TThere are many types of friendship. Most adults in the United States say they have friends who occupy certain niches in their liveslike gym buddies or work friendsThese relationships can come and go as life circumstances change, fading when someone changes jobs or loses interest in a shared hobby.
And then there are close friends, those with whom you lean on in difficult times and know on a deeper level. Many adults in the U.S. say they have only a small handful of friends fall into this category.
Even rarer are true forever friends, those who stick by you for decades—through jobs, moves, relationships, fights, losses, and life stages—and may even feel like family. But what makes a friendship strong enough to stand the test of time in this way?
“Common personality 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 that there are seven areas of overlap that are particularly important for forming lasting friendships: speaking the same language, growing up in the same neighborhood, similar career trajectories, and shared hobbies, points of view, sense of humor, and taste in music. Every pair of close friends may not have all of these things in common, but the more they have in common, the stronger their relationship is likely to be, Dunbar says.
Read more: How to Make Friends as an Adult – At Every Stage of Life
Despite the popular belief that opposites attract, research actually shows that “we prefer people who are very similar to us,” he says.
Research Jeffrey Hall, director of the Relationships and Technology Lab at the University of Kansas, also believes that people need to spend a lot 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 closely connected than those who keep things superficial.
“When you establish a strong, intimate relationship with someone, consistency is key to maintaining it,” says Aminatou Sow, co-author of the book. Great Friendship: How We Maintain Closeness to Each Other with friend Ann Friedman. Friends who are down on their luck don't necessarily have to see each other all the time, but research shows that maintaining friendships is important, Sou says.
Reassuring the future — making it clear to your friend that you want them in your life for the long haul — and developing shared rituals are good ways to do this, she says. A “ritual” can be as simple as regularly sending memes or scheduling a monthly phone call. Or it can be borrowed from the realm of family and romance: taking an annual vacation with a friend, celebrating birthdays and life events together, even marking the anniversary of your friendship. “It’s the little things that keep the magic alive,” says Sow.
And that’s “magic,” according to Sou. She doesn’t think science has all the answers when it comes to close friendships or why some relationships last forever. “You can’t predict who you’re going to fall in love with,” romantically or platonically, she says. “Some of it is mystery and magic, and some of it is hard work.”
Hall agrees that there’s an ineffable quality to some of the best friendships. Science suggests that it takes a long time to build a strong bond, “but what’s really weird,” he says, is that once people become best friends, they can go months or even years without talking and still pick up where they left off. Sometimes, “once you’ve had a really strong friendship,” Hall says, “it never stops being that way.”
How do true best friends explain their long-term relationships? TIME spoke to a pair of friends to find out.
Amy Cohn, 69, who lives in New York, and Madeline Rudin, 69, who splits her time between Florida and Connecticut, have been friends for 65 years.
MR: We grew up across the street from each other in New York City. We met on the playground and went to kindergarten together the next day. We ended up going to school together for 14 years. We just clicked.
AK: I never felt like I fit in well at school, so having a best friend meant everything. Madeline got me through the first 18 years of my life. It was always us against the world.
MR: It got more difficult when we went to college in different states, but we wrote letters. I visited once or twice, and we saw each other when we were home on school breaks.
AK: But then we had a long period where we didn't talk. I came out to Madeline when I was 21, and she was amazing. But I had a few separate bad experiences with straight people, and I ended up getting sucked into the gay community in New York City in the late 20s and 30s. We didn't talk during that time.
MR: I made other friends. I wasn't mad at her; I just decided we were going to grow apart. Then one day, out of the blue, Amy sent me an email.
AK: There is no good answer as to why it took me so long to do this. I finally did it because I was with family for Thanksgiving and my cousin asked about Madeline. The phone rang right after I sent the email and it was Madeleine. I didn't know email traveled that fast!
MR: The day we met again for lunch, I remember crying. It felt like years had been wasted because we just immediately connected again. It never stopped after that.
AK: In many ways, we are very different – I love sports and being active, and Madeline doesn’t. But there is a level of trust and unconditional acceptance that underpins everything. I know I can tell Madeline anything, and if she doesn’t agree, there is no judgement. Whenever something really good or really bad happens in my life, I want to tell Madeline immediately. She just gets me. If I think I’m funny, she thinks so too.
MR: I feel the same way. I would say the same about her! I have had a lot of health issues over the years too and Amy has been there every step of the way. She is my rock, besides my son. I know she won't get tired of me being sick. She is just so supportive.
AK: We are very open about how important we are to each other. We say, “I love you” a lot. We have verbalized that we will be there for each other forever, and that, at our age, is incredibly comforting.
Another piece of advice I gave my daughter was, “Realize that all my friends are idiots on any given day.” I mean, I don't like everything they do, but I still love them.
MR: I told my son, “You can have friends for different reasons.” Not all of my friends want to do everything I want. For a while, I felt like I had to be as close to everyone as everyone else. Then I realized, “No, I don't.” But with Amy, I don't have any of those problems.