What Identical Twins Reveal About Personality, Individuality, and the Self

Approximately .2 percent of the earth’s population, or 14,000,000 people worldwide, are identical twins.

Despite their fairly commonplace occurrence, identical twins remain a source of mystery and inherent intrigue for our collective imagination. They have been the subject of countless cultural studies, stories, mysteries, and fables. The uncanny similarities that twins can exhibit, even when raised separately, raise endlessly fascinating questions: Is there some undetectable telepathic pull that binds these bodies—two parts of the same initial embryo—together no matter the distance? Is it coincidence, or evidence of fate?

Examining the cases of identical twins with disparate upbringings provides the key to answering these questions, even offering insights into the age-old debate of nature vs. nurture.

The Minnesota Twins Study (MTS), a 20-year watershed investigation conducted between 1979 and 1999, attempted to address these compelling inquiries. As one of the most extensive, in-depth twin studies in history, it included 56 fraternal and 81 identical pairs. The larger umbrella project resulted in over 170 individual studies, ranging in topics from IQ, to sexual orientation, to gum health.

Dr. Nancy Segal, a Professor of Psychology at California State University, Fullerton, and Founder and Director of The Twin Studies Center, worked on the study for nine years. Later, Segal wrote a book about the experience, entitled “Born Together–Reared Apart: The Landmark Minnesota Twins Study.”

Reflecting on the most significant takeaways from her experience, she tells BTR, “The fact that identical twins raised apart were as similar in personality as identical twins raised together shows us that shared environments do not make people alike.”

Segal recalls the story of Jack and Oscar, two twins who she interacted with through the course of the study. Despite being raised separately and experiencing strikingly different childhoods, they still grew up to be eerily alike. While Jack was raised as a member of the Jewish faith, in Trinidad, his twin Oscar grew up in Nazi Germany, where he was part of the Hitler Youth. When Jack and Oscar finally met, many of the idiosyncrasies they displayed in their own lives corresponded to an astounding degree.

“They both used to sneeze loudly in elevators thinking that was very funny,” Segal muses. “They both had a lot of weird habits, like washing their hands before they used the toilet. They both used to read books back to front, they both were very patient, both very high-tempered. They had a lot of similarities … they both dealt with many different situations in similar ways.”

Unsurprisingly, though both of the twins expressed an interest in politics, their interpretations and analyses of the same events were inconsistent.

“The difference between them, of course, was their understanding of politics and history, but they both realized that were it reversed, they would’ve been the other person,” explains Segal. For her, Jack and Oscar’s experience illustrates how our basic temperaments and personality tendencies may be influenced by genetics, while our environment influences the content.

The study of twins has revealed that even a person’s happiness, which no doubt is a defining facet of personality, has a genetic source.

Tanya Lewis, a science writer with familiarity of the MTS, explains to BTR that the research ultimately revealed an inextricable link between genetics and our fundamental attitudes.

“The Minnesota Twin Study has shown that certain measures of happiness do have a genetic component,” she says. “Twins raised in separate environments reported similar levels of happiness.”

Personality is complex. Often we perceive it to be something completely our own–unique, unpredictable, and formed by our personal experiences. Though the MTS reveals that this is not entirely the case, to a certain extent, there is truth in this perception. According to Segal, personality relies only 50 percent on genetic influence.

“Half the variation from person to person is because of differences in your genes,” she says, “and half is your environment. It’s not like you can take a person and slice their personality into genetic parts or environmental parts–you can’t do that.”

However, by bringing together and examining genetically indistinguishable humans with distinctly different experiences and considering them two sides of the same coin, Segal ostensibly does get the sensation of separating a person into their respective genetic and environmental parts.

There’s nothing quite like uniting two completely separate, yet inextricably related people.

Segal reflects, “When you watch the twins [meeting for the first time] it’s like this uncontrollable moment of glee. They become children in a way, when they touch each other’s faces and they dance around…it’s just marvelous to witness.”

Coming face to face with oneself is indeed an immensely powerful event. The moment of meeting is so unbelievable that for some, it’s utterly overwhelming.

“Sometimes they look at each other’s faces and they look so much alike that they have to turn away,” says Segal. “It’s almost like its too intense.”