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“For so many women, the process of becoming requires two,” Julie Buntin writes in her novel Marlena, in which a 30-something woman obsesses over the memory of her childhood best friend. “It’s not hard to make out the marks the other one left.” The fictional relationship is based on Buntin’s memories of her own glamorous, drug-addicted best friend, whose liver gave out in her early 20s. “Lea was the kind of person you join Facebook to stalk,” Buntin wrote in an Atlantic piece about her inability to stop monitoring Lea’s Facebook page, years after her status updates ceased.

Marlena is part of a slew of recent books focusing on a woman’s complex, lifelong link to her first real friend. Elena Greco — the narrator of Elena Ferrante’s Neapolitan novels — marries into a cosmopolitan family and makes a name for herself as a writer, but can’t shake the feeling that she is less deserving than her naturally brilliant best friend, who never left the poor neighborhood where they grew up. In her memoir The Hot One, Carolyn Murnick (an editor at New York) explores how her relationship with her childhood best friend shaped her sense of identity even after their paths diverged — even after a cross-country move and an underlying rivalry drove them apart, and even — especially — after Ashley was murdered at 22. Ashley was charismatic, unself-conscious; Carolyn was bossier, more of a misfit. Both had brothers and Jewish dads. It was Ashley — who had Hollywood-good looks and was dating Ashton Kutcher when she died — who had been “the hot one,” leaving Carolyn to take up the mantle of the “smart one.”

The impact of a child’s relationship with her caregiver is well-known: Any number of online quizzes and best-selling self-help books will tell you that the type of relationships you form as an adult and your very way of being in the world can be traced back to the way you related to your parents. Did you have an “anxious” attachment? You’re doomed. Avoidant? You’re doomed. Secure? Congrats! But what about the first relationships kids choose for themselves? Writers may be ahead of psychologists here, but researchers are coming to appreciate how formative those bonds are.

“An underlying assumption of these attachment theorists is that the earlier relationships result in an ‘internal working model’ or ‘relational schema’ that guides how adolescents navigate and experience future relationships,” explains Amy Bellmore, a psychology professor at the University of Wisconsin-Madison. Those attachments can be with peers as well as parents. “Early adolescence is when we see the emergence of an understanding of intimacy and the growing importance of feeling trust in relationships.” In the prepubescent years of middle school, girls tend to peel off into intense, exclusive pairs, while boys are more likely to hang out in groups, an escalation of a gendered pattern that emerges in early childhood. “Whether it’s socialized or whether there’s a genetic predisposition, it’s something we see throughout the lifespan,” says Mitch Prinstein, a psychology professor at the University of North Carolina and author of Popular: The Power of Likability in a Status-Obsessed World.

It’s also a developmental sweet spot. “There’s something about the pubertal years when those relationships get far more emotionally intimate, especially for girls,” says Prinstein. “They are more likely to tell their friends things rather than parents. They rely on each other for coping.” Eleven- and twelve-year-olds are capable of a more mature intimacy than younger kids, but haven’t yet been bruised by the breakups and rejections that make adults more cautious. Their identity is still forming, their boundaries fluid; they can try on the hobbies, preferences, even the personality of a favorite friend. Through sleepovers and play dates, they become privy to the idiosyncrasies and private rhythms of a second family unit, expanding their world and challenging their ideas of what’s normal.

Part of the intensity of these friendships is situational: A burgeoning capacity for empathy coincides with a flood of new opportunities and freedoms. In middle school, kids have a larger pool of potential buddies to choose from, and can finally pick their own friends, based on criteria more meaningful than who’s on their bus route or who their parents approve. “Friendship is particularly important during middle school because supervision from adults declines,” says Brett Laursen, a psychology professor who has studied adolescent peer pressure and the longevity of middle-school friendships. “They have a lot more latitude in terms of their choices of what to do and who to spend time with.”

And the transition to middle school can be turbulent. “They need help navigating this brand new world where people aren’t looking over their shoulder all the time,” says Laursen. “When you were little and you went to the museum, your teacher would tell you you have to stick close to that buddy they assigned you. Nobody’s assigning you a buddy, but you want to stick close to somebody as you navigate adolescence.”

There is often something passionate, even proto-romantic about those best-friend bonds. While it should never be assumed that a close relationship between two women is homoerotic — just ask Oprah, who is perennially interrogated about the nature of her friendship with Gayle — it shouldn’t diminish the validity of a friendship to acknowledge a pseudo-sexual undercurrent, either. Harry Stack Sullivan, an influential early-20th-century psychiatrist, argued that same-sex, one-on-one prepubescent friendships are practice for later heterosexual romance. According to Sullivan, a shift takes place sometime around the ages of 8 to 10, as children begin to form more reciprocal, emotionally intense relationships, and to consider the needs and desires of their friends. Instead of focusing solely on “what should I do to get what I want,” he wrote, the preadolescent starts to think about how to contribute to the “feeling of worth-whileness of my chum.” And in thinking about how the friend perceives her, the preadolescent gains a new understanding of herself: “One gets a look at oneself through the chum’s eyes.”

Though not all of Sullivan’s ideas stand up to modern research, the gist of this one does. “The pattern for our behaviors in romantic relationships seems to be predicted by the kinds of friendships that we had and the things that happened to us within those friendships,” Prinstein says. “People who had difficulties with their friendships might have difficulty, growing up, establishing supportive relationships. Our relationships with our peers are the most important in adolescence, so it makes sense that they would hold a special power years later.”

As consuming as those friendships are in the moment, though — and as sincerely as 10-year-olds believe that they will last forever — they tend to be transient. “You don’t really know who you are or what you’re looking for,” Laursen said. His study, which followed hundreds of kids from the beginning of middle school until the end of high school, found that only one percent of friendships forged in seventh grade made it to graduation. The friendships fell apart as their individual identities took shape: they were most likely to drift apart if one friend became more successful — whether academically or socially — than the other. Yet the legacy of those bonds can endure. I know: I haven’t spoken to my middle-school best friend — my “chum,” as Sullivan might say — in years, but I still Google her every so often. Maybe next time, I won’t feel as weird about it.