Humans and chimps had a common ancestor 4-6 million years ago. Humans’ common ancestor with orangutans lived a far more distant 15 million years ago. This may be why chimps' personalities are like humans’ in many respects, and why orangs' personalities are like humans’ only in some.
Or not. How much like humans are chimps and orangs, anyway? Are they more similar to us than the puppies we dress in doll clothes so we can circulate cute pictures on the Internet? Are they more similar than cows that look pensive or chipmunks that “complain?”
Such are the questions that intrigued the University of Edinburgh's Alexander Weiss, lead author of a study in the journal Animal Behavior. Working with fellow scientists from Kyoto University and the University of Arizona, Weiss set out to determine whether humans think their primate cousins are human-like because they really are human-like or because humans tend to anthropomorphize, projecting similarities out of amusement, over-identification, or (let’s face it) egocentricity.
Weiss’s team asked 230 people visiting zoos to rate individual chimps and orangs on 40-50 different measures, each with a 7-point scale. Then the researchers used two original statistical methods to eliminate observers’ bias. When they did they found that the personality ratings assigned to non-human primates were stable regardless of who did the observing and rating. In other words, chimps seem capable of being temperamentally neurotic, extroverted, conscientious, agreeable, and/or open to experience probably because they really are, just like us. Orangs? Mmmm … not so much—at least not on measures of conscientiousness and openness to new experience. But then that common ancestor died a very long time ago.