His experiment, published in the journal Psychological Science, was simple, but ingenious. He worked with two severely amnesic patients, whose memory had been damaged by illness and who had difficulty recalling things that happened more than a minute before, and fed them a meal. At least 1o minutes later, he fed them another. And at least 10 minutes after that, he fed them a third. He repeated the experiment on three separate occasions, and each time the same thing happened: They eagerly ate the food that was served to them. One of the participants even announced, after having a third lunch, that he planned to "go for a walk and get a good meal."
"Without their memory, what they had eaten previously had absolutely no impact on how much they ate a second and even third time," said David Just, a professor of behavioral economics at Cornell who studies consumer food choices. "It was fascinating; this widely held assumption didn't really hold up."
The part of the patients' brains that triggered satiety and hunger didn't seem to function without the patients actually remembering having eaten. There was something that could be more important than the physiological effects of eating — than the activation of taste buds and digestion of calories. Rozin didn't respond to a request for comment. In the study, he wrote that memory was probably "a substantial contributor to the onset or cessation of eating of a meal."
Today, despite Rozin's work, and other research that suggests our bodies aren't the masterful eating compasses we want them to be, the misconception persists — people still like to believe that their stomachs are good at telling them when to eat and when to stop.
"We aren't rigged like that," said Just, who believes the misunderstanding plays an important role in the prevalence of overeating.
It's natural that our eating patterns ebb and flow, depending on circumstance. Some days we eat more than others, and that's fine so long as we adjust for the stints of gluttony. But adjusting is less a matter of responding to feelings of fullness than it is one of remembering what we had earlier in the day, or week, and our memories aren't reliable in this way.
We might not eat three meals in close succession like Rozin's test participants, but the fried chicken we had for lunch isn't as likely to change what we eat later that day or week as we might think.
"We're just not very good at remembering what we've eaten previously," Just said. "And even when we are, we're not very good at compensating for it."
All the little things that matter
Memory is just one of the subtle but powerful factors that affect eating habits. Some of these are simple and fairly straight forward, like the size of the plates we use, which have been shown to change how much we eat, or the presence of a television, which has been shown to do the same.
"I don't think people are totally unaware of these sorts of things, but I'm not convinced they realize how much they influence the amount they eat," said Traci Mann, who teaches psychology at the University of Minnesota and has been studying eating habits, self-control and dieting for more than 20 years. "They definitely add up. If you're eating 10 or 15 percent more at every meal, that's going to add up."
But other influences are far less obvious. A recent peer-reviewed study found that something as innocuous-seeming as the size of a table can affect how people perceive the food that's placed in front of them — the larger the table, the harder it is for people to discern that they have been served less food, and the more easily they are satiated.
Subtle social cues, meanwhile, have an impact, too. A separate study, published last month, found that diners who were served by overweight waiters tended to "order significantly more items." Yet another, published in 2014, found the size of the people someone eats with alters the quantity and quality of the food they choose — when accompanied by an overweight friend, people tend to opt for more and less-healthy food.
"The people around us greatly influence how much we eat," Just said. "We know, for instance, that when people go through a buffet line, they are very reactive to the person in front of them. If the person in front of you takes a lot of food, not only do you feel like you have a license to do the same, you actually feel like it's expected, like you need to do the same or that you'll look strange if you do something different."
"Our natural impulse is to fit in, and fitting in often means eating more than we should or would otherwise want to," he added.
But our natural impulse is also, quite simply, to eat. When we eat, Mann said, it's not our stomachs that make us stop. That, she explained, happens later, after we're finished. And it's part of why convenience can be such a dangerous thing.
"The easier it is to eat, the more you're going to eat — it's that simple," said Mann, author of "Secrets From the Eating Lab," a look at why diets don't work. "If you've just gone grocery shopping, you're going to eat more; if you're at a buffet, you're going to eat more; if food is sitting within an arm's reach, you're going to eat more."
Just has found much of the same in his own research at the Cornell Food and Brand Lab, which he helps run. "Something as silly as putting food behind a cabinet door instead of on the counter can have a huge impact," he said.
Training ourselves to eat less
The good news is that there are ways to counteract our tendency to put too much food into our faces. One of the simplest is putting barriers in between ourselves and our food — placing cereal boxes inside cabinets, sweets in less easily accessible places, and snacks at more than an arm's distance.
But there are others, some of which have been shown to be surprisingly effective. One of the most useful of these, according to Mann, is what is often referred to as mindful eating. "When people start to eat with more focus or attention, they tend to eat less," she said. "It helps to track not just every meal, but every bite. When people are mindful of everything they put in your mouth, they're more in touch with each meal."
Another approach, unlikely as it sounds, is committing to eat things that taste better. "I know it's counterintuitive, but when things are more delicious you tend to eat less because you're satisfied more quickly," Just explained. "Whereas when something is acceptable but doesn't taste all that great, you'll end up eating more to achieve the same level of satisfaction. There is ample evidence for this."
All of the tricks, maneuvers and annoying adjustments we need to make in order to watch our waistlines, however, are the result of an uncomfortable truth that Rozin helped shed light on almost two decades ago.
"We like to think that we're rigged to think about and control what we eat and how much we eat," Just said. "But the truth is that we're rigged to eat whenever the opportunity rises."
"In that sense, we're really no different than dogs," he added.