of my office. My own anecdotal experience does make me wonder, though, if there’s yet another facet to the issue of willpower depletion.

My department holds monthly lunch seminars that feature free food, and dessert is always included. I’ll be paying attention to the speaker, and then someone will pass around the tray of brownies. I might manage to hand the tray to the next person without taking one, but then I will remain acutely aware of the brownie tray’s exact location. It usually comes to rest on a table at the side of the room, and then every minute or two for the rest of the seminar I’ll briefly consider slipping over to grab a brownie. I might manage to resist their call, and I might even manage to get back to my desk without taking one, but if there are leftover brownies sitting by the water cooler for the rest of the day I’m going to end up eating one (or three).

By contrast, a certain colleague of mine has been known to bring in homemade brownies as a thank-you for people who help her grade the portions of multiple-choice final exams. She does not seem tempted by the brownies — in fact, I’ve seen her sit at a table for hours with a half-full tin of brownies in front of her and refrain from eating a single one.

My hypothesis is that this colleague is not actually experiencing the dozens of brownie decisions that I am. Her brain is not presenting her every two minutes with the “how about a brownie?” choice that I’m facing during the lunchtime seminars. If researchers could figure out how to reduce the number of choices our brains perceive, then we might have a shot at reducing obesity rates. But so far, the research just seems to be explaining why it’s so hard for people to stick to healthy diets.