A new study of identical twins raised apart in South Korea and the United States sheds light on how genetic, cultural, and environmental factors interact to shape human development. The research, published in the journal Personality and Individual Differences, provides a rare opportunity to explore these dynamics in a transnational context, where the twins grew up in entirely different cultural settings and family environments.
“I have studied identical twins reared apart for many years. They pose a simple, yet elegant experiment for disentangling genetic and environmental influences on human traits. This case was unique in that the twins were raised in different countries,” said researcher Nancy L. Segal, a professor and director of the Twin Studies Center at California State University in Fullerton. Segal is also the author of the book Deliberately Divided: Inside the Controversial Study of Twins and Triplets Adopted Apart.
The twins in question were born in 1974 in Seoul, South Korea. At the age of two, one of the twins got lost during a visit to a market with her grandmother. Despite her family’s extensive efforts to locate her, including distributing flyers and appearing on a television program for missing persons, she was never found. She was eventually taken to a hospital 100 miles away, diagnosed with measles, and subsequently placed into foster care. She was later adopted by a family in the United States. Meanwhile, her twin sister remained with the biological family in South Korea.
For decades, neither twin knew of the other’s existence. However, in 2018, the twin who was adopted in the United States (referred to as US in the study) submitted a DNA sample as part of South Korea’s program for reuniting family members. This led to the discovery of her biological mother and, to her surprise, a twin sister who had been raised in South Korea (referred to as SK in the study). The twins met for the first time in 2021, almost 47 years after being separated.
In the new study, the twins were evaluated across a range of psychological and behavioral measures, including family environment, general intelligence, nonverbal reasoning ability, personality traits, cultural values, self-esteem, mental health, job satisfaction, and medical history. They also participated in structured interviews about their life experiences, providing a comprehensive look at how their different rearing environments shaped their development.
Not only did the twins experience different cultures growing up, they also were raised in very different family environments. The twin raised in South Korea grew up in a relatively stable, supportive, and cohesive family. This environment encouraged personal growth and provided a nurturing backdrop. In contrast, the twin raised in the United States reported growing up in a stricter, more religiously oriented household that had higher levels of conflict.
These different family environments align with cultural distinctions between the two countries. South Korea, with its more collectivist culture, places a strong emphasis on family harmony and group cohesion. The United States, known for its individualistic values, tends to prioritize personal independence and self-reliance. These cultural differences were also reflected in the twins’ value systems.
The twin raised in the United States showed more individualistic tendencies, favoring personal autonomy and equality. In contrast, her sister raised in South Korea expressed values more aligned with vertical collectivism, reflecting the hierarchical and collective nature of South Korean society.
“Genes have a more pervasive effect on development than we ever would have supposed — still, environmental effects are important. These twins showed cultural difference in some respects,” Segal told PsyPost.
The researchers found “striking” differences in cognitive abilities. The twin raised in South Korea scored much higher on tests of perceptual reasoning and processing speed, with a 16-point difference in IQ between the two. Such a gap is larger than typically observed among identical twins raised together, where the average IQ difference is about 7 points.
The researchers suggested that this discrepancy might be partially explained by US’s history of concussions, which could have impacted her cognitive functioning over time. US had suffered several concussions throughout her life, which are known to affect cognitive abilities, particularly in areas such as reasoning and processing speed.
Both twins showed similar strengths in verbal comprehension and working memory, suggesting that while some cognitive differences emerged, their genetic makeup still strongly influenced certain cognitive traits.
Despite being raised in very different environments, the twins had remarkably similar personalities. Both scored high on measures of conscientiousness, indicating that they are organized, responsible, and goal-oriented individuals. They also had low scores on neuroticism, suggesting emotional stability and resilience. These similarities align with previous research on twins, which shows that personality traits tend to have a strong genetic component.
In terms of mental health, both twins exhibited similar profiles. Their scores on clinical measures of anxiety, depression, and other mental health issues were within the normal range, indicating good mental health. This finding is notable given that US had experienced more family conflict and challenges growing up, which might have been expected to result in higher levels of psychological distress. However, her mental health remained comparable to that of her sister raised in South Korea, suggesting that genetic factors may have played a protective role in both twins’ emotional well-being.
Another area where the twins showed strong similarity was job satisfaction. Despite having very different careers—one as a government administrator in the United States and the other as a cook in South Korea—both reported high levels of satisfaction with their work. This may reflect their shared high levels of conscientiousness, a personality trait often linked to job satisfaction. Interestingly, the twin raised in the United States scored slightly higher on intrinsic job satisfaction, which measures personal fulfillment from one’s work. The researchers speculated that this might be due to differences in the nature of their jobs, with US working in a more stable, long-term professional role, and SK working in a job that required more flexibility and adaptability.
When it came to self-esteem, the twins were nearly identical, both scoring high on the Rosenberg Self-Esteem Scale. This finding was somewhat surprising, particularly for US, who had grown up in a more conflict-ridden home environment and had experienced significant life challenges, including divorce. The fact that both twins had such similar self-regard despite these differences underscores the strong role of genetics in shaping self-esteem.
While this study offers valuable insights into the effects of genetics, culture, and environment on human traits, it has some limitations. The sample size is extremely small, as the study focused on just one pair of twins. While the findings are intriguing, it is difficult to generalize from a single case study.
“We need to identify more such cases if they exist,” Segal said. “And we still do not understand all the mechanisms involved from the genes at the molecular level to the behaviors we observe every day.”
The study, “Personality traits, mental abilities and other individual differences: Monozygotic female twins raised apart in South Korea and the United States“, was authored by Nancy L. Segal and Yoon-Mi Hur.
