In a new study published in PeerJ, researchers at the Lincoln Park Zoo attempt to move beyond categorizing chimps as either "human reared" or "mother reared." Instead, they looked at a full spectrum of chimp vs. human interaction. On one end of the spectrum, chimps were completely isolated from members of their own species for the first four years of life, living instead with humans. On the other, they had little or no interaction with humans.

It wasn't all bad news: Surprisingly, the researchers didn't find increased aggression or anxiety in the chimps towards the human end of the spectrum. But they saw big differences in social grooming behavior (that is, where chimps groom each other), which scientists believe to be of incredible importance in chimpanzee communities.

"Honestly, it's surprising that we saw any differences at all, given that some of them had been in healthy social groups for decades," lead researcher and director of the Fisher Center for the Study and Conservation of Apes at Lincoln Park Zoo Steve Ross said, "but the biggest difference was definitely in rates of social grooming, and that was disturbing to see."

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For chimps, grooming is an expression of friendship and a way of getting to know new individuals. To a chimp, Ross said, a failure to reciprocate grooming behavior would be like a failed attempt at a handshake in the human world. "If you ignored someone reaching out for a handshake, there'd be this strange tension," he said. "And chimps can be very aggressive when that kind of tension arises, so you don't want that happening."

To Ross and his colleagues, the study drives home the point that keeping a pet chimp can be ill-advised. "Even performers tend to be less influenced by humans than pets are, because if a company is training animals for commercials or TV, they rarely just have one chimp," Ross said. "Opposition to chimpanzee ownership usually focuses on short-term outcomes, like how well they're being treated in captivity. But to people that take them on after, it's clear that the effects can last much longer."

And unfortunately, chimps usually become too large and aggressive to keep in the home. When kept as pets into their adulthood, they can become a serious threat to their owners' safety. Removal to a sanctuary is the best practice, and Ross has transferred more than 30 primates from private homes and businesses to healthy habitats through his program ChimpCARE.