That's been the assumption anyway, that Americans overeat because we are getting so much darn pleasure from all the tasty, unhealthy foods in our path.

Films and books have been built around the idea that we overeat to get pleasure, including "Supersize Me" (2004) by documentary filmmaker Morgan Spurlock, "The End of Overeating" (2010) by former FDA commissioner David Kessler and bestseller "Salt, Sugar, Fat" (2014) by Pulitzer Prize-winning New York Times journalist Michael Moss. They all deliver more or less the same message — that overeating is a national love affair with highly palatable foods, one some of us just can't quit.

It's so intrinsic to what we believe about eating that no one ever bothered to check it out. And it's wrong.

That's the finding of a surprising new National Institutes of Health-funded study out of Yale University this week on the deep-brain psychology of eating. As reported in a paper under review for the International Journal of Obesity, a team of scientists at the Modern Diet and Physiology Research Center in New Haven placed 110 subjects of differing weight in a functional MRI machine. They then fed them tasty milkshakes through straws. They then had them use a trackball to fill out a questionnaire considered "the gold standard instrument for how much people like things," said Dr. Dana Small, Ph.D., lead author.

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On the MRI readout, researchers observed a reward circuit lighting up in the heaviest people, a familiar brain pathway that scientists had always assumed signaled pleasure. But on their questionnaire designed to test pleasure, the test subjects were reporting something else entirely. Specifically: "we found no evidence," the authors wrote, "for a relationship linking (obesity) and the perceived liking, wanting or intensity of the milkshakes." In other words, "the reason obese people are obese," says Small, "is not because they like food any more than anybody else. It's not because obese people are hedonists."

Small, who studies the neuroscience of taste and perception, said testing the question of pleasure and eating has been on her mind for some time.

"One thing I had noticed in my experience since the mid 1990s is that I had failed to find evidence for conscious perceptions of liking, for pleasure," said Small, who is professor of psychiatry and psychology at Yale. "I had failed to observe in my work or other people's work evidence that there's a relationship with obesity.."

Instead of pleasure, Small says overeating is likely triggered by unconscious processes, metabolic signals carrying the body's perception of the energetic value of a food based on prior experience. These signals can cause conditioning within the brain over time. The study adds fuel to a debate dividing obesity research into two camps.

On one side is a long-running "junk food" line of thought, whereby clinicians assert that some foods are just too tasty. On the other side, clinicians say sugar and carbohydrates trigger insulin, and this locks away energy, creating a state of perpetual messages signaling hunger. The first is a conscious process, while the second is unconscious. Small believes both are happening at the same time.

"I believe that the dichotomy is not accurate. A lot of these metabolic signals actually (change) the brain's response to the food, and that determines how reinforcing it is. The two systems are integrated." Also, she says, most animals base their eating decision on unconscious drives that do not include pleasure.

"These are very old brain systems, and are working differently with modern energy-dense foods. ... Everyone will say and I will say it as well: avoid processed foods, that's the simple lesson. There's more and more research saying it's not the nutrients itself, the problem is having foods that our physiology is not evolved for. They are kind of super strong rewards."

"I thought it was a great study," said Jocelyn Lebow, Ph.D., a psychologist who treats eating disorders at Mayo Clinic. "In this culture there's a pervasive belief people with obesity are that way because of some sort of personal failing, and that the rest of us are disciplined while they are gluttons who can't resist the same urges.

"It gives us the green light to be shaming and critical of people who are in larger bodies. It also stops us from asking the right questions. We go down the road of trying the same interventions that we know don't work. We know 95-98 percent of diets fail and so we say, 'well you must not have done it well enough.' We're not researching the things that matter. There are other mechanisms at work. This isn't just a behavioral problem."