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Everyone knows that people put on weight because they consume more calories than they burn. But as the medical community struggles to get a handle on obesity in the USA, a growing body of research is delving deeper to find out more about the psychology behind the numbers.

Although people might be inclined to think of nutritionists or dietitians, obesity is "one of the big common public health issues that falls right in the heart of psychology," says psychologist Paul Rozin of the University of Pennsylvania in Philadelphia.

Among a host of questions aimed directly at the psychology of eating are why Americans are eating more than they used to; whether some foods can really be addictive; and whether more people than in the past are genetically predisposed to pack on pounds.

Rozin, who has studied humans and food for 30 years, is one of dozens of psychologists who will share their latest findings and theories this weekend at the American Psychological Association's annual meeting, starting Thursday in Orlando. Obesity is one of the themes.

"Obesity has been much more resistant to treatment than any reasonable person would have thought 50 years ago," he says.

There's no question that Americans are heavier than ever before. More than one-third of adults are considered obese and almost 17% of teens and kids fit the category, according to the most recent federal data, released earlier this year.

In fact, food is everywhere at any time, and advertising is an additional lure, says psychologist Kelly Brownell, director of the Rudd Center for Food Policy Obesity at Yale University in New Haven, Conn. "We've been completely retrained to think that large portions are acceptable, that eating throughout the day is acceptable, that eating late at night is acceptable, that eating in the car is acceptable," he says. "All the boundaries that would put limits around eating have been exploded."

Among the most-blamed culprits are intense food marketing toward consumers and less physical activity.

But research scientist David Allison has some other ideas. A psychologist who directs the Nutrition Obesity Research Center at the University of Alabama-Birmingham, Allison points to other contributors, such as too little sleep and advanced maternal age, which some research has shown can increase the chances of overweight kids.

Allison's new research, online in Frontiers in Genetics, finds that people with higher body mass index tend to partner with those of similar BMI and may predispose their offspring to obesity. Using Danish height and weight data collected for hundreds of thousands of children at age 13, researchers were able to find 37,792 spousal pairs who married between 1945 and 2010. They then calculated couples' BMIs. The study he co-wrote confirms that those with higher BMIs tended to pair up and suggests the implications for heavier offspring.

"It starts concentrating the genes for BMI within families," he says.

He and others also are looking at cognitive demand. Early findings suggest we may be draining our brains because "we have more cognitively demanding lifestyles."

Significant increases in the prevalence of obesity occurred over the past 30 years, when computers and technology use exploded, Allison says. Being constantly available to others means we are so often occupied with mentally involved tasks that we're on cognitive overload. And that, he says, may be wearing out our self-control to resist food temptations.

Food could be the fuel we need, the reward we want or maybe both, he says. But "if those mental activities lead to increased food intake, that could be a major driver of why we're taking in more food," he says.

"That's not to say any of us want to give up our computers or stop engaging in mentally demanding activities. But we may want to say, 'Are there ways to alter our lifestyle that might protect us?' "

'The buffet effect'

Another area of research focuses on food itself. Studies by Barbara Rolls, director of the Laboratory for the Study of Human Ingestive Behavior at Penn State University in University Park, Pa., have found that something as seemingly innocuous as more variety actually encourages overeating. She says pleasure from eating a food declines while eating. But if other foods at the meal have different tastes, aromas, shapes and textures, instead of stopping eating, people shift to another food that remains appealing.

"It's the buffet effect," she says. "If you go to a place with 50 different kinds of foods, you're going to eat more than if there was just a few."

She co-wrote a study in the August Journal of the Academy of Nutrition and Dietetics in which researchers found that participants ate more vegetables when served three types (broccoli florets, baby carrots or snap peas) at a meal than when served the same amount of just one, even if it was a preferred vegetable. The 66 adults got pasta and cooked vegetables once a week for four weeks; amounts were carefully measured.

Among the more controversial topics to be discussed is food addiction. Some research suggests certain foods "hijack" the brain in ways that resemble addiction to drugs or alcohol.

"Nobody would say food addiction is like morphine, but it does get similar effects," Brownell says. "So the question is, 'Should some of these constituents of food be limited because they're hijacking the brain?' "

Brownell's center is at the forefront of food addiction research. One of its studies, published last year in the Archives of General Psychiatry, gained attention for finding that food cues activated the same brain areas in those who score high on food addiction measures as drugs or alcohol do in those addicted to them. Lead author Ashley Gearhardt, a psychologist, will present research on food addiction in children this weekend.

She also has compiled data on the foods people report stimulate the most addictive response. Generally, she says, they are higher in sugar, fat and salt.

"Ice cream, chocolate and pizza were our three big culprits," says Gearhardt, who this fall moves to the University of Michigan in Ann Arbor as an assistant professor.

"In recent years, science has begun to provide support for food's addictive properties, and food addiction has gained attention as a contributor to obesity," says Rebecca Puhl of the Yale center. "But there has been no research examining how the public perceives food addiction, and whether it is believed to be a legitimate addiction or disease," an area of her latest work.

Still, many are successful at losing weight and keeping it off. Rena Wing, a professor of psychiatry and human behavior at Brown University in Providence, says psychologists have had an influence on the field of obesity.

She has been studying people who have been successful at maintaining long-term weight loss in a national study tracking more than 10,000 people who have lost at least 30 pounds and kept it off at least a year.

Stigma doesn't help

"We have a lot of measures of their behavior," she says, even MRIs showing how people respond to pictures of food. "The pattern of brain responses in successful weight losers suggests they are restraining their responses to the food cues. They exhibit a lot of cognitive control when looking at the pictures - more than normal-weight or overweight people."

But for those who struggle with the pounds, there is continued stigma, says Puhl, director of Research and Weight Stigma Initiatives at the Rudd Center. She says the public may think "maybe a little bit of stigma is not such a bad thing - that maybe it will motivate people to lose weight and provide an incentive for weight loss."

But she says the opposite is true. "When people are stigmatized because of their weight, they are more likely to engage in unhealthy behaviors, like binge eating and unhealthy weight-control practices. They actually increase their food consumption and have lower motivation for physical activity," she says. "They are at increased risk for numerous psychological consequences that includes depression, anxiety, poor body image, low self-esteem and, unfortunately, suicide."

Weight stigma surfaced just last week over Australian Olympic swimmer Leisel Jones when a newspaper in her home country published photos suggesting she had put on weight and polled readers online about whether she was fit enough to swim. Readers were outraged, and the poll was removed within hours.

"We live in a culture that has placed a premium value on thinness," Puhl says.

Rozin's work as a cultural psychologist focuses on how culture frames eating behavior. For a paper comparing French and American eating cultures, published last year in the journal Frontiers in Psychology, he found that Americans emphasize quantity over quality, have a higher preference for variety and prefer convenience in food. He says the French enjoy food more than Americans do - yet are thinner.

"They eat more fat than we do. They don't snack. They have a very strong food culture - which we don't have - as to what a proper meal is. The meal is a real occasion to sit down and relax and spend time together - and not eat too much."

By Sharon Jayson, USA TODAY