Researchers infiltrated a fast-food Chinese restaurant and found up to a third of diners jumped at the offer of a half-size of the usual heaping pile of rice or noodles, even when the smaller amount cost the same.

Giant portion sizes are one of the culprits behind the epidemic of bulging waistlines, and nowhere is the portion-creep more evident than in restaurants with French fry-heavy meal deals or plates overflowing with pasta. Now scientists are tapping into the psychology of eating to find ways to trim portions without people feeling cheated — focusing on everything from the starchy sides to the color of the plates.

"The small Coke now is what used to be a large 15 years ago," laments psychologist Janet Schwartz, a marketing professor at Tulane University who led the Chinese food study. "We should ask people what portion size they want," instead of large being the default.

Restaurants are paying close attention, says prominent food-science researcher Brian Wansink of Cornell University. His own tests found children were satisfied with about half the fries in their Happy Meal long before McDonald's cut back the size, and the calories, last year.

"We'll be seeing some very creative ways of down-sizing in the next couple of years," predicts Wansink, author of Mindless Eating: Why We Eat More Than We Think.

But let's call it "right-sizing," says Duke University behavioral economist Dan Ariely. Right-size suggests it's a good portion, not a cut, he says.

Couldn't you just get a doggie bag? Sure, if you've got the willpower to stop before your plate is mostly clean. Lots of research shows Americans don't. We tend to rely on visual cues about how much food is left, shoveling it in before the stomach-to-brain signal of "hey wait, I'm getting full" can arrive.

So Schwartz and Ariely tested a different approach: Could we limit our own temptation if we focus not on the tastiest reason we visited a restaurant — the entree — but on the side dishes? After all, restaurants can pile on calorie-dense starches like rice or pasta or fries because they're very inexpensive, filling the plate so it looks like a good deal, Schwartz says.

A popular Chinese franchise at Duke University, with a mix of students, staff and visitors to the campus hospital, allowed the researchers in at lunchtime.

In the serving line, customers pick the rice or noodles first. The standard serving is a whopping 10 ounces, about 400 calories even before ordering the entree, says Schwartz. There was no half-size option on the menu board.

In an experiment, customers were offered the same meal with a full serving of rice or half serving. (Aline Michelle Grüneisen/Duke University/Associated Press)

In a series of experiments, servers asked 970 customers after their initial rice or noodle order: "Would you like a half-order to save 200 calories?" Those who said yes didn't order a higher-calorie entree to compensate. Weighing leftovers showed they threw away the same amount of food as customers who refused or weren't offered the option.

A 25-cent discount didn't spur more takers. Nor did adding calorie labels so people could calculate for themselves, the researchers report in this month's journal Health Affairs — concluding the up-front offer made the difference.

Anywhere from 14 per cent to 33 per cent chose the reduced portions, depending on the day and the mix of customers.

Even 200 fewer calories can add up over time.