The key to keeping a resolution is willpower, obviously. But what is willpower? Is it some mysterious quality that some people have and others don’t? Is it a virtue we can build in ourselves with practice? Is it what separates saints from sinners?

None of the above, say some scientists. According to Roy F. Baumeister, a social psychologist at the University of Florida, willpower is simply a form of mental energy, fueled, like all brain functions, by glucose in the bloodstream. And that means that like any other form of mental energy, it can be used up.

Baumeister, in a 2007 experiment, gave students an attention-taxing task (watching a boring video while ignoring words at the bottom of the screen), then rewarded them with a glass of lemonade. Half got lemonade made with real sugar, while the others got lemonade sweetened with Splenda. They were then given tests of self-control—and the students who had drunk Splenda-sweetened lemonade consistently performed worse. Their willpower was literally unfueled.

Baumeister has co-written a book on the subject, Willpower, with John Tierney, science columnist for the New York Times. He calls this state of mental fatigue “ego depletion,” and there’s really nothing we can do about it: it’s just the way our brains work. So the real key to keeping resolutions, Baumeister and others believe, is, as Jonah Lehrer put it in a recent article for Wired.com, “to recognize the inherent weakness of the will.”

Nothing displays that weakness better than New Year’s resolutions. A 2002 study by John C. Norcross and other psychologists at the University of Scranton found that by the end of January 26 percent of resolvers had broken their resolutions. Half had broken them by March. By July, that had risen to 56 percent. A 2007 survey found that eventually 88 percent of all resolutions end in failure.

Bad statistics perhaps, but there’s actually a flip side. Sure, only 44 percent of those who made resolutions continued to cling to them by July, but only four percent of a control group who had the same goals (i.e., losing weight) had made progress in that same amount of time. Resolutions, in other words, made it ten times more likely people would actually change what they wanted to change.

And despite the odds, some people do succeed at sticking to efforts at self-improvement. How do they do it?

A new study says it’s not by any great feat of willpower, of which they have no more than anyone else. Rather, it’s by application of careful strategy.

In this study, led by Wilhelm Hoffmann at the University of Chicago, 205 participants in Wurtzburg, Germany, received specially designed smartphones. Over a week, they were pinged seven times a day and asked to report whether they were experiencing a strong desire: if so, they were then asked to describe it, how strongly they felt it, and whether it caused an “internal conflict.” If it did cause a conflict, they were asked about their ensuing success at controlling it: did they successfully thwart their desire to, say, eat a whole container of ice cream?

About half the desires were reported as causing internal conflict. In about 40 percent of those cases, the subject attempted to actively resist the desire. Resistance was not futile: only 17 percent of those desires that were resisted were acted upon, whereas 70 percent of desires that were not resisted were consummated.

The key finding, though, was that the best way to thwart self-conflicting desires isn’t through the application of weak willpower, but by avoiding temptation in the first place. As Lehrer puts it, “unsuccessful dieters try not to eat the ice cream in their freezer, thus quickly exhausting their limited willpower resources,” whereas “those high in self-control refuse to even walk down the ice cream aisle in the supermarket.”

The latest scientific findings, to be sure: but what it all boils down to for me is an old saying I heard many times growing up: “The spirit is willing, but the flesh is weak.”

If you don’t want to yield to temptation, better to avoid it altogether: and maybe, just maybe, you’ll actually keep your New Year’s resolution.

(The photo: A dessert table at the International Festival of Wine & Food, Banff Springs Hotel.)

The fact is, we’re all influenced by the people around us…and we often think of that influence as a bad thing.

As the Bible puts it, “Evil companions corrupt good morals.” And other kinds of companions can have other effects.

For instance, an analysis of 12,067 people that appeared in The New England Journal of Medicine in 2007 revealed that you are more likely to be obese if your best friend is obese. (Overstuffed siblings or spouses also makes a difference, but the greatest negative effect comes from fat friends.)

To a certain extent, it seems, obesity is a kind of societal disease, rather like the mysterious fainting disorders Victorian women seemed to be heir to, or the bizarre disorder koro, which in South China convinces men that their genitals are shrinking inward and they will die once they disappear.

Which they don’t, of course, but the affliction is real even if the symptoms are imagined.

High anxiety, depression, anorexia and bulimia are also contagious, and if you believe all these things are being foisted upon us by a secret government agency, perhaps you have fallen prey to another contagious state of mind, paranoia.

But not all social contagions are bad.

Back around Christmas of 2008 I wrote a column on the discovery that happiness is contagious, spreading rather like influenza. It seemed a particularly fitting column for the holiday season…and the newest research along those lines seems particularly fitting for a post-holiday column, because now comes word that self-control is also contagious.

At the University of Georgia, psychology professor Michelle vanDellen and colleagues were able to measure the effect in the laboratory, through a variety of studies over two years.

In one, they randomly assigned 36 volunteers to think about a friend with either good or bad self-control. They found that those thinking about a friend with good self-control squeezed a handgrip (a standard method of measuring self-control, since you have to force yourself to keep squeezing it as your hand gets more and more tired) longer than those who were asked to think about a friend who had poor self-control.

In a second test, 71 volunteers were again randomly assigned to either watch other people exert self-control by choosing to eat a carrot rather than a cookie when presented with both options, or fail to exert self-control by eating a cookie rather than a carrot. The mere act of watching those acts of good and bad self-control altered the volunteers’ performances on a later test of self-control.

In a third experiment, 42 volunteers were randomly assigned to list friends with both good and bad self-control. Then, as they were working their way through a computerized test designed to measure self-restraint, the computer would flash, for just 10 milliseconds–too fast to be read, but enough to bring the names to mind–the names of those friends. Volunteers who were flashed the name of a self-disciplined friend did better on the test than those who were flashed the name of a non-self-disciplined friend.

VanDellen believes that the influence is great enough that hanging around with friends who exhibit self-control could help us keep from eating an extra high-calorie snack at a party, or drive us to go to the gym for a workout at the end of a hard day at work. It’s only a nudge, but sometimes a nudge is all that’s needed.

And just why are we so susceptible to peer pressure?

Because we’re primates, and, as science writer Meredith F. Small puts it, “A primate’s day is all about everyone else.”

Chimpanzees and other primates spend all their time touching each other, keeping track of each other, and building interpersonal relationships…and in a very real sense, so do we. We are constantly influenced by those around us…and influence them, in turn.

Rather alarming, if you ask me. I think I’ll just sit in my office from now on and not talk to anyone.

I wouldn’t want to catch anything.