I have been studying friendship in both humans and monkeys for over 50 years. My research has been particularly interested in two areas: the limits on the number of people with whom you can have meaningful relationships, and how those relationships are built and maintained.
I have noticed that the interest in research about friendship has grown dramatically in the past few years. There has also been more interest in how to make new friends. This is probably due to the pandemic, as people were concerned about not being able to see their friends—with good reason. If you don't see them, research indicates they go off and make friends with somebody else.
How many friends do we need?
In the last 15 years we have learned that the number and quality of close friendships in your inner circle is the best predictor of your future psychological and physical health and wellbeing. This inner core layer, which we refer to as the "support clique" or "shoulders-to-cry-on" are the cavalry that will drop everything the moment you ring them up and say, "My world is falling apart." They'll do it without even thinking about it.
We published a big study in 2021—a prospective survey across 13 EU countries—that looked at whether the number of friends you have in that inner circle could predict symptoms of depression in two years' time.
We found that future symptoms of depression are minimized if you have about five friends in your "support clique." Either side of that number—whether you only have two, or try to have several—and the number of symptoms tends to increase. We devote 40 percent of our total social effort to our "support clique," in terms of time and emotional capital, so if you spread that too thinly, close friendships can't form.
While five is the optimum for the number of friends in your close circle, you can have up to 150 meaningful relationships, including with family members. These are the relationships where, if you bumped into them at 3 a.m. in a departure lounge in Hong Kong, you wouldn't hesitate to go up and clap them on the back and say, "How are you doing? Haven't seen you in ages." It's a reciprocal relationship and if you asked them to do you a favor, they would do it—maybe a little reluctantly, if they're further out in your friendship circle, but they would do it because of the sense of obligation that people have in those layers.
We initially predicted the figure 150 off the back of an equation relating the size of groups in monkeys and ape species to the size of their brains. When I stuck the size of the human brain into this equation, it predicted about 148.
So then we spent a lot of time, initially in libraries, looking for data on natural human groups, and then sampling real human relationships by looking at telephone databases: who you phone over the course of a year and, more recently, who you post to on Facebook or social media. We got people to write down all the people they have a meaningful relationship with, whom they've seen within the past year. And very consistently the average is around 150, over about 20 different samples.
This appears to be a cognitive limit—it's almost as though we only have 150 slots in our brain for handling relationships of significant emotional quality. Beyond 150, you get into the world of acquaintances, which might include, for instance, your work colleagues or your friendly local barista. That acquaintance layer runs up to 500 people, so there's a lot of room in there.
While 150 is the average number within a population, there's a fair amount of variability between individuals, reflecting personality and age. You tend to have more friends when you're younger—between 200 and 250—and you whittle them down to 150 in your 30s. Usually that's when babies come along, so people trim off the less important friendships and focus on the ones that really matter.
Once you're over the age of 65, this number tends to decline linearly. We start to lose friends, either because they move away to be nearer to their children or grandchildren, or because they die, and we don't have sufficient motivation and energy to go round and fill up the empty slots.
How to make a friendship last
Maintaining a friendship boils down to spending time with people in the flesh. That's partly because there's something much more meaningful about having a person in front of you, but also because of the physical contact during an in-person conversation. With close friends and family, there are usually hugs, stroking of arms, and pats on the shoulder—which goes back to basic primate "grooming." Physical contact on the flesh triggers the brain's endorphin system, which makes you bond.
If you don't see somebody and you don't have all these stimulating experiences, the friendship will gradually decay over the course of a few months. It takes about three years for somebody to go from being a good friend, whom you used to see a lot, to becoming an acquaintance.
Social media—as well as phone calls, texts and Skype—slows down the rate at which a friendship will decay over time. It's not going to stop it completely, though: meaning once in a while you really need to make the effort to meet up in person, to reset the relationship.
You need to see your close circle of friends at least once a week. We found this stat by accident in the very first study we did, 27 years ago. We wanted to know how many friends people had in each of their circles and arbitrarily used, "Who have you seen in the last week?" and "Who have you seen in the last month?" as our benchmarks. They actually turned out to be the defining criteria for the two layers of friendship.
The further out you go into the solar system of your social world, the less frequently you need to see people. So you might only have to see people in the outer layer once a year.
The activities we do with friends play a role in how we maintain friendships. This is slightly different for men and women, as we found out in a 2015 study. For women, friendship relies heavily on conversation: the more you talk, the longer a relationship will survive. For men, conversation has zero effect on whether their relationship will last. That's because men's social world is more activity-based, revolving around things like climbing mountains as a team or going out for drinks.
I hasten to add that romantic partners are costly, and a 2015 study suggested that people who are in a new romantic relationship sacrifice a friend and family member from their inner circle. This is because people devote much more time to romantic relationships in the early stages, so they have to sacrifice two friendships to make room. It doesn't continue at that level forever, and you often go back to having five friends, although usually several years down the line.
How to make friends as an adult
Lockdown caused an increase in loneliness and isolation, particularly for women. I think that caused relationships to weaken, and people ended up with holes in the inner layers of their network. I suspect that's why we saw such a high response rate in a recent survey, where two-thirds of respondents said they were looking for new friends to add to their inner circle.
So, how do you find new friends? The best way of meeting new people is to join some kind of activity club. I particularly recommend singing, as you get an instant friendship hit. We call it the "ice-breaker effect." We ran an experiment in 2015 with new singers and we found that, after an hour's singing, they came out feeling like they'd known the other people all their lives. That's because singing releases endorphins, and singing in a group ramps up the endorphin effect.
Exercising socially, singing, dancing, having conversations that make us laugh, and eating together have a similar effect. All of these activities trigger the endorphin system in the brain, which makes you feel very relaxed, content, at peace with the world, and trusting of those you are with. This state helps us bond with people and form relationships. You get a short-term hit in terms of it making you feel good, and a long-term hit in that it reinforces the friendship.
It takes 34 hours of commitment for someone to transition from an acquaintance to a friend, according to a recent study I worked on. In that time, you're sizing up people's suitability and assessing where they stand on a bunch of criteria, which we call the "seven pillars of friendship."
Our friendships are heavily dominated by what's known as homophily, or seeking out those who are similar to ourselves. The seven pillars essentially distill down into: growing up in the same area; speaking the same language; having the same education or career trajectory; having the same hobbies and interests; having the same political and religious views; having the same musical tastes; having the same sense of humor.
The more pillars you have in common, the closer the friendship tends to be. When you decide where someone lies on the seven dimensions, you adjust the time you devote to them. You see them as often as is suitable for the degree of similarity. We've seen from telephone data in a separate study that you can predict which layer of friendship a new person is going to be in, from the frequency with which you contact them within the first month of meeting them.
Robin Dunbar is a professor of Evolutionary Psychology at the University of Oxford, and the author of Friends. Dunbar recently collaborated with the company Fisherman's Friend on their study on friendship, which was published on October 6, 2022.
All views expressed in this article are the author's own.
As told to Katie Russell.
