Identical Twins Raised Apart in Different Countries Have Huge IQ Difference

Scientists have found that two identical twins raised in different countries have a huge IQ difference.

A report published in the journal Personality and Individual Differences, assessed the differences between one twin raised in the United States and the other in South Korea.

While the report said that "twins reared apart are rare," this particular pair—born in South Korea—was separated at age two in 1976, after one became lost in a supermarket. Despite the family's efforts to find the lost twin, she ended up in foster care and eventually came to live with an adoptive family in the United States.

Eventually, the adopted sister was reunited with her twin in 2018. The two underwent scientific comparisons for their intelligence as well as their personalities and medical histories.

Twins
A stock photo shows baby twins. A pair of twins raised separately in the U.S. and South Korea were found to have a "substantial" difference in their IQs. kckate16/Getty Images

Researchers found that the twins had "substantial differences" in their IQ.

Tests revealed that the adopted twin had an IQ 16 points lower than her long-lost sister. This contradicts existing research that says identical twins usually have no more than 7 points difference in their IQ.

While researchers are not certain what caused the drastic IQ difference, the study noted that the adopted twin had a more disruptive upbringing than the sister, who had been raised within her biological family.

The twin living in South Korea described a more loving family environment than her sister, who suffered disruption in her home life, including her adoptive parents divorcing.

However, despite the difference in IQ between the pair, their personality scores were almost identical.

The study said that similarities between the twins were "evident" in "personality, self-esteem, mental health, job satisfaction and medical life history." While their IQ difference was notable, their similarities in personalities were less surprising, and in line with existing research on twin genetics.

The study said it was particularly notable that the twins were both very conscientious, meaning they were well-organized and striving for success. This, in particular, indicates the role that genetics is thought to have in crafting a person's personality, as the adoptive twin showed these traits despite her difficult upbringing.

The study said that the findings enhance understanding of the influences of "genetic, cultural, and environmental influences on human development."

One of the authors of the report is Nancy Segal, a professor and director of the Twin Studies Center at California State University in Fullerton.

"The findings show, that despite cultural differences, some traits remain remarkably similar in identical twins, demonstrating genetic effects, " she told Newsweek. "Most striking to me—both were short and did not reveal their height to people!"

Segal also said that the study sheds new light on the nature vs nurture debate.

"The study shows in a dramatic way (identical twins raised indifferent cultures) that genetics influences many traits, but all traits are a complex mix of genetics and environments," she said. "Also, not all traits are equally affected by genes and environments—so it is impossible to say if nature or nurture is more important—it is specific to specific traits. Also, genes do significantly affect mental ability, but in individual cases like these twins, the effects may be less salient."

This article has been updated to include quotes from Nancy Segal.