SAN ANTONIO, Texas—Eating might seem, principally, like a simple, primal act. We get hungry; we eat; we're full. But surprising new research suggests that our habits, previous experiences, and our desire to conform to social norms helps determine not only how much we eat, but also how full we feel later on. The findings were presented September 24 at the Obesity Society Annual Scientific Meeting in San Antonio, Texas.

As research about portion and serving-plate size shows, we are inclined to finish what is in front of us—and to serve enough food to fill much of their plates. But to effectively address the obesity problem we need to be thinking even more deeply about the psychology of eating, argued Jeff Brunstrom of the University of Bristol.

Whether we realize it or not, we tend to have different feelings of satiety depending on how much—and what—we perceive to have eaten. For instance, one study found that if you give study subjects smoothies with identical amounts of fruit but tell some people it had more fruit and others it had less, those who thought they had consumed more will continue to report feeling fuller a few hours later. Similar effects have been seen if you give people the same food but call it a "meal" for some and a "snack" for others or label it "high-calorie" for some and "low-calorie" for others.

These external influences all affect what Brunstrom calls "expected satiety," or how much we anticipate a certain food will fill us up. And this is where previous experience plays an important role. "How do we decide how many calories to put on our plates?" Brustrom asked. "That's a very complicated question," he noted, but "memory and portion size are closely related." As an example, he suggests thinking of how full you might expect to feel after eating 100 calories of chocolate versus 100 calories of a baked potato. Although they each technically have the same energy content, chances are we would expect different levels of fullness from each.

To better understand how people make these often-unconscious judgments, he and his colleagues ran an experiment asking subjects to view two side-by-side pictures of food items on identical plates. The food portion on the left, say slices of pizza, remained static, while the food item on the right, say chicken tikka masala, slowly increased in size until the participant said that they would expect to feel about the same level of fullness after eating either plate. The researchers found that the simple volume of food on the plates was important in determining expected fullness. But volume didn't tell the whole story. Another factor, especially for children but also for adults, is familiarity. "We expect novel foods to confer relatively little satiation until experience teaches us otherwise," Brunstrom explained.

He argues that all of our tens of thousands of previous meals have "taught" us about what to expect at mealtime, both in terms of portion size and in terms of how full we will be afterward. So if we can re-learn to serve smaller portions for meals and to pick nutritious foods that will leave us feeling satisfied afterward, we might be able to reprogram our automatic eating behaviors.

Another way to improve food size selection is to play on our deep-seated desire for social conformity, noted Jason Riis, of the Harvard Business School. "We're very sensitive to social norms," he said, noting examples that when people know most other people are doing things, whether being an organ donor or reusing hotel towels for the duration of their stay, they are much more likely to engage in that behavior, too.

A simple trip to a chain restaurant will often show most other diners devouring portions that are much too large and calorie-heavy than would be recommended for a healthful diet. But very rarely do people explicitly request smaller portions. Riis and his colleagues have completed studies at Panda Express, a chain Asian fast food restaurant, showing that customers will often accept an offer to reduce the size of a calorie-filled side dish (such as rice) to cut more than 200 calories out of their meal (some also received a small discount on the meal for doing so). The offer was much more successful than menu calorie labeling at getting customers to eat lower-calorie lunches.

This downsizing option, modeled off of the infamous "supersize" era of McDonald's meal upsells, has yet to catch on outside of the experimental realm. But Riis hopes that calling these more appropriately proportioned portions "right-size"—or something to that effect—will eventually "become part of the standard script" at restaurants "so you don't feel like a weirdo" having to ask for a smaller helping. It would take a lot of work and adjustment, he noted, but "changing defaults to smaller portions" could make a big difference in how much we end up consuming when we go out to eat—which could also translate to more healthful portions at home, too.