Twins grew apart, then were compared — The results are shocking

Medical journals profile only a few instances of twins growing up separately, so this present case provides rare medical and sociological insights.

 monozygotic twins (photo credit: Felipepuntocl/Wikimedia)
monozygotic twins
(photo credit: Felipepuntocl/Wikimedia)

A pair of identical twins who grew up in separate countries showed unexpectedly large gaps in their cognitive abilities, despite very similar personality traits. A comparison of these sisters’ characteristics brings new light to an ancient debate: Does environment and/or genetics influence intelligence most?

These twins were born in Seoul, South Korea, in 1974. The sisters became separated at age two when one twin got lost in the market. Despite her parents' appearance on a TV show about missing persons, the young girl was eventually adopted by a couple in the States, the IFL website reported.

Growing up in America

Growing up in the US, the adopted twin didn’t know that she had siblings until she did a DNA test; the company referred her to South Korea's family reunification program in 2018. Two years later, she was notified that she had a twin, and another brother and sister .

After being reunited, the twins completed a series of tests to assess their intelligence, personality profile, mental health and medical history. Somewhat surprisingly, results revealed that the IQ of the twin who grew up in the US was 16 points lower than that of the sister who grew up in Korea.

The findings, published in the journal Personality and Individual differences, contradicts previous studies on monozygotic twins, which noted an average IQ difference of no more than seven points. Referring to the unexpectedly large gap between the two sisters, researchers said: "It’s amazing that the twins showed significant differences in cognitive abilities that were linked to a strong genetic influence.”

Nature or nuture

It’s hard to say if this gap was caused by the twins' different upbringings, although researchers note that the sister who grew up in the US suffered from three previous concussions which may have affected her cognitive abilities. They added that the overall configuration of the twins' personalities was similar, consistent and with moderate personal genetic effects in adulthood.

"It’s worth noting that the two twins received a significantly high score in conscientiousness, indicating that they’re both deliberate, well-organized, compliant and achievement-oriented," the researchers said. The fact that these similarities continued despite the different life experiences and home environments of the sisters is interesting, emphasizing the role that genetics plays in determining a person's temperament.

For example, while the twin who grew up in Korea described the family home as loving and harmonious, the adopted sister reported difficulties in education, which was colored by constant conflicts and her adopted parents’ divorce. Despite this, the two women had identical scores for self-esteem and very similar mental health profiles.

The two also shared aspects of their medical history, since both had surgery to remove ovarian tumors. Yet the twins were divided over the ideologies embedded in their culture, with the sister who grew up in the US exhibiting a more individualistic view while the one in Korea had more collectivist values.

Twins being raised in different places is rare and more cases like this need to be investigated before decisive conclusions can be drawn. Nevertheless, this study provides some fascinating new insights into the genetic, cultural, and environmental factors which influence human development.