When it comes to keeping your New Year's resolutions, don't rely on your willpower.

That's the main take-away message from three experts in human behavior from Carnegie Mellon University's Social and Decision Sciences Department.

Willpower is the concept embodied in the Protestant work ethic and the Horatio Alger stories: the notion that with enough grit and determination, we can overcome all obstacles and temptations.

Nonsense, said Carnegie Mellon professors George Loewenstein, Carey Morewedge and Golnaz Tabibnia.

If decades of social psychology research have taught us anything, Dr. Morewedge said, it's "how much we discount the extent to which we're influenced by the situation around us."

Each year, Dr. Loewenstein said, he loses about 10 pounds during an annual camping trip, not only because he's more active but because he's removed from temptation. When he returns, the pounds come back on, and "when I'm working at home during the holidays, and there's food instantly accessible, it's hopeless."

On the other hand, he said, he got a compensating boost at school this year when the campus food trucks were moved much farther from his office, "and I found that often, I'd just skip lunch because it was such a pain to get to the food trucks."

In general, Dr. Loewenstein said, "people are tremendously overoptimistic about their future ability to exert self-control," which is a major reason many people can't keep their resolutions.

From a psychological vantage point, "willpower" may represent the part of our brain we use to deal with unexpected situations or emergencies, Dr. Tabibnia said.

But the behaviors that most people want to address with New Year's resolutions -- dieting, exercising, not smoking and not drinking -- involve a different part of the brain, which she calls the "habit system."

"The habit system develops slowly and is involved in learning any kind of skill, like driving. You take small steps at first, and you slowly do these tougher things over time and then you get so good at it you don't even have to think about it anymore. And in fact, addiction ends up hijacking this system, and that's why it becomes so hard to overcome" behaviors like overeating or smoking.

Trying to use willpower to conquer bad habits is sort of like using a squirt gun against a brick wall.

What might work much better, Dr. Tabibnia said, is to set incremental goals that a person can reach successfully, and "replace the bad habits with good habits. So instead of quitting bad foods cold turkey, you might replace them incrementally with good food."

Another way of changing our environments is to find something we need to do every day and adapt it to meet our goals.

So instead of buying a gym membership -- which many people purchase but fail to use -- it might be better to "leave your car at home and walk to work or use public transportation," Dr. Morewedge said, because it's something you have to do every day, as opposed to getting out of your chair and heading for the gym.

Another key to keeping resolutions is having a supportive social network, the researchers said, and realizing that our social groups can be powerful forces -- for success or failure.

"If you're surrounded by people who are smoking," Dr. Loewenstein said, "it's very difficult to stop smoking, and people eat much more when they're in the presence of other people who are eating, especially if they're eating a lot."

But it's just as true that "other people can help you to stick with your resolutions," he said.

Even with a smart plan and helpful friends, the researchers said, it can be tough for people to keep their resolutions because of three other psychological tendencies: the hot-cold empathy gap, hedonic adaptation and the peanuts effect.

"The hot-cold empathy gap is the idea that when you are in one emotional state, it's very difficult to imagine yourself in another," Dr. Loewenstein said.

"So when you're not hungry, it's very difficult to imagine what you'll feel or do when you are hungry, but when you are hungry, it feels like you're going to be hungry forever."

Many people make dieting resolutions right after they have indulged in overeating during the Thanksgiving or Christmas holidays, he said. But the problem is that "it's real easy to agree to diet when you're not hungry," but it doesn't help you later on when you're starving.

Hedonic adaptation is our tendency to quickly become desensitized to whatever progress we have made, Dr. Morewedge said.

"If your target goal is losing five or 10 pounds, and you reach that and it becomes your new reference point, you no longer think about how you were once 10 pounds heavier. You just think about maintaining your current weight and that becomes a struggle," he said.

And the peanuts effect, Dr. Loewenstein said, is the reason why we often fail to keep our resolutions after succeeding for a short period.

It crops up whenever we tell ourselves that just one cigarette or just one dessert won't make that much difference, and that we can start fresh the next day.

"If you look at the things that people have problems with, they share that property -- no one cigarette is going to give you cancer, and no one meal is going to make you fat, but those things are just like potato chips -- they quickly add up."

Because we share these weaknesses, the professors said, we need to focus on finding the right combination of people and environments to overcome our worst tendencies.

"This is the reason why surrounding ourselves with peers who hold the same values we aspire toward can be helpful," Dr. Tabibnia said, "because they can help us discover why this new lifestyle might be pleasurable -- that vegetables can be tasty, or walking to work can actually be more pleasurable than driving a car."

It's also why exercising alone in the comfort of our homes often fails, Dr. Loewenstein said. His Highland Park neighborhood has a yard sale every year, he said, "and I can tell you that one of the most ubiquitous items out for sale is exercise equipment."

Exercising with others or as part of a daily obligation are two ways of overcoming this tendency to let the treadmill gather dust in a spare room.

"I think the conclusion we're coming to," he said, "is that to have a successful New Year's resolution, we really should be resolving to change our situation, and not resolving to change ourselves."

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