YOUTUBE, TJADOThe idea that emotions can spread from person to person is not new. But recent research is starting to uncover the physiological mechanisms behind such “emotional contagion.” A study published this month (May 9) in Psychological Science, for example, showed that infants dilate or contract their pupils in response to depictions of eyes with the corresponding state, suggesting that emotional contagion may develop early in life. A 2014 study found that mothers could pass emotional stress on to their babies in a largely unconscious way. Together, the findings add to a growing body of research revealing the role of this phenomenon in human interactions.

“One of the most important things as a human species is to communicate effectively,” said Garriy Shteynberg, a psychologist at the University of Tennessee, Knoxville, who has shown that emotional contagion is enhanced in group settings. In order to do that,...

Scientists have shown emotions can be passed between humans (and other animals) by various sensory modalities, including visual, auditory, and tactile.

Christine Fawcett of Sweden’s Uppsala University and colleagues recently studied pupillary contagion—changing one’s pupil size in response to seeing another person’s pupils dilate or contract—in 6- and 9-month-old babies. The researchers showed the infants images of black circles of varying sizes designed to look like pupils, while they recorded the infants’ own pupil sizes. They also showed the infants images of black squares as a control.

Infants in both age groups increased the size of their pupils in response to seeing the large black circles compared with the small ones, Fawcett’s team found. The researchers saw no such difference in pupil size when the babies were shown large versus small squares. “We show that [emotional contagion] does possibly develop very early,” Fawcett told The Scientist.

Black circles aren’t the same as real eyes, of course, but Fawcett said the team sought to use controlled stimuli. “We are looking at doing more realistic versions of the study with photographs [of human eyes],” she added.

Sara Waters, now of Washington State University, and colleagues sought to measure whether stress was contagious between mothers and their infants. Her team recruited 69 mothers and their 12- to 14-month-old babies. The researchers separated the mothers from their babies and had the former study participants perform a stressful task—delivering a short speech to a small panel that would provide a positive evaluation, negative evaluation, or none at all. The researchers then reunited the mothers with their babies for a period of time, and monitored both mom’s and baby’s heart rates and other physiological measures. The researchers also observed the infants’ behavior upon being reunited.

Waters and colleagues found that the infants' physiological stress levels mimicked those of their mothers. The babies whose mothers underwent evaluation after giving their speeches were more likely to avoid strangers, compared with those whose mothers received no evaluation. And infants whose mothers received negative evaluations showed the strongest mirroring of their mothers’ stress, the researchers found.

The findings suggest “you can experience emotion outside of an interaction and bring it with you to that interaction,” study coauthor Tessa West of New York University told The Scientist. “You can spread your emotion to a partner in ways that are completely outside your awareness,” West added. “The fact that infants are ‘catching’ emotions suggests [the contagion] can be very low level.”

The mechanisms behind such emotional contagion are hotly debated in the field.

The most popular model, developed by social psychologist Elaine Hatfield and colleagues, suggests that people tend to synchronize their emotional expressions with those of others, which leads them to internalize those states. This suggests, for example, that the act of smiling can make a person feel happiness.

As to what may be going on in the brain when this happens, some research suggests that emotional contagion may engage the default mode network—a set of brain circuits that are active when an individual is not engaged in any particular task, but may be thinking about him or herself or others, noted Richard Boyatzis of Case Western Reserve University. When this network is activated, a person may be picking up on emotional cues from others, he told The Scientist. And “the speed at which you pick it up is probably the most important issue going on,” as it suggests that this process is largely unconscious, Boyatzis said.

Guillaume Dezecache of the University of Neuchatel, Switzerland, cautioned against putting too much stock in models of emotional contagion.

“My main issue with this concept of ‘emotional contagion’ is that it suggests that emotions always are contagious . . . that emotional communication is always replicative,” Dezecache wrote in an email. “As a matter of fact, emotional [communication] is much richer than that. We don’t necessarily become happy upon perceiving happiness in others. This depends on the identity of the person we interact with as well as the nature of our interaction.”

C. Fawcett et al., “Pupillary contagion in infancy: evidence for spontaneous transfer of arousal,” Psychological Science, doi:10.1177/0956797616643924, 2016.

S. Waters et al., “Stress contagion: physiological covariation between mothers and infants,” Psychological Science, doi:10.1177/0956797613518352, 2014.

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