They also were taught to estimate a person's body-mass index, or BMI, on sight. Body-mass index is the ratio of a person's weight to their height, and doctors use it to gauge whether a person is overweight.
The results of the study revealed key differences in how thinner and heavier people approached a buffet.
"Skinny people are more likely to scout out the food. They're more likely to look at the different alternatives before they pounce on something," Wansink said. "Heavy people just tend to pick up a plate and look at each item and say, 'Do I want it? Yes or no.'"
In other words, Wansink said, thin people tend to ask themselves which dishes they most want out of all the choices offered, while heavier people ask themselves whether they want each food, one at a time.
Thin people also were about seven times more likely to pick smaller plates if they were available than those who were heavy.
Those behaviors also appeared to help people eat less. People who scouted the buffet first and used a smaller plate also made fewer trips to the buffet, whatever their weight.
There were other key differences in how thinner and heavier people acted, Wansink said. Thin people sat about 16 feet farther away from the buffet, on average, than bigger people. They also chewed their food a little longer -- about 15 chews per mouthful for those who were normal weight compared with 12 chews for those who were overweight.
Those behaviors weren't associated with taking fewer trips to the buffet, but researchers think they may be habits that help thinner people regulate their weight.
"The interesting thing was that almost all of these changes were unconscious to the person making them," Wansink said. "They essentially become habits over time."
A nutrition expert who was not involved in the study praised the research, but questioned whether these strategies might really be powerful enough help.
"As with all of Wansink's observations, these are insightful and useful," said Dr. David Katz, director of the Yale University Prevention Research Center, in New Haven, Conn. "But in some ways, they are like looking for the reasons why some people got wet sooner than others when the Titanic went down. The bigger issue was: The ship was sinking, and everyone was in the same boat."
Katz said the best advice for dieters might be to avoid a buffet's temptations in the first place. "By all means, survey the scene and choose a small plate," he said. "But, better yet, avoid the all-you-can-eat buffet altogether."