In a very rare event, identical (or monozygotic) twin girls were raised in different countries, and the studies following their reunion challenge long-held beliefs about nature vs. nurture. Despite differences, one brought up in the United States and the other in Korea, studies found they shared many personality traits. T

he most surprising finding was that their cognitive abilities varied greatly, with the American twin at an IQ level 16 points lower than her sister.

The twins were born in 1974 in Seoul, South Korea, and through an unfortunate set of events, one girl was adopted by a family in the U.S.

After reuniting via a DNA program, the twins reported very different childhood experiences. The American twin grew up in a harsh household, marked with conflict, and divorced adopted parents. The Korean sister describes a harmonious and loving upbringing.

The study, published in the journal Personality and Individual Differences, tested the twins’ intelligence, personality profiles, mental health, and medical history.

Speaking to the significant IQ gap, the study authors write, “it is striking that the twins showed substantial differences in cognitive abilities that have been linked to strong genetic influence.”

It’s unknown if this discrepancy was due to different childhoods, including that the US twin suffered three concussions that may have affected her cognitive abilities. Other assessments found “the overall configuration of the twins’ personality was similar, consistent with literature on moderate genetic influences on personality in adulthood.”

The twins’ exceptional circumstances reveal interesting information about how genetics, culture, and, individual experiences impact human development, but more exploration into monozygotic twins is needed to draw scientific evidence about nature vs. nurture.