You can tell a lot about a person from their body. And I don’t just mean how many hours they spend at the gym, or how easy it is for them to sweet-talk their way out of speeding tickets. For the past several decades researchers have been studying the ways in which the body reveals properties of the mind. An important subset of this work has taken this idea a step further: do the ways our bodies relate to one another tell us about the ways in which our minds relate to one another? Consider behavioral mimicry. Many studies have found that we quite readily mimic the nonverbal behavior of those with whom we interact. Furthermore, the degree to which we mimic others is predicted by both our personality traits as well as our relationship to those around us. In short, the more empathetic we are, the more we mimic, and the more we like the people we’re interacting with, the more we mimic. The relationship between our bodies reveals something about the relationship between our minds.

The bulk of this research has made use of clever experimental manipulations involving research assistant actors. The actor crosses his legs and then waits to see if the participant crosses his legs, too. If so, we’ve found mimicry, and can now compare the presence of mimicry with self-reports of, say, liking and interpersonal closeness to see if there is a relationship. More naturalistic evidence for this phenomenon has been much harder to come by. That is, to what extent do we see this kind of nonverbal back and forth in the real world and to what extent does it reveal the same properties of minds that seem to hold true in the lab?

A recent study conducted by Ivan Norscia and Elisabetta Palagi and published in the journal PLoSONE has found such evidence in the unlikeliest of places: yawns. More specifically, yawn contagion, or that annoyingly inevitable phenomenon that follows seeing, hearing (and even reading) about another yawn. You’ve certainly experienced this, but perhaps you have not considered what it might reveal to others (beyond a lack of sleep or your interest level in their conversation). Past work has demonstrated that, similar to behavioral mimicry, contagious yawners tend to be higher in dispositional empathy. That is, they tend to be the type of people who are better, and more interested in, understanding other people’s internal states. Not only that, but contagious yawning seems to emerge in children at the same time that they develop the cognitive capacities involved in empathizing with others. And children who lack this capacity, such as in autism, also show deficits in their ability to catch others’ yawns. In short, the link between yawning and empathizing appears strong.

Given that regions of the brain involved in empathizing with others can be influenced by the degree of psychological closeness to those others, Norscia and Palagi wanted to know whether contagious yawning might also reveal information about how we relate to those around us. Specifically, are we more likely to catch the yawns of people to whom we are emotionally closer? Can we deduce something about the quality of the relationships between individuals based solely on their pattern of yawning? Yawning might tell us the degree to which we empathize with, and by extension care about, the people around us.

To test this hypothesis the researchers observed the yawns of 109 adults in their natural environments over the course of a year. When a subject yawned the researchers recorded the time of the yawn, the identity of the yawner, the identities of all the people who could see or hear the yawner (strangers, acquaintances, friends, or kin), the frequency of yawns by these people within 3 minutes after the original yawn, and the time elapsed between these yawns and the original yawn. In order to rule out alternative explanations for any contagion the researchers also recorded the position of the observers relative to the yawner (whether they could see or only hear the yawn), the individuals’ gender, the social context, and their nationality.

Sure enough, yawn contagion was predicted by emotional closeness. Family members showed the greatest contagion, in terms of both occurrence of yawning and frequency of yawns, and strangers and acquaintances showed a longer delay in the yawn response compared to friends and kin. No other variable predicted yawn contagion. It seems that this reflexive, subtle cue exposes deep and meaningful information about our relationship to others. Many studies have shown that we preferentially direct our nobler tendencies towards those with whom we empathize and away from those with whom we do not. The ability and motivation to share other people’s experiences and internal states is crucial for cooperation and altruism and seems to be the defining deficiency when we dehumanize and behave aggressively. Remember this the next time you let out a big one at lunch and your friend continues to calmly chew his sandwich.

Are you a scientist who specializes in neuroscience, cognitive science, or psychology? And have you read a recent peer-reviewed paper that you would like to write about? Please send suggestions to Mind Matters editor Gareth Cook, a Pulitzer prize-winning journalist at the Boston Globe. He can be reached at garethideas AT gmail.com or Twitter @garethideas.