We often turn to dogs and to chimpanzees to understand our species. Chimpanzees are our closest relatives (with bonobos), while centuries of selective breeding have turned dogs into a species uniquely suited to comprehend our own social cues. If anybody can help us understand contagious yawning, it's them. This week, primatologists Matthew Campbell and Frans de Waal of the Yerkes National Primate Research Center at Emory University added a new chapter to the ongoing story of contagious yawning.

But to understand their findings, its worth looking back at the history of contagious yawning research and the ongoing controversy over whether it reflects empathy or not.

There was once a time when scientists believed that the function of yawning was to cool the brain, or to relieve stress. Sometimes it is. But researchers quickly realized that yawning was a rather curious behavior; it was contagious. When you watch someone yawn, you're more likely to yawn in response, than when you watch another person do just about anything else. That means yawning has a social component. Indeed, humans with developmental and personality disorders that feature social deficits also show less susceptibility to the yawn contagion. In addition, contagious yawning is elicited more strongly by familiar individuals than by strangers. That's true not just for humans, but for chimpanzees, bonobos, and gelada baboons. While contagious yawning hasn't been studied in rats, mice, elephants, and birds, there is a link in those animals between familiarity and empathy-related behaviors as well.

While it is clear that yawning has a social component, definitive proof of a connection with empathy has been a bit harder to come by.

In 2009, psychologist Ramiro Joly-Mascheroni showed that human yawns are contagious for dogs. But later that year, Aimee Harr was unable to replicate that finding. In 2010, Sean O'Hara and Amy Reeve uncovered more evidence in support of contagious yawning for dogs, but not nearly as robust a pattern as Joly-Mascheroni found. Karine Silva provided a partial answer, in 2012, as to why some researchers seem to find contagious yawning while others didn't. She discovered that dogs were more likely to yawn after hearing the yawn of a familiar human than of an unfamiliar human. That actually makes a good deal of sense: if yawning is tied to empathy, then dogs might be more willing to empathize with familiar people than with strangers.

The story for primates hasn't been any more straightforward. In 2011, Campbell and de Waal turned to chimpanzees. If empathy underlies contagious yawning, then chimps would be more likely to yawn when watching an ingroup member yawn but not when watching an unfamiliar chimp yawn. In chimp society, by definition, unfamiliar chimps are members of a social outgroup. Indeed, their data fit their hypothesis: chimps yawned more after watching videos of familiar chimps yawning.

But then last year, Elainie Alenkær Madsen made things more complicated. She looked at juvenile and infant chimpanzees at a sanctuary in Africa, and rather than using videos of chimpanzee yawns, she used human researchers as the main stimulus. She found that the juvenile chimps could catch human yawns, but infants were not susceptible to the human yawn contagion. Did the infants not respond because the yawns were human rather than chimpanzee? It's impossible to tell, so it's hard to compare it with Campbell and de Waal's findings with mature chimpanzees. Another complication is that the juveniles yawn regardless of whether the human was familiar or unfamiliar. Is that because chimpanzees treat humans differently from other chimpanzees? Or is it because they're juveniles, who are not yet firmly situated within complex chimpanzee society? We have to teach our own human juveniles to be wary of strangers; maybe it's the same for chimps.

As I wrote last year: comparing this study with the earlier one by Campbell and de Waal may be inappropriate, as Madsen herself acknowledges, writing, "adult chimpanzees have only been tested on yawn contagion when viewing the yawns of conspecifics, while young chimpanzees have only been explicitly tested with respect to [human] yawn contagion.” In other words, it might be apples and oranges.

It is apples and oranges no longer. This week, Campbell and de Waal reported a new study in the journal Proceedings of the Royal Society B. They still used videos, as videos allow each chimpanzee to view the exact same stimuli, but this time they provided chimps with videos of familiar and unfamiliar human yawns, as well as the yawns of gelada baboons. Would the familiarity effect that they found in their 2011 study extend to humans, in contrast to Madsen's finding? Would chimps treat familiar humans as they do familiar chimps, and treat unfamiliar humans as they do outgroup chimps? And where would the baboons, which comprises an entirely unfamiliar species, fit into the picture?

