Researchers seized a rare chance to examine the personalities and cognitive abilities of identical twin girls separated early in life and raised in different countries. While striking similarities emerged, the analyses also uncovered notable differences that might not appear under typical circumstances.
Cases of twins raised apart are exceedingly rare, particularly across countries and cultures. A study published in the journal Personality and Individual Differences details the behavioral, physical, and medical similarities and differences in a pair of monozygotic twins reared separately. Monozygotic twins originate from a single fertilized egg, sharing identical DNA.
Born in South Korea in 1974, the twins were separated at age two when one got lost in a market and was taken to a hospital dozens of kilometers away. Despite exhaustive searches by her biological family, she was adopted by an American couple. The sisters reunited in 2020 after the US twin submitted DNA to a program reuniting lost South Korean children. Researchers then conducted extensive tests and interviews.
The study revealed numerous similarities. "Similarities were evident in personality, self-esteem, mental health, job satisfaction, and medical history," the authors note. Yet the twin raised in South Korea outperformed her sister in perceptual reasoning and processing speed, with a 16-point difference on cognitive tests.
The cause of this gap remains unclear. The US twin experienced three concussions in adulthood, which may have played a role, though certainty is impossible. Their home environments differed markedly: the American household had more conflict and less freedom. Broader cultural contrasts—individualism in the US versus collectivism in South Korea—likely influenced outcomes.
This case challenges us to reassess how much of our intelligence stems from genes versus environment. A single pair limits firm conclusions, but accessible DNA testing may enable more such reunions and richer data for future research.
