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Constant Cravings

What keeps us from eating all day isn't how full we'd feel, it's our memory of already having eaten.

Chocolate ice cream, macaroni and cheese, Mom's meatloaf: we all have foods we crave, we could eat them all day. But what keeps us from doing so isn't how full we'd feel. It's our memory of already having eaten.

Paul Rozin and colleagues hoped to determine why we sit down to meals when we do. Is it because we're physiologically hungry? Or because we're mentally aware of not having dined for a few hours?

Rozin, a professor of psychology at the University of Pennsylvania, and his team recruited two patients who were densely amnesic, meaning that they had virtually no memory of events that had happened more than a minute ago. First, the researchers placed a meal that the subjects professed to enjoy (a Swanson TV dinner for one, cheese-rice casserole for the other) in front of them. When the patients had finished eating, their plates were taken away and they were asked to sit and chat with the researchers. Between 10 and 30 minutes later, Rozin asked if they wanted a meal. Both memory-impaired subjects readily accepted the second lunch -- and even a third meal. Before the second and third meal, the two men only vaguely recalled that they'd eaten earlier, and couldn't say what they'd consumed or when it had been. In fact, 20 minutes after eating much of his third lunch, one subject claimed that he'd like to "go get a good meal." Two control subjects without global memory deficits, however, ate only the first lunch offered and were genuinely surprised and puzzled when asked if they wanted a second.

This finding, says Rozin, suggests that what keeps us from eating constantly isn't a lack of hunger, but our knowledge that we just ate. So without our culturally determined three meals a day, we'd likely be content to snack around the clock.