Are smiles really contagious?

Facial expressions are so infectious, they can change from person to person in a fraction of a second, according to a new study.

Researchers reported that when we see an expression on another face, it triggers what is called "sensorimotor simulation." This is when we imitate the other's expression, helping us comprehend and react to their emotion.

This reaction takes place in just a few hundred milliseconds and happens automatically, researchers say. The facial muscles doesn't always move, but the significant parts of the brain are still activated. The muscle reaction helps the brain access memories of similar emotional states, according to the study.

"It's kind of like an extreme version of putting yourself in someone else's shoes," said lead researcher Adrienne Wood, a social psychologist at the University of Wisconsin, in the study's news release.

"This is intriguing at many levels. Looking at the bigger picture, it seems, each time you smile at a person, their brain urges them to return the favor," says Dr. Chandragupta Vedak, a psychiatrist at Advocate Good Shepherd Hospital in Barrington, Ill. "The symbiotic relationship this exchange creates allows both to release feel good chemicals in the brain and activate reward centers."

The researchers suggest this is a better understanding of how we "can infer complex mental states from even the slightest facial expression" that could help develop new treatments for people with social disorders.