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Friendship may sometimes feel complicated, but it turns out that recognizing your true friends can be surprisingly simple. There are some fundamental elements that every close bond — including those with family and romantic partners — shares: To call someone a friend, the relationship must be long-lasting, it must be positive, and it must involve cooperation. That’s the three-legged stool that friendship rests on, say the evolutionary biologists I’ve been reporting on. Similar personalities or a shared sense of humor do matter, but they’re less essential than you might think. Remove a leg of that stool, though, and your “friend” may not be there for you when you need them—and being able to count on someone in a crisis, say the researchers, is the whole reason we have friends in the first place.

Getting Serious About Friendship

This stripped-down definition of friendship is the result of a shift in how scientists think about social behavior. For a long time — centuries, really — serious scientists didn’t study friendship, because they didn’t much respect it. It was too squishy, too ephemeral, too hard to define and measure (and defining and measuring are essential to the scientific process).

But then primatologists — yes, the researchers who spend their time observing monkeys and apes — noticed that the animals they watched seemed to have friends, too. A debate ensued about whether it was appropriate to use the word “friend” when talking about animals. In the end, the scientists decided to call it like they saw it, and what they saw was friendship.

How could they tell? Because in animals, the researchers could measure the interactions between individuals, over the lifetimes of generations of baboons and macaques and chimpanzees. From those years of observation, they developed — for the first time — the basic, standard definition of a close social bond I’ve already cited. “Friendship is a long-term, positive relationship that involves cooperation,” says psychologist Robert Seyfarth of the University of Pennsylvania, who, with his wife, biologist Dorothy Cheney, spent decades observing baboons and vervet monkeys in Africa and is a pioneer in the field.

Animals actively work to build these friendship bonds. They do it, the theory goes, to avoid being eaten by predators. Multiple studies, by Seyfarth and others, have shown that animals with the strongest social networks live longest and have greater reproductive success. “What friendship is about at the end of the day,” says evolutionary anthropologist Robin Dunbar of the University of Oxford, “is creating small-scale, intensely bonded groups that act as protection [to life’s] stresses.”

It’s not hard to see the logic in that for humans as well. Studies have shown that people who are socially connected also live longer. Seen this way, the three essential ingredients of friendship make a lot of sense.

Investing Time

The more time we spend with our friends — talking, sharing a meal, going to a movie — the closer we usually become. Monkeys spend time together by grooming each other — for as much as 20 percent of their waking hours. But they don’t groom with all members of their troop equally; they pick and choose, just as we do, building bonds over time. All of it, says Dunbar, lays groundwork to help us cope with what lies ahead, whether it’s a lion or a lay-off. “If you need a shoulder to cry on,” says Dunbar, “you need to prepare that shoulder ahead of time.”

Grooming consists of picking dirt and bugs out of one another’s fur, but it also triggers endorphins and oxytocin, the neurotransmitters, or chemical messengers, that make monkeys feel good and bond them more closely to each other. Human activities like laughing, singing, dancing, and storytelling — those things we do with friends — trigger the same neurotransmitters. In other words, there’s biology behind the feeling that a real friend is someone you want to spend time with, and who makes you feel good. By contrast, unpleasant interactions set off our stress responses.

Being Helpful

All that investing of time and good feeling only pays off if, eventually, your friend is there to help when you need it and vice versa — to bring dinner over in a crisis, to help you move. The two of you must cooperate. When you don’t — when a relationship feels too lopsided — it often fades away, or it should, because you need to invest in someone else.

On average, research shows that most of us have about five people in our lives who meet these three criteria, and those are the people we count as our most intimate friends. Who are yours? And are you doing everything you should to ensure that you will be able to count on each other when necessary?