Growing up, my identical twin sister Amanda and I were rarely called by our names. “Earl girls” was an easier choice, thanks to our conveniently rhyming last name. My twin great aunts, Patience Rosenbaum and Penelope Pestronk, recall their family calling them “Twin.” And my family friends Ellen Levy and her identical twin were “Ms. Levy” until they graduated from the University of Michigan in 1990.

According to Live Science, identical births account for approximately 0.4% of the global population, making them a rare phenomenon. Stories of twins raised apart and reunited tend to be even more compelling narratives. This June, "The Accidental Twins," a Netflix documentary about two sets of twins from Colombia, joined the rich collection of films on pairs separated during infancy.

Identical, or monozygotic, twins form when a single fertilized egg splits into two, resulting in individuals with the same genetic makeup. Twins have played an invaluable role in studying the impacts of genetics versus environment in human development. The renowned 1990 article “The Minnesota Study of Twins Reared Apart” proved the strong role of genes in determining human behavior, personality and intelligence.

Netflix’s documentary and the Minnesota study focused on twins already separated at birth. But during the '60s and '70s, child psychiatrist Peter Neubauer deliberately separated multiples at birth to study the nature versus nurture debate, a longstanding psychological discussion about whether genetic predispositions or environmental factors more strongly shape development. The study faced severe criticism for its lack of informed consent and the psychological distress it caused participants.

"The idea that two people born together should be separated affects us all deeply. It goes against what really should be," said Nancy Segal, American psychologist and contributor to the Minnesota study. "It's like having two identities."

Segal said she believes depriving twins of each other would be taking away a part of their selfhood.

Twins who are lucky enough to grow up together get a lifelong companion. Grant Drake, my great uncle and brother to Rosenbaum and Pestronk, struggled with loneliness and depression when he was younger. He recalls his parents having a rocky relationship and the family being distant.

"The twins always had someone to talk to. I envied them for that," he said.

As a former psychiatrist, Drake also said he was not the only family member facing depression. While Rosenbaum and Pestronk shared almost everything growing up, from their looks to their friends, their mental health differed. Pestronk developed depression as a young adult, primarily due to family struggles and a lupus diagnosis in college. But Pestronk said she thinks some of her depression stemmed from an identity crisis as an identical twin. During college, she often compared herself to Rosenbaum, the more outgoing and tenacious one.

"She was always so comfortable in social situations. Why wasn't I?" Pestronk said.

As Pestronk grew older, she found effective therapy and accepted her unique identity. Her path to individuality began soon after college when she moved to New Haven with her husband to work as a pediatrician. Meanwhile, Rosenbaum stayed in Ann Arbor, Mich., to pursue a career in health policy and finance, and she did not get married for another decade.

For other twins who grow up together, this process can take longer.

"I didn't realize who I was until I was 30," said Eileen Pearlman, a licensed therapist and identical twin. "I finally learned to recognize my own needs instead of matching my twin's."

Pearlman, who specializes in twin issues, finds the "separation and individuation process" the most challenging aspect of being a twin. According to an Italian study published in Scientific American, twins begin their relationship in the womb as early as 14 weeks into gestation. Researchers observed the movements of twin fetuses in five pregnancies, finding that they intentionally made more contact with one another than with themselves or the uterus walls.

Whereas a "singleton" child develops independently in the uterus, twins become comfortable with constant contact with their counterparts, a feeling that carries over postnatally. To break attachment with their mothers, babies are usually given a transitional object – a blanket or teddy bear that soothes them when their mother is absent. For identical twins, their twin becomes their transitional object.

"Twins have to learn how to separate from each other in addition to just their mother," Pearlman said. "It's a process that must happen for a twin to become independent."

Twin codependency can extend into adolescence and even adulthood, so parents often try to foster individuality early. My parents separated me and my twin after my preschool teacher noticed that my sister was becoming a spokesperson for both of us: she answered for me and even wrote my name.

"The idea was to give you more room to have your own identity and develop your own relationships," said my mom, Joanna Drake.

However, Segal finds that class separation can harm twins more than it can help them.

"It's already difficult for twins to separate from their parents and home. Isolating them from each other adds an extra burden, one we don't impose on singleton children," she said.

She advises parents to take a more moderate approach to building their twins’ individuality and opt to put them at different tables and play groups. Often, twins just need to see each other to feel comfortable.

A recent article by The Conversation supports Segal's view, showing little evidence that twins perform better academically in separate classes. A 2022 Canadian study found that keeping twins in the same primary classes positively impacted their behavior and social interactions.

"Having a twin is a protection," said Bennett C., a Highland Park attorney and identical twin to Alan C.

The two brothers were inseparable during high school and few could tell them apart from each other. But as Alan C. moved across the world, their contact diminished. Now 72, they see each other once every 10 years. Bennett C. has asked his brother to move from Tucson, Ariz., to be closer to him, but no relocation has been made yet.

"He's not as open about his feelings," Bennett C. said. "I feel like a piece of me is missing."

Child development research was far less advanced in the '50s and '60s when Bennett C., Alan C. and my great-aunts grew up. As a result, their families and friends treated them as a unit until they went to college.

"It was back in the era when it was fashionable – almost required – to dress them alike," Drake said.

Rosenbaum said twins forced togetherness was more about convenience.

"It was easier to stick the twins together,” she said. “If one of us had measles, stick the other twin with her so we both get it at the same time."

Despite being born 20 years later, identical twins Ellen Levy and Laura Jana experienced similar upbringings. They scored the same on tests, played doubles in high school tennis and both attended the University of Michigan.

Apart from their physical appearances and voices, Levy and Jana hardly live the same life. Jana is a pediatrician and educator in Omaha with three adult children. Levy is a Silicon Valley entrepreneur with one child, whom she gave birth to at 39.

"It frustrates me when people joke that we're so similar because I'm so aware of all our differences now," Levy said. "I think it's just laziness at a high level."

These three pairs of twins exemplify how varied the twin experience can be. While Rosenbaum and Pestronk live in different cities – one in Ann Arbor, Mich., and the other in Washington, D.C. – they have remained best friends. They call each other at least once a day, discussing everything from what they ate to their relationship issues. Both have never felt the urgency to develop such a close bond with anyone else apart from their husbands.

"I'm not sure it's possible to achieve that same level of closeness and intimacy in any other relationship," Rosenbaum said.

Levy and Jana are also grateful for their relationship. Like Rosenbaum and Pestronk, they appreciate having someone who consistently understands them, even through disagreements. They said they have become more empathetic because of it.

"People find it surprising that we're not super close and don’t talk on the phone every day," Jana said. "We almost don't say anything because it shatters people's image of twins."

Of course, some twins share few ties beyond genes. Others purposely choose different lifestyles because of their similarities. Amanda and I are so alike that I sometimes feel like we're telepathic – we often blurt out the same thing simultaneously or unintentionally wear the same color. I've always tried to avoid these habits, fearing people will think I'm too close to my sister. But I'm learning that our similarities do not diminish my individuality. Pearlman's advice resonates with me: "To force yourself to be different, that's not being yourself either. Being yourself is choosing what you want. If your twin happens to choose the same, what are you going to do?"

When asked what it's like to be an identical twin, Bennett C. usually responds by asking what it's like to be a single person. I also find it hard to explain – there is nothing to compare it to since it is all I've ever known. I often worry that I will always need my twin. But, unlike those twins who were purposely separated at birth for research, I am lucky to have her in the first place.

"What I say to people is imagine if you were never lonely," Pestronk said. "Even when you're not with them, you know they’re there."