There are some surprising results to report after researchers took advantage of a rare opportunity to study identical monozygotic twins who were separated early in life before being raised in different countries.
In this case there was a significant 16-point difference between the siblings, because IQ has been shown to be up to 80 percent heritable.
There were a lot of similarities between the pair, but the differences were notable, suggesting that there needs to be a rethink of how much of our intelligence is down to our genes and how much is down to the environment that we are raised in.
The researchers write that similarities were evident in personality, self-esteem, mental health, job satisfaction, and medical life history.
In contrast to previous research, the twins' general intelligence and non-verbal reasoning scores showed some marked differences.
The twins were separated at the age of two after one of them got lost in a market. Despite efforts by her family to find her, the lost sibling was adopted by a couple in the US after she was taken to a hospital 100 miles away from her family home.
The US twin gave a sample of his saliva as part of a program to find children lost to families in South Korea. The pair were contacted by researchers and put through a series of tests.
The twin raised at home in South Korea scored more highly in terms of perceptual reasoning and processing speed, despite many similarities.
The reason for the scores is not clear. The researchers note that the US twin suffered three concussions as an adult, which made her feel different. It is not certain if this has affected the scores seen in the cognitive tests.
The family homes the twins were raised in were different from each other and they were in completely different parts of the world. The researchers found that there was more conflict and less freedom in the US home than in South Korea.
The twins were raised in very different environments, aside from their different countries and cultures.
The study supports the idea that certain behavioral traits can stay the same, even when kids are brought up in different environments, and that both twins scored highly in terms of their levels of conscientiousness and self-esteem.
In terms of national culture, the US is more individualism and less collectivist than South Korea. Some of the personality scores reported may have been affected by cultural differences.
It is important not to draw too many conclusions from one pair of twins, but the findings do make fascinating reading, and the spread of easily accessible DNA testing means it is likely that more long-lost twins will be discovered in the years to come.
Nancy Segal from California State University is the first author of the study.
We don't understand all the mechanisms involved from the genes to the behaviors we observe.
The research has been published.
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