Identical twins separated as toddlers and raised in the US and Korea have ‘striking’ IQ difference

A set of identical twins who were separated as toddlers and raised in different countries have provided interesting insight into the nature/nurture debate.

The two girls were born in 1974 in Seoul, South Korea, where they lived together until they were two years old.

It was at that age that their maternal grandmother took them to a market, where one of the twins became separated from her family and got lost.

After being spotted by strangers, the lost twin was taken to a hospital approximately 100 miles from her home, before going to live with a foster mother.

The twin had no recollection of being lost, and she was eventually transferred to an adoption agency who organized her adoption by a family in the US.

Fast-forward a few decades to 2018, when the lost twin submitted a DNA sample as part of a program in South Korea to reunite family members.

Her DNA went into the system, and in October 2020 she learned that not only was she a twin, but a sibling to a brother and another sister.

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Following her discovery, researchers from both the US and South Korea administered tests and interviews to the twins to assess their intelligence, personality profiles, mental health, and medical history.

Overall, the researchers found the makeup of the twins' personality was similar and 'consistent with literature on moderate genetic influences on personality in adulthood'.

This came in spite of the fact that the twin raised in Korea reported having a harmonious family home, while the sister in the US experienced conflict and the divorce of her adopted parents.

“Notable is that both twins are distinctively high on Conscientiousness, indicating that both are purposeful, well-organized, dutiful, and achievement-striving," the researchers wrote in the findings, which were published in the journal Personality and Individual Differences.

The similarities between the pair helps to highlight the role of genetics in developing personality, though the researchers were surprised to find 'substantial differences' in the IQ of each twin.

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Previous studies of identical twins indicated an average IQ difference of no more than seven points. However, the results in this case found that the twin raised in the US had an IQ 16 points lower than her sister.

"It is striking that the twins showed substantial differences in cognitive abilities that have been linked to strong genetic influence," the researchers wrote.

The gap between when the twins were separated and when the tests were conducted makes it hard to say whether their upbringings were responsible for the difference in IQ, though the study did note that the sister raised in the US had

suffered three concussions in her life, which may have impacted her cognitive capacities.

Differences which could be more clearly linked to their upbringing included their ideological outlook, with the sister raised in the US adopting a more individualist view while the sister who remained in Korea had more collectivist values.