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The Evolution of True Friendship

Some interesting implications of our evolved friendship detectors.

St. Thomas Aquinas famously mused, “There is nothing on this earth more to be prized than true friendship.” I recently came across this quote, and it seemed like an easy statement to agree with. But the more I thought about true friendship, the less I felt like I understood.

So I did what any scholar of human nature would do and continued to scroll through quotes on the internet. One idea kept popping up: true friends are around when times are tough. “Friends show their love in times of trouble and not happiness,” wrote Greek philosopher Euripides. “A real friend walks in when the rest of the world walks out,” quipped radio host Walter Winchel. And I even found this gem, attributed to the sometimes brilliant Justin Bieber: “friends are the best to turn to when you’re having a rough day.”

I went bleary-eyed reading pages of quotes like these, which usually means that the nugget of wisdom isn’t a particularly deep insight into human behavior. Of course we want our friends to be around when times are tough. That’s what friends are for! But why? Why do we place such emphasis on our friends being there during tough times? This seemingly simple question is actually part of a deep evolutionary puzzle.

The puzzle starts with acknowledging that friendships exist because two people can benefit from the relationship. It makes us uncomfortable to think of our friends in these terms, as people who can benefit us (e.g., “my friendships aren’t about what I get in return”). But let’s be honest, if you aren’t getting some benefit from your friends, you should probably get some new friends.

The real problem with the quid-pro-quo of friendship isn’t that it’s distasteful, it’s that it sets up a nasty paradox: we most need our friends’ help when we are least likely to repay them. In other words, when we are, sick, sad, or broke—that’s when we really need our friends—but being in such a sorry state means that we’re least capable of returning the favor. This creates the unfortunate incentive for our friends to abandon us when our need is greatest. And this happens. We call them fair-weather friends: people who disappear when we get sick, depressed, or lose our jobs, and then reappear when we’re back to normal. But as horrible as fair-weather friendship seems, from an evolutionary perspective, the real question is: Why aren’t all friends fair-weather friends? This is exactly how it works among other animals; a deer running from a predator doesn’t double back to help a friend. So what’s so different about human friendship?

The solution to the paradox, of course, is that true friends don’t abandon us in times of need because they know that when we return to health, happiness, or gainful employment, we will repay them the favor. This is a valuable arrangement to have, but the important thing to recognize is that true friendship wasn’t inevitable. It could have been otherwise, and a number of key pieces of human psychology had to evolve before true friendship could exist in its current form. Specifically, the biggest thing our species needed was a way to separate fair-weather friends from true friends. How do we do this?

In the same way that our eyes evolved to be sensitive to salient cues in our environment, our friendship detectors have evolved to be particularly sensitive to salient cues about whether our friends will abandon us in times of need. This isn’t to say that our friendship detectors only focus on being abandoned in tough times; we choose our friends based on many different characteristics, like who devotes the most time to us, who adds the most happiness to our lives, and who is the kindest, the funniest, or the best looking. But the point is that there was intense evolutionary pressure not to be abandoned in tough times, and this has a profound effect on how human friendship works.

Here are three of the most interesting consequences of our evolved friendship detectors:

1. Indebted to Those Who Help Us in Times of Need. The most obvious consequence is that we feel indebted to those who help us during tough times. We know this, of course, but consider how extreme we are in this regard. We all know people who are in friendships that are otherwise not very healthy, but they remain in them because said friend helped them at some critical juncture (e.g., “Liz was there for me when on one else was.”). We can also do hundreds of nice things for a happy friend, but all is forgotten when we drop the ball on a sad friend. This behavior only makes sense if we recognize that our friendship detectors are especially tuned to make sure we’re not abandoned at the worst possible time. In sum—for better and for worse—the strength of our friendships is often less about the total amount of happiness we add to our friends’ lives, and more about whether we were there for them in specific times of need.

2. Seeking Friends Who Think We Are Unique. Have you ever noticed how our friends really appreciate the skills we have but they do not (e.g., “Gertrude is so great, she’s such an amazing DJ, our parties would suck without her,” “Greg does such an awesome job planning our vacations—what would we do without him?” etc.). At first, it seems obvious why our friends would notice our unique traits—they are right there on display for anyone to see—but the hidden evolutionary logic of our friendship detector suggests another reason: perhaps we choose our friends in the first place based on who is most likely to appreciate our uniqueness. Why? This is exactly the strategy one would adopt to minimize the risk that our friends would ditch us in times of need. If we pick friends who value our unique traits, then we are hard to replace (or so they think), and so our friends are more likely to be there for us when it counts. In sum, no friendship is unconditional and when it comes time to ask for help, it’s better to be irreplaceable, and so we evolved to make friends with people who view us as unique.

3. The Difficulty of Modern Friendships. This implication is more speculative, but the nature of our friendship detectors might also help explain why so many people find it difficult to form close friendships. We are all living longer, healthier, and safer lives than at any time in recorded history, and this is a good thing, but it also means that many of our friendships have never been tested. Indeed, to know who really has your back, there needs to be some diagnostic event that separates true friends from fair-weather friends. We’ve all struggled at points in our lives, but modern friendships more often involve going to the mall than summiting Everest, exploring a harsh new frontier, or going to war. In sum, it takes a watershed event to separate true friends from fair weather friends, but these events are rare and so many of our friendships remain in true friendship limbo.

It might seem odd to take an evolutionary perspective on a question like true friendship. After all, friendships are a matter of personal taste, they change over the lifespan, and vary across cultures. But if there is one enduring truth about friendship, it’s that we want our friends to be there in times of need. This seems obvious to us, but it’s interesting to think that it could have been otherwise. It could have been that we judge our friends by who spends the most time with us, or who brings joy to our lives most often. Or it could have been that all friendships were fair-weather friendships that we ended as soon as we stopped getting proportional returns.

But our model of friendship is different. We give our friends our bottom dollar, put our lives on hold to visit when they are sick, and forget our own problems to help them with theirs. And because this is how friendship works, it has a number of consequences, including making us constantly on alert for possible signals of abandonment in times of need, motivating us to make friends with those who value our unique traits, and sometimes feeling uncertain about who are true friends really are. All this is part of the hidden evolutionary logic of true friendship.


Tooby, J. & Cosmides, L. (1996). Friendship and the Banker’s Paradox: Other pathways to the evolution of adaptations for altruism. In W. G. Runciman, J. Maynard Smith, & R. I. M. Dunbar (Eds.), Evolution of Social Behaviour Patterns in Primates and Man. Proceedings of the British Academy, 88, 119-143.