Psychologists found a “striking” difference in intelligence after examining twins raised apart in South Korea and the United States

A new study of monozygotic twins raised apart in South Korea and the United States provides unique insight into how genetic, cultural, and environmental factors influence human development. The new research has been published in the scientific journal Personality and Individual Differences.

“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.

The twins were born in 1974 in Seoul, South Korea. One of the twins became lost at age two after visiting a market with her grandmother. She was later taken to a hospital that was approximately 100 miles away from her family’s residence and diagnosed with the measles. Despite her family’s attempt to find her, she was placed into the foster system and ended up being adopted by a couple residing in the United States.

She later discovered she had a twin sister after submitting a DNA sample in 2018 as part of South Korea’s program for reuniting family members.

In the new study, the twins completed assessments of family environment, general intelligence, nonverbal reasoning ability, personality traits, individualism-collectivism, self-esteem, mental health, job satisfaction, and medical life history. They also completed structured interviews about their general life history.

Not only did the twins experience different cultures growing up, they also were raised in very different family environments. The twin who remained in South Korea was raised in a more supportive and cohesive family atmosphere. The twin who was adopted by the U.S. couple, in contrast, reported a stricter, more religiously-oriented environment that had higher levels of family conflict.

The researchers found “striking” differences in cognitive abilities. The twin raised in South Korea scored considerably higher on intelligence tests related to perceptual reasoning and processing speed, with an overall IQ difference of 16 points.

In line with their cultural environment, the twin raised in the United States had more individualistic values, while the twin raised in South Korea had more collectivist values.

However, the twins had a similar personality. Both scored high on measures of conscientiousness and low on measures of neuroticism. They also had a similar level of satisfaction with their job, even though their occupations were quite different — a government administrator and a cook. The twins also had similar mental health profiles and had identical scores on the measure of self-esteem.

“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.

“We need to identify more such cases if they exist,” she added. “And we still do not understand all the mechanisms involved from the genes at the molecular level to the behaviors we observe every day.”

Segal is also the author of the book “Deliberately Divided: Inside the Controversial Study of Twins and Triplets Adopted Apart.”

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.

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