When someone yawns, you often yawn too. Feelings can spread in the same way. Psychologists call this emotional contagion. People copy other people's faces without noticing.

In one experiment, people saw a smiling face for only 0.03 seconds. They did not know they saw it, but their cheek muscles still moved a little. Feelings may also spread on social media, but the effect there was very small.

Yawns are famously contagious, but so are smiles and irritation. Psychologists call this emotional contagion. According to Elaine Hatfield and her colleagues, people unconsciously copy others' facial expressions, voices, and postures, and this copying may make their feelings more similar.

In a 2000 experiment in Sweden, participants were shown happy or angry faces for only about 0.03 seconds, too quickly to notice. Even so, their cheek muscles reacted to smiles and their brow muscles reacted to angry faces. Emotions may spread online, too.

In 2014, Facebook researchers adjusted the news feeds of about 690,000 users for one week. People who saw fewer positive posts used slightly fewer positive words themselves. However, the change was tiny, and the study was criticized because users had not given individual consent.

A yawn can spread through a room, and emotions may travel in a similar way. Elaine Hatfield and colleagues proposed that emotional contagion begins with automatic mimicry. We copy the facial expressions, voices and postures of people around us, and this feedback may nudge our own feelings in the same direction. The first step has solid support.

In a 2000 study, Ulf Dimberg and colleagues flashed happy or angry faces for about 30 milliseconds and then masked them, so participants were unaware of having seen them. Electrodes still picked up small responses in the cheek muscles to smiles and in the brow muscles to frowns. Whether mimicry actually changes mood is less certain, because large replications of facial feedback effects have found them small or inconsistent.

Contagion has also been studied online. In 2014, Facebook altered the news feeds of roughly 690,000 users, and those shown fewer positive posts wrote slightly fewer positive words. The effect was extremely small, and the experiment drew strong ethical criticism. Network studies claiming that happiness spreads between friends face a different problem. Similar people tend to become friends and share environments, and the same methods could make acne or height appear contagious.