Brain scans used to study the minds of young children show that by age 3, they have begun developing brain networks used to understand the beliefs and feelings of others, according to researchers at the Massachusetts Institute of Technology who published a study Monday.

The study, published in the journal Nature Communications, looked at the theory of mind. The theory is defined as understanding others in the social world, such as inferring their beliefs, desires, and intentions.

“People are really good at thinking about other people,’’ said Hilary Richardson, an MIT graduate student who is the study’s lead researcher. “Understanding the mental states of others is really what separates us from other species.’’

Richardson said the study bridges behavioral and neuroimaging studies to show evidence that children as young as 3 show activity in brain scans when they attempt to understand a social interaction and the motivations behind a person’s actions. Functional MRIs, which were used in the study, are not usually performed on young children because it can be hard to get them to sit still, Richardson said.

“We made a lot of innovations — practice scans before real scans, the experiment was six minutes long instead of 20 minutes or more, and we showed them a Pixar short — because kids move less when they’re watching a movie,’’ Richardson said. “We also let them hold a massive stuffed animal.’’

Children were shown the animated short “Partly Cloudy,’’ which features two characters who have some tense moments in their friendship and working relationship but resolve their problems at the end.

Researchers spent four years gathering data from 122 children ages 3 to 12. They scanned the entire brain, focusing on two networks known to be used during theory-of-mind thought processes. In particular, they studied whether children understood false- beliefs theory, which is understanding that people can be motivated by beliefs that are untrue.

Scans revealed that in children as young as 3, networks used in false-belief theory were active, becoming stronger in children as they got older. Whether the children could correctly answer questions about false beliefs after the film did not necessarily correlate with whether theory-of-mind networks were active, Richardson said.

“Despite failing the tasks, it’s not like these kids aren’t reasoning about these things,’’ Richardson said.

Richardson said researchers previously believed the networks used in theory-of-mind reasoning were not developed until at least age 4, but the brain scans in the study show evidence that disproves that.

“These brain regions have been developed and continue to develop,’’ Richardson said.

MIT said in a statement that in the future, researchers would like to study the brain of autistic children to see how their theory-of-mind network develops.

Laney Ruckstuhl can be reached at laney.ruckstuhl@globe.com. Follow her on Twitter @laneyruckstuhl.