The twins were first taken from each other, then taken from their country. Abandoned during China’s one-child policy, they found themselves scattered around the world.
As they grew up they unwittingly found themselves a unique test for a key question: where does intelligence come from? How much is nature? How much is nurture?
The answer, according to a study published on this rare group of separated identical twins, is that the genetic component of intelligence predominates. Even when raised in different households, in some cases different countries, identical twins converge on the same IQ scores — and seem to do so more the older they get.
“As genetic factors kick in, the environment drops out,” said Nancy Segal, of California State University. “So they become more alike with time.”
Segal, who researches twins, came across the separated pairs from China after being approached by a mother. “She told me she’s looking for literature on how to raise one twin. I told her that doesn’t exist. Then she told me her story,” she said.
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A consequence of China’s one-child policy was the abandonment of thousands of girls because boys were more highly prized. “Some families wanted to get rid of them. So parents would give [them] up. And among these, thousands of baby girls who were abandoned there were twins.”
These twins were, in turn, separated. Parents from other countries looking to adopt them would often be assigned only one. Segal has worked to track down both halves in 15 instances where identical twins were separated in this way, then used them to understand aspects of intelligence. “The mum who initially contacted me knew a couple of other families. And then sometimes I’d read about them in the newspaper. Then families would contact me once they heard what I was doing.”
Of the 15 pairs, 14 are girls. It is not many but, she said, given how unusual it is, enough. “Even though it’s a small sample, it’s a much more powerful sample than twins raised together.”
Twins are useful in behavioural research because they share DNA and usually share an upbringing. With identical twins, nature and nurture are the same; with non-identical twins, who share only half their DNA, nature is different. By comparing the two, researchers