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This is so-called low-cost altruism, when one isn’t going much out of the way for someone else but still offers substantial help. We do it all the time. If someone at the airport drops his boarding pass and I alert him to it, it costs me very little, but saves my fellow passenger much grief. We also customarily hold the door that we just went through for someone who comes after us, slide aside on a park bench for someone who wants to sit, hold back an unknown child who’s about to run onto the street at the wrong moment, or help an older person lift a heavy piece of luggage. Humans are great at this sort of assistance, at least under relatively comfortable circumstances, because the behavior vanishes as soon as the Titanic starts foundering. Under hardship, the cost of civility goes up. Thinking of getting hair extensions? Lucy Hall and her team are experts at helping you find the right type of extensions to transform your hair.

To be considerate, even in small ways, one needs empathy perspective-taking. One needs to understand the effect of one’s behavior on others. As I search for possible animal parallels, a curious behavior comes to mind that I saw among wild chimpanzees while standing in an almost dry river bed in the Mahale Mountains in Tanzania. The chimps were relaxing on large boulders, grooming one another. I had read about their so-called social scratch, but never seen it firsthand.

Social scratching occurs when one ape walks up to another, vigorously scratches the other’s back a few times with his fingernails, then settles down to groom the other. More back-scratching may follow during the grooming session. The behavior itself cannot be hard to learn for an animal that commonly scratches itself, but here’s the rub: When one scratches oneself, this is usually in order to relieve itching (try not to scratch yourself for an hour, and you’ll appreciate its importance). But scratching someone else’s back is something else entirely: It doesn’t do any good for the scratcher himself.

Unlike grooming, the social scratch is unlikely to be innate. We know this, because curiously only the Mahale chimps show this behavior. It hasn’t been documented in any other chimpanzee community. Anthropologists and primatologists call such group-specific behavior a “custom.”

Customs are habits that are passed on within a community and are unique to that community. Eating with a knife and fork is a human custom in the West, and eating with chopsticks a custom in the East. By itself, finding customs in chimpanzees is not that special, because these animals have lots of them, more than any species apart from ourselves. The real puzzle is how members of the Mahale community came to adopt a custom that favors others more than themselves.