Growing up as one of six siblings—the third oldest, and the second of three girls—Carlita Gay loved the distinction of a big family and that everyone was exposed to so many personalities. Though she saw her family less after moving away from her hometown, going to therapy as an adult helped Gay, now 33 and an executive assistant in New York, understand “how much of a refuge my siblings can still be” because of their deep context and shared history. In particular, they were some of the few people who could understand her experience of growing up biracial in a “mainly white” part of Minnesota. “I had a perspective of ‘Maybe I’m alone’” in trying to make sense of how her racial identity developed, Gay told me, but over time she realized that her siblings could relate to both that general experience and how it played out within their family. “If anyone could understand my experience the most,” she said, “it might be them.”

Not many people have five siblings like Gay does, but 82 percent of American kids do have at least one. The prototypical sibling relationship has two distinct phases. First, the kids’ connection is embedded within the family system and shaped by their parents. Then they start becoming independent, eventually leaving home and building their own lives. In these later years, the sibling bond is an intriguing mix of involuntary (nobody chooses their siblings) and voluntary (drifting apart from a sibling is generally considered less concerning than divorce or estrangement from a parent).

In other words, siblings are forced together, and then suddenly they’re not. The independence of adulthood—when proximity is no longer required and the obligations lessen—creates opportunities for siblings to build, repair, or discard the relationships of their youth, to stay stuck in or break free of the roles they played as children.

If there’s one thing that most people think they know about sibling relationships, it’s the influence of birth order. Experts I spoke with for this article wanted to correct that misconception: It’s a myth. Forget it. No matter how many kids post on TikTok lamenting “middle-child syndrome,” birth order doesn’t have that much influence on personality.

But even if the exact ordering of the kids doesn’t matter, siblings do shape one another’s lives deeply. During the teen years, brothers and sisters are as influential as peers, and far more so than parents, in guiding decisions about whether to drink and do drugs, according to Shawn Whiteman, an expert in adolescent development and a professor at Utah State University. In one study, subjects who had conflict or distance in their relationships with siblings before age 20 were more likely to be depressed at age 50. It’s worth noting, however, that a lot of sibling research is done on the specific formation of two siblings no more than five years apart, giving short shrift to wider age gaps and bigger families.

[Read: Are siblings more important than parents?]

The experts also pointed to a factor that does have a big effect on sibling relationships, and that contains the potential to poison the dynamic: favoritism. Katherine Jewsbury Conger, a UC Davis family-studies professor, has been interviewing sibling pairs—both together and separately—since they were in ninth grade; they are now adults. Through these interviews, Conger found that siblings remembered who shared more interests with Mom and who preferred to play ball with Dad. Being treated differently based on specific hobbies or perceived need, such as whether a sibling needed extra help with homework, was fine, but “if they felt like Mom or Dad or both of them played favorites with one of the siblings, that’s where you saw conflict or tension arise in the relationship,” Conger told me.

Favoritism-based feelings of resentment tend to peak before the teen years. When kids enter high school, they become more independent, spending more time with their own friends and less time supervised by their parents. At this point, the voluntary nature of sibling relationships starts to emerge and “you actually have to put effort forth to have a relationship,” Nicole Campione-Barr, a psychologist at the University of Missouri, told me. What happens in this period can set the stage for how siblings relate in adulthood.

For Mimi Gonzalez, a 27-year-old who hosts a podcast about grief and mortality, a traumatic event during the crucial period of adolescence changed her relationship with her older sister, Imahni. Like Carlita Gay, Gonzalez comes from a big family—five girls, ranging from 28-year-old Imahni to a 4-year-old. When Mimi was 6, Imahni, her stepsister, came into her life. Both curly-haired with freckles, the girls were often mistaken for twins. Though they fought constantly, they were close, too, and spent most of their time with their best friend, Meghan.

Meghan died by suicide when the girls were 15; grief from the loss caused Mimi and Imahni to grow apart. Imahni began to retreat and “distanced herself a lot” compared with before, Gonzalez told me.

