In 1976, a two-year-old infant was found by herself unattended in a market in Seoul, South Korea. She was taken to the hospital where she was diagnosed with measles, and since she couldn’t be identified, the little girl was placed in the foster system. Later, an American couple adopted her and raised her in the United States. Only decades later, as a grown adult, would the woman learn that she had an identical twin sister — and the two are surprisingly different in many ways, researchers found in a new study.

Culture may matter much more than previously thought

What explains academic performance or our personality? Enter the age-old “nature vs nurture” debate. No one can deny that education and upbringing play critical roles in the formation of individuals, whose quality — or lack thereof — have been found to be consistent predictors of success in adulthood.

But genes, which can shape intelligence, and athletic performance, or predispose people to diseases, also play major roles in our development. But by what amount? Half-half maybe?

It’s safe to say that the weight of each factor cannot be established with accuracy. People are very complicated and answering this question would require a strictly controlled randomized experiment, and as we all know life doesn’t really work like that.

However, the South Korean twins provide a unique opportunity. After the sister in the US submitted her DNA in 2018 as part of South Korea’s program for reuniting family members, there was a match and scientists immediately jumped on board to perform some assessments on the twins.

Nancy L. Segal, a professor and director of the Twin Studies Center at California State University in Fullerton, assessed each twin’s family environment, personality traits, world views and value system, medical history, as well as things like general intelligence and mental health.

Over the course of their investigation, Segal and colleagues learned that the South Korean twin was raised in a supportive and cohesive family. Her adopted sister was not abandoned; her disappearance was an accident due to her grandmother’s negligence who basically lost her at the market. Meanwhile, the sister who grew up in the US was raised by a family in a stricter, more religious-orientated household where there was more conflict.

Perhaps not that surprising, the sisters’ almost polar opposite family environments shaped radically different models of how they view the world. The sister raised in South Korea had more collectivist values — morals and social behaviors that favor the group over the interests of its individual members — while the one who grew up in the US had individualistic values.

But what was striking was the huge difference in cognitive abilities between the two sisters. The woman raised in South Korea scored considerably higher on intelligence tests than her sister from the US, with an overall IQ difference of 16 points.

Previously, a 2001 study out of the University of California at Los Angeles had 10 pairs of identical and 10 pairs of fraternal twins go through a battery of tests that examined 17 separate abilities, including verbal and spatial working memory, attention tasks, verbal knowledge, motor speed and visuospatial ability. The researchers also performed MRIs to scan each volunteer’s brain. When they compared each pair of twins, the researchers were amazed by how similar their brains were to one another, both in terms of structure and cognitive abilities, leading them to conclude that IQ is inherited.

Anecdotally speaking, identical twins often seem to have similar life trajectories. Scott and Mark Kelly are identical twins and both extraordinarily managed to become NASA astronauts, traveling to the International Space Station. The new study, however, suggests that one’s family environment can drastically alter a person’s life — even among twins.

“In contrast with previous research, the twins’ general intelligence and non-verbal reasoning scores showed some marked differences. Adding these cases to the psychological literature enhances understanding of genetic, cultural, and environmental influences on human development,” the researchers wrote in their study that appeared in the journal Personality and Individual Differences.

Of note was the fact that the two reunited South Korean sisters had very similar personalities, with both scoring high on measures of conscientiousness and low on measures of neuroticism. Their self-reported levels of satisfaction with their jobs was also very similar, despite one working as a cook and the other as a government administrator. The two also had similar mental health profiles and identical scores on measures of self-esteem.

These results highlight once more that both nature and nurture are important, with the caveat that this is a study on only one pair of subjects. We’d have to find dozens if not hundreds of estranged twins to really dive deep into this discussion. Perhaps there are more waiting to be found — something that isn’t that far-fetched considering more and more people are having their DNA sequenced.