Two people are beginning a weight-loss program. One is an optimist and the other a realist -- perhaps even a pessimist. Who has the better chance of losing weight?

According to a new study from Doshisha University in Japan, it's the pessimist. People who began a six-month program of nutrition, exercise, and counseling were less likely to lose weight if they had an optimistic personality.

This at first doesn't make sense. We usually associate optimism with greater success and better health. There's plenty of research showing that optimists have stronger social relationships, are happier, and even live longer. So why wouldn't optimism help people lose weight?

In this case, all of the participants in this study were obese, which meant they had a real challenge ahead of them. Most people who are obese have been overweight for years, even most of their lives. The causes of their obesity are likely complex, including behavioral, psychological, social, and medical factors.

Optimists may underestimate the setbacks and pitfalls they will face when trying to lose weight. They may also overestimate how much weight they will lose, how quickly, and how the weight loss will improve their lives. This can turn a success experience into a failure experience. If you expect to lose five pounds a week and lose two, you'll be disappointed. You may criticize yourself or the weight-loss plan. You may be so unhappy that you comfort yourself with food or find you can't muster the energy to exercise.

On the other hand, if you expect to lose 1-2 pounds a week and you do, that's a totally different experience. You'll feel encouraged, have greater faith in yourself and the program, and be motivated to continue. Same weight loss, same success, different trajectories for long-term weight loss.

Previous research has also shown that higher expectations of benefit can backfire when you're trying to change a behavior. If you expect that losing weight (or quitting smoking, or exercising, or learning a new skill) will fix a relationship, help you find a job, or get rid daily stress, and it doesn't, then your motivation to maintain the behavior plummets. People who expect the most rewards from changing a behavior -- that is, unrealistic rewards -- are least likely to succeed.

This doesn't mean that every aspect of optimism is self-defeating. When optimism is accompanied by a strong sense of self-efficacy -- that is, when you believe you have the inner strength to handle challenges -- you may maintain hope and preservere in the face of setbacks. And when optimism is realistic -- for example, you believe that losing weight will give you more energy in the day, but not necessarily that it will fix all of your problems -- the focus on positive long-term outcomes can help you resist short-term temptations.

So if you have a sunny disposition, don't start looking for the storm clouds. This study doesn't so much doom optimists to failure as it provides good food for thought.

When you think about what it will take to make a behavior change, and what benefits you will receive, are you being realistic? Have you considered how you will respond emotionally and behaviorally when you experience a setback? Do you have a plan, or social support, that will help you recommit? If you think that a behavior change will transform your entire life, can you focus first on the most likely benefits, and celebrate those moments when they happen?

This kind of realistic optimism should carry you through challenges and help you savor the rewards along the way.

Study: Saito, H., Kimura, Y., Tashima, S., et al. (2009). Psychological factors that promote behavior modification by obese patients. BioPsychoSocial Medicine 3:9 (in press).

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