This year, Imahni became pregnant. “It kind of opened the door,” Gonzalez said. Talking about the coming baby became a way to initiate other conversations and then, slowly, talk about Meghan and other losses, and how the two women would relate going forward. “Most of the core memories we have is when we were young, playing outside, going to the beach, going camping, and just not having these adulting responsibilities,” Gonzalez said. “And we both made it clear that we can still create core memories even though some of the people we love aren’t here. At least we have each other and she’s bringing new life into the world. We’re really trying to work on our relationship, and trying to become closer.”

[Read: Why are people weird about only children?]

Conger calls what happened between Mimi and Imahni “dynamic re-centering”: the process of siblings coming together later in life to reevaluate and rewrite their bond. During childhood, kids see their parents as the focus of the family, so re-centering, then, refers to centering the sibling bond instead, and changing that way of relating.

Many factors affect who is most likely to build, or rebuild, a relationship. One of the most important predictive factors, according to both interviews and survey data, however, is warmth. Parents may lament how much their kids bicker, but conflict at least shows that the siblings are engaged with one another. High conflict is not necessarily a problem if there is also high warmth; more worrying are relationships that are dismissive and uninvolved.

Still, Conger suspects that the majority of siblings are likely to go through some type of dynamic re-centering. “I’m not going to say there’s not a couple of 40-year-olds out there who don’t still have rivalries,” Michele Van Volkom, a psychologist at Monmouth University, told me, but she thinks that time and maturity are powerful in wearing down old barriers.

Major life events and transitions, such as marriage, pregnancy, birth, and death, can all shake up a family. Though most of the research on dynamic re-centering has focused on its more positive possibilities, these big events and their associated stresses can drive people apart as well. For example, those favoritism-based resentments that experts mentioned can reemerge when siblings need to come together to provide caregiving, according to Megan Gilligan, a human-development professor at Iowa State University.

[Read: It used to be okay for parents to play favorites]

For Mimi and Imahni, Meghan’s death drove them apart initially, but the pregnancy kicked off a process of reevaluating their relationship. Conger believes that a common catalyst for this sort of revived attention is a new shared experience—for example, someone having their first child might reach out to a sibling who already has kids for input and advice.

Carlita Gay said that circumstances surrounding their father’s illness deepened the relationship between her and her brother Ryan, who is 36 and a therapist in Minnesota. The six kids had gotten along when they were young but were not particularly close. “All my siblings are very sensitive people—my parents included—and I think that sensitivity sometimes had us be guarded with each other and not as vulnerable with our emotions,” Carlita explained.

Five years before he died, their father moved in with Ryan, the oldest of the siblings. It had always been hard for Carlita to get in touch with their parents, but with Ryan there to answer her calls, she was able to talk with her parents more frequently. “Ryan wasn’t shy to give that insight into what was happening with both my parents, and it was especially helpful because I couldn’t get there for two of the years with COVID,” she said. “These are really important things that someone gives you access to. Being out of town, it just meant a lot to me to have that plug in.”

Ryan remembers giving Carlita updates on their father, and he said that that became a way for them to have conversations about their own lives, relationships, and hopes. Now they talk on the phone every weekend and frequently send each other voice memos. Ryan is not only a bridge to her parents but to different parts of her life too. Two years ago, he visited Carlita in New York, and now he is the only person from their family to have met her cats and seen her apartment from that time. “I no longer have those cats because they’re with my ex, and he saw them at seven weeks [old],” she said. “That did hit me. I was like, ‘I’m so glad you did that.’ It’s like grief. When someone is gone or animals are gone, it’s really cool when people have met them.’”

Carlita was realizing something that many sibling researchers emphasize: that for a lot of people, sibling bonds are the longest relationships of our lives. We know siblings before we meet our partners (and before we have our own kids), and we’ll know them after our parents die. “I tell my students this all the time,” said Van Volkom, the Monmouth psychologist. “There’s only one person who’s been there from the beginning. Not my friends, not my spouse, not anybody but my sibling.” Siblings are a living part of someone’s history and a form of memory that lives outside of ourselves. And because every enduring relationship can be frustrating, layered with decades of misunderstandings and baggage, sibling bonds have their own complexity. But the longevity and changing nature of the relationship offers siblings a choice: to let that history define the bond or to use the past as a foundation from which a new way of relating can grow.

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