To figure out what makes humanity so distinctly human, it's helpful to identify how we differ from our closest relatives. Whether or not apes experience empathy is an ongoing question (as is figuring out whether they act altruistically by choice, a debate we've covered before).

In a study published today in PeerJ, researchers compare the "yawn contagion" phenomenon between bonobos (the apes most closely related to humans) and ourselves. You know how it works: When someone yawns next to you, it sometimes seems impossible not to follow suit.

Bonobos experience the same impulse, and previous research suggests that yawn contagion is a basic form of empathy. An "emotional contagion," which is when some feeling is transmitted from one person to another, is what makes you more likely to smile when being smiled at, or feel tired when those around you seem worn out. Since yawning is so easy to measure, the researchers were able to compare the contagion effect directly between humans and apes.

Humans didn't have more contagious yawns than bonobos when they were surrounded by strangers, but they did yawn more when patient-zero of the yawn plague was a family member or friend. The stronger the emotional tie between the subjects, the researchers reported, the faster and more consistently humans yawned. To bonobos, all yawns seemed to be equally catching.

The researchers believe this suggests that the basic level of empathy that humans experience — our instinctive connection to other humans, even if they aren't friends or family — isn't all that stronger than what our ape cousins feel for one another. But when it comes to the strongest of our social bonds, particularly the complex ties we share with family members, we may have evolved quicker and more consistent empathetic responses.

Of course, that's just one possible explanation. And this is just one of many evolutionary questions our ape relatives have presented.