What makes yawning spread from one person to another?

You're chatting with someone when they let out a yawn. You start to wonder if they’re losing interest in the conversation, only to catch yourself yawning shortly after. Then, someone else nearby notices and yawns too, creating a ripple effect as the yawn passes along.

But what causes yawning to be so contagious?

Yawning is often contagious — it's a response triggered by suggestion. You don't have to witness someone yawn to catch the behavior yourself; just hearing a yawn or even reading about it can provoke the same reaction. It's likely that you'll find yourself yawning at least once as you read this. However, the spread of yawns may be influenced by more than just suggestibility.

Why Do Yawns Spread So Easily?

Experts agree that yawns are contagious, though the precise reason remains unclear. One theory points to social mirroring, suggesting that yawning could have evolved as a mechanism for enhancing group awareness or aiding transitions within a group.

Studies also show that some individuals are more prone to contagious yawning than others. However, the psychological factors that influence why some people are more susceptible to this phenomenon are still not well understood by scientists.

There are two primary types of yawns: contagious yawning and spontaneous yawning.

Spontaneous yawns are triggered by physiological factors, such as body temperature — yawning serves as a mechanism to cool the brain, lowering its temperature — and shifts between wakefulness and sleep. On the other hand, contagious yawns remain a bit of a puzzle.

Yawning in Primates

Yawning may have various purposes, which could differ across species. Humans aren't the only creatures that yawn — even fish do. Some animals, including chimpanzees, bonobos, and dogs, also exhibit contagious yawning like humans.

A study in Kyoto, Japan, observed six captive chimps. These chimps watched videos showing other chimps yawning, as well as videos of chimps merely opening their mouths without yawning. Out of the six, two chimps yawned contagiously multiple times.

Interestingly, much like young children under the age of 5, three chimp infants showed no reaction to contagious yawning [source: MSNBC]. This is around the same age when children start developing empathy for others.

Bonobos, too, display contagious yawning. However, whether a bonobo will yawn in response to seeing another bonobo yawn largely depends on their proximity to the other — yawns from friends and family are far more likely to trigger a contagious response.

Humans exhibit the same behavior, which has led some researchers to suggest that contagious yawning may be connected to empathy [source: Demuru].

Autism and Contagious Yawning

If contagious yawning stems from empathy, it wouldn't occur until the ability to empathize has been developed. But what if empathy never comes? A study led by cognitive researcher Atsushi Senju set out to explore this question.

Individuals with autism spectrum disorder are often considered emotionally developmentally impaired. They may struggle with forming connections with others and find it challenging to experience empathy. If this is the case, researchers theorized that they would not respond to contagious yawning.

Yawning Behavior in Children with Autism

To test this hypothesis, Senju and his team placed 48 children, aged 7 to 15, in a room with a television. Of these, 24 had been diagnosed with autism spectrum disorder, while the remaining children had not. Similar to the Kyoto chimp study, the children were shown short videos of people yawning and of people opening their mouths without yawning.

While the children with autism showed no response to either type of video, the children without autism yawned more frequently after watching the videos of people yawning [source: BPS].

However, there might be an alternative explanation for Senju's results. Autistic individuals tend to focus more on the mouths of those they interact with. Yet, contagious yawning is believed to be triggered not by mouth movements, but by changes around the eyes of the yawning person.

One possible explanation for why autistic individuals are less prone to contagious yawning is that they may simply miss the visual cues that typically trigger it.

The phenomenon of yawning in autistic adults.

Yet, this idea is challenged by another study. Conducted by researchers at Yale University, the study analyzed how autistic adults reacted when watching emotionally intense scenes from the movie, "Who's Afraid of Virginia Woolf?"

The researchers observed that those autistic individuals who focused on the eyes of the characters showed no stronger emotional response compared to those who watched the mouths. This suggests that contagious yawning is more than simply a reaction to visual cues; even those who concentrated on the eyes found little emotional significance in them [source: Yale].

Despite these findings, the scientific community has yet to establish a clear connection between empathy and contagious yawning. A 2017 systematic review revealed "a pattern of inconsistent and inconclusive evidence regarding the connection between contagious yawning and empathy" [source: Massen].

The Theory of Instinctual Yawning

A plausible explanation for both the act of yawning and its contagious nature may lie in the social gatherings around the watering hole on the African savannah tens of thousands of years ago.

Some researchers suggest that yawning is an automatic response to stress: by yawning, we boost the blood circulation to the brain, which in turn makes us more alert.

Contagious yawning might have served as a subtle form of communication among early humans, signaling the presence of a nearby predator like a hungry lion. As fear is an emotion we can all relate to, yawning could have been a signal to spread that sense of danger.

Now, consider this: how many times have you found yourself yawning?