A pair of identical twins who were raised in separate countries have displayed unexpectedly large disparities in cognitive abilities while exhibiting highly similar personality traits. A comparison of the monozygotic sibling’s characteristics sheds new light on the age-old nature/nurture debate.

Born in Seoul, South Korea, in 1974, the sisters became separated at the age of two when one twin got lost at a market. Despite her parents’ appearance on a television program about missing persons, the young girl could not be reunited with her family and ended up being adopted by a couple in the US.

Growing up stateside, the adopted twin was unaware that she even had any siblings until she submitted her DNA to South Korea’s program for reuniting family members in 2018. Two years later, she received word that she not only had an identical twin, but an older brother and sister too.

Having been reunited, the twins then completed a series of tests designed to assess their intelligence, personality profiles, mental health, and medical history. Somewhat surprisingly, results revealed that the IQ of the twin raised in the US was 16 points lower than that of the Korean-raised sibling.

The finding, published in the journal Personality and Individual Differences, contradicts previous studies on monozygotic twins, which have indicated an average IQ difference of no more than seven points. Commenting on the unexpectedly large gulf between the two sisters, the study authors write that “it is striking that the twins showed substantial differences in cognitive abilities that have been linked to strong genetic influence.”

Whether this discrepancy was caused by the twins’ different upbringings is hard to say, although the researchers note that the sister raised in the US had suffered three previous concussions, which may have influenced her cognitive capacities.

Continuing their assessment of the pair, the researchers go on to reveal that “the overall configuration of the twins' personality was similar, consistent with literature on moderate genetic influences on personality in adulthood.”

“Notable is that both twins are distinctively high on Conscientiousness, indicating that both are purposeful, well-organized, dutiful, and achievement-striving.” That these similarities persisted despite the sisters’ contrasting life experiences and home environments is interesting, and highlights the role that genetics play in determining a person’s temperament.

For instance, while the twin raised in Korea described growing up in a loving and harmonious family home, the adopted sister reported a harsher upbringing, colored by regular conflict and the divorce of her adopted parents. Despite this, the pair had identical scores for self-esteem and highly similar mental health profiles.

The pair also shared aspects of their medical histories, with both having undergone surgery to remove tumors from their ovaries. However, the twins differed with regard to their culturally imprinted ideologies, with the US-raised sibling demonstrating a more individualist outlook while the sister raised in Korea had more collectivist values.

Understandably, instances of twins being raised separately are rare, and more cases like this need to be studied before any firm conclusions can be drawn. Nonetheless, this study provides some fascinating new insights into the genetic, cultural, and environmental factors that influence human development.