The findings are remarkably straightforward. Chimpanzees were able to "catch" a yawn from familiar and unfamiliar humans alike, just as Madsen's juvenile chimps did. But they didn't yawn contagiously in response to baboon yawns. That suggests that chimpanzees might treat humans as a unique sort of other species - at least for those chimpanzees who live in a sanctuary where they are cared for by humans.

To really understand the pattern, Campbell and de Waal combined this data with the data from their 2011 paper. The chimpanzees' response to ingroup members was statistically indistinguishable from their response to familiar and unfamiliar humans, suggesting that these chimps treat all humans as if they were members of their ingroup. "For our subjects, a different species (but one they have a history of positive experiences with) was more potent at eliciting empathy-based contagion than outsiders of their own species," they write. But that may be because the chimps have lived for so long inside a sanctuary with no exposure to unfamiliar chimps, while being routinely exposed to new humans. "Students come, complete their studies and leave, and care staff gain and lose members in the normal course of people changing jobs. The chimpanzees may have been conditioned to take a positive view of humans in general, not just the ones that they know." Not that the chimps can't distinguish familiar from unfamiliar humans more generally, only that the distinction isn't reflected in contagious yawning.

Meanwhile, their response to baboon yawns was the same to their response to unfamiliar chimpanzees. But a deeper look reveals an important distinction, which is that the chimps spent more time looking at the videos of the unfamiliar chimpanzees. In fact, they watched the unfamiliar chimp videos with more attention than any of the human, familiar chimp, or baboon videos. "Outgroup chimpanzees possibly elicited a hostile response," they say, referencing the notion that all unfamiliar chimpanzees are automatically thought of as enemies by wild chimps. That could have interfered with any possible empathy-related response. By contrast, the gelada baboons were simply thought of as socially meaningless. A similar outcome, in terms of contagious yawning, but for a very different reason. If their hunch is correct, then it seems reasonable to say that "contagious yawning with strange chimpanzees was actively thwarted, whereas with geladas it was not there to begin with."

Where does that leave empathy? The data actually suggests that empathy may underlie contagious yawning, since the human stimuli remind us that familiarity alone can't account for the results. But empathy isn't given out to everyone. An individual needs social experience to lubricate the wheels of empathy, which these chimpanzees had for humans but not for baboons. Campbell and de Waal ask whether exposing their chimpanzees to gelada baboons would increase familiarity and thus lead to the passing of yawns between the species. More intriguing, they wonder: "could experience change the way chimpanzees respond to outgroup chimpanzees?"

If so, what would that mean for our own species and the way we interact with one another? A principle in psychology called the "mere exposure effect" holds that exposure itself is enough to facilitate increased liking for a previously unfamiliar individual. Could that also facilitate empathy? It's an important question to ask, because despite the fact that human culture is immeasurably more complex than chimpanzee society, the stakes are high. If nothing else, this research points out that "flexible social engagement was probably already present in the most recent common ancestor with chimpanzees," Campbell and de Waal say.

They conclude on a hopeful note. "This flexibility opens a door to examining how we can modify who chimpanzees will form an empathy-based connection with and how strongly. Understanding this flexibility in social engagement may help explain the proximate mechanisms that allow for switching between cooperation and competition within chimpanzee and human societies."

Campbell M.W. & de Waal F.B.M. (2014). Chimpanzees empathize with group mates and humans, but not with baboons or unfamiliar chimpanzees, Proceedings of the Royal Society B: Biological Sciences, 281 (1782) 20140013-20140013. DOI: 10.1098/rspb.2014.0013

Header image: Katie yawning. Second image: Rita watches a video stimulus. Video: First Rowena and then Liza watch videos of familiar chimpanzees yawning. All via Yerkes National Primate Research Center, Emory University, used with permission.