Researchers have taken advantage of a rare opportunity to study identical (aka monozygotic) twins who were separated early in life, before being raised in different countries by different families – and there are some surprising results to report.
Whereas IQ has been shown to be up to 80 percent heritable – with twins usually scoring roughly the same on cognitive tests – in this case there was a substantial 16-point difference between the siblings.
There were, of course, lots of similarities between the pair, but the differences were also notable, suggesting that there needs to be a rethink of how much of our intelligence is down to our genes and how much of it is down to the environment that we’re brought up in.
“Similarities were evident in personality, self-esteem, mental health, job satisfaction, and medical life history,” the researchers write in their paper.
“In contrast with previous research, the twins’ general intelligence and non-verbal reasoning scores showed some marked differences.”
The pair of twins were born in South Korea in 1974, and were separated at the age of two after one of them got lost in a market. After being taken to a hospital that was 100 miles away from her family home, and despite efforts by her biological family to find her, the lost sibling was eventually adopted by a couple in the US.
The twins were reunited in 2020 after the US twin provided a DNA sample as part of a program to track down children lost to families in South Korea. The pair were then contacted by researchers, and put through a series of tests and interviews.
Despite many similarities – including in the areas of mental health and job satisfaction – the twin raised at home in South Korea scored more highly in terms of perceptual reasoning and processing speed.
While the scores are clear, however, the reason for them isn’t. The US twin suffered three concussions as an adult, the researchers note, which made her feel like a “different person”. However, it’s impossible to say for sure whether this has affected the scores seen in the cognitive tests.
What’s also worth taking into consideration is that the family homes the twins were raised in were not at all similar, besides being in completely different parts of the world. There was more conflict and less freedom in the US home compared with the one in South Korea, the researchers report.
“The twins were raised in very different environments, aside from their different countries and cultures,” write the researchers.
What the study also backs up in terms of nature versus nurture is the idea that certain behavioral traits can stay the same, even when kids are brought up in different environments; both twins scored highly in terms of their levels of conscientiousness and self-esteem, for example.
Generally speaking, the US is more individualistic and less collectivist in terms of national culture, compared with South Korea. The researchers think that these cultural differences likely had an impact on some of the personality scores reported.
While it’s important not to draw too many firm conclusions from one pair of twins, the findings do make fascinating reading – and the spread of easily accessible DNA testing means it’s likely that more long-lost twins will be discovered in the years to come, giving scientists more data to work with.
“We need to identify more such cases if they exist,” evolutionary psychologist Nancy Segal from California State University, the first author of the study, told PsyPost.
“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 research has been published in Personality and Individual Differences.