The findings are clear: as we get older, we tend to be more and more selective in friendship, choosing our circles of trusted friends and positive relationships. According to a study carried out by primatologists and psychologists at Harvard University, chimpanzees also show this same behavior. Could studying our "ancestors” help us to be better understand this social selectivity, which can potentially contribute to healthy aging?

Research has already shed light on the importance of social bonds throughout our lifetime; and, in particular, how they can help improve cognitive health. In humans, aging is characterized by an increased focus on pre-established positive and close social interactions. One of the reasons for this is a positivity bias, which as we get older "prompts" us to pay more attention to "positive relationships rather than ones that bring tension or conflict." According to the authors: “the origin of this social aging pattern is therefore a central issue both for evolutionary perspectives on the life-course, and for promoting wellbeing in old age.” While this socio-emotional selectivity can be partially attributed to a different relationship with time (the future is not perceived as finite when we are young), this does not explain everything.

To better understand why we become increasingly picky about our friends as we age, Alexandra G. Rosati and her colleagues decided to determine whether the phenomenon could be observed in wild chimpanzees, one of our closest relatives. The study was based on nearly 78,000 hours of observation, spread over nearly 20 years. Indeed, between 1995 and 2016, in Kibale National Park (Uganda), the researchers observed the social interactions of a community made up of 21 male chimpanzees, ages 15 to 58, with an average of 10.6 years of data per individual and 141.6 days of observation per year. The study focused on male chimpanzees because they have a tendency to form denser social bonds and to demonstrate them more frequently than females.

What did the scientists observe? As they age, chimpanzees spend more and more time grooming other chimps with whom they already have an established mutual relationship. In addition, though older chimpanzees liked to be alone for longer periods, they also showed a preference for interacting with other older chimps and “mutual” friends (friends with whom they had a balanced relationship). Thus, aging chimpanzees have more mutual friendships characterized by a close and balanced relationship (0.6 one-sided friendships vs. 3 mutual friendships in 40 year-old males), while younger males tended to have more lopsided relationships (2.1 one-sided friendships vs 0.9 mutual friendships in 15 year-old males).

The data analysis allowed the researchers to see the similarities between humans and chimpanzees as the two species age. According to the authors: “Older males were more likely to spend more time alone and showed a preference for interacting with — and grooming — chimps they deemed to be more important social partners. And like older humans looking for some peace and quiet, the chimpanzees showed a shift from negative to more positive interactions as they reached their twilight years.” In other words, as we age, we seek peace and tranquility (a preference the authors characterize as a positivity bias).

Alexandra Rosati and her colleagues point out that chimpanzees adopt behaviors similar to humans in terms of their social selectivity, even though, unlike humans, they lack an understanding of death. It is likely there is another explanation as to why their relationships become more positive as they age.