There are many flavors of friendship. Most U.S. adults say they have pals who fit into specific niches in their lives, like gym friends or work friends. These relationships may come and go as life circumstances change, fading away when someone switches jobs or loses interest in a shared hobby.

Then there are close friends, those you lean on in hard times and know on a deeper level. Many U.S. adults say they have only a small handful of friends who fit into this category.

Rarer still are the true forever best friends , those who are by your side for decades on end — through jobs , moves , relationships , fights , losses , and life stages — and may even come to feel like family . But what makes a friendship durable enough to stand the tests of time in this way?

Shared traits, interests, and backgrounds help a lot, says Robin Dunbar, an evolutionary psychologist and author of Friends: Understanding the Power of Our Most Important Relationships.

Dunbar’s work suggests there are seven areas of overlap that are particularly crucial in forming a solid friendship: speaking the same language, growing up in the same area, having similar career trajectories, and sharing hobbies, viewpoints, senses of humor, and tastes in music. Every close friend pair may not have every one of these things in common—but the more they share, the stronger their relationship is likely to be, Dunbar says.

Despite the cliché that opposites attract, research actually suggests “we prefer people who are very similar to us,”he says.

Research by Jeffrey Hall, director of the Relationships and Technology Lab at the University of Kansas, also finds that people need to spend lots of time together—at least 300 hours—to become true best friends. And, Hall says, friends who express their deepest thoughts and emotions to each other tend to become more tightly bonded than those who keep it surface level.

Once you’re solidly close with someone, consistency is key to staying that way, says Aminatou Sow, who co-wrote the book Big Friendship: How We Keep Each Other Close with her friend Ann Friedman. Ride-or-die friends don’t necessarily have to see each other all the time, but research does suggest friendship maintenance is important, Sow says.

Assurances about the future—making clear to your friend that you want them in your life for the long haul—and developing shared rituals are good ways of doing that, she says.

A “ritual”can be as simple as regularly sending memes or scheduling a monthly phone catchup. Or it can be borrowed from the realms of family and romantic relationships: taking an annual friend vacation, celebrating birthdays and life events together, even marking your friendship anniversary. “These are small things that keep the magic alive,”Sow says.

And it is indeed “magic,”in Sow’s view. She doesn’t think science has all the answers when it comes to close friendship and why some relationships last forever. “You don’t predict who you fall in love with,”romantically or platonically, she says. “Some of it is mystery and magic and the rest of it is hard work.”

There is an ineffable quality to some best friendships, Hall agrees. Science suggests it takes a lot of time to build a strong bond—“but what’s very weird,”he says, is that once people become best friends, they may go months or even years without talking and still pick up right where they left off. Sometimes, “once a very strong friendship has been created,”Hall says, “it never really stops being that way.”