The result "demonstrates the need for adults to make sure they sit and eat with children and help to support children to recognize their hunger and fullness," said Samantha Ramsay, an assistant professor of Foods and Nutrition at the University of Idaho, who was not involved in the research.
Ramsay added, the findings don't mean that children should not serve themselves at mealtimes.
Major nutritional and medical organizations have endorsed the practice of letting kids help themselves to food out of communal dishes.
There are a number of benefits to so-called "family style" service, including the development of social skills, such as taking turns, practicing fine motor skills, such as working a fork or spoon, and learning independence, Ramsay said.
Another presumed benefit is that kids will not overeat.
Past research has found that children eat more when their meal portion is larger.
"So the thought (was) that allowing children to self-select (food portions), as opposed to being served portions, which tend to be larger than age-appropriate, children may potentially consume more in line with what they actually need," said Jennifer Savage Williams, associate director of the Center for Childhood Obesity Research at Penn State University the new study's lead author.
Savage Williams and her colleagues wanted to test whether that was the case, so they experimented by giving kids macaroni and cheese, either already dished out or from a self-serve bowl.
The study included 63 three- to five-year-olds.
During the lunchtime in which the researchers chose the serving size, the kids were given a large, 400 gram serving -- close to a pound -- of macaroni and cheese.
During another lunch intended to mimic family-style eating, the kids were allowed to serve themselves from a bowl that contained 400 grams of macaroni.
In both cases, the kids ate about the same amount of macaroni and cheese, roughly 200 grams.
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