To eat or not to eat -- that would seem to be the question for people who want to lose weight. But a dieter's decision to eat is often determined by powerful environmental cues that he or she is probably not even aware of. But daily weighing can help combat those forces, reports a new Cornell study published in the journal Public Health Nutrition.

"We're slaves to our environment," said David Levitsky, professor of nutritional sciences and of psychology at Cornell, who wrote the review article with graduate student Carly Pacanowski. "We're getting fatter clearly because we're eating more, not because we're exercising less. So the question is, why are we eating more? It's because of these unconscious mechanisms, and the only solution is to become more conscious of all these factors and resist."

Levitsky and Pacanowski analyzed hundreds of articles on the factors that influence eating behavior. They found that the obesity epidemic in the United States is due to factors we are not usually aware of, from whether others around us are eating to what we think others consider normal portions. Other powerful forces include how much food costs, how quick and easy it is to get food, how much time we have to eat and other such factors over which we have little or no control.

However, portion size emerged as one of the most powerful links to overeating. "And it's not just the amount you put on your plate, but also the package size from which the food comes determines how much you will eat," Levitsky said.

Social forces, such as seeing others eat, are also strong stimulants, Levitsky said, and these have taken on more strength in the past 50 to 60 years, as restaurant dining becomes more frequent. The faster others eat, the faster you will eat; and restaurants tend to serve larger portions than one would at home. One reason they do is because people are more likely to return to a restaurant if they perceive a good value -- an important factor for people who are particularly cost sensitive, he said.

But weighing yourself and graphing the results on a daily basis can help counter these forces, the study suggests, because it boosts one's awareness of unconscious eating and the gradual creep of weight gain over time. "When you start to see your weight go up, that's a trigger: Watch what you're eating, skip the desserts, maybe even skip a meal," Levitsky said.

His 2006 study showed, for example, that freshmen who weighed themselves every day gained no weight during a semester, but those who did not gained almost 4.5 pounds.

Levitsky adopted this practice himself 10 years ago and has lost -- and kept off -- 20 pounds ever since. "Stepping on that scale every day makes one aware of all these stimuli out there that are beckoning you to eat a little bit more, and it makes you aware that you have the power to do something about it."

He also suggested that the government play a role in combating the obesity epidemic by subsidizing fruits and vegetables. That would make low-calorie foods cheaper, making it more probable that people would buy and eat them more frequently, he said.

The research was supported by Cornell.