Posted on May 12, 2022

Psychologists Found a “Striking” Difference in Intelligence After Examining Twins Raised Apart in South Korea and the United States

Eric W. Dolan, PsyPost, May 7, 2022

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.


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.


“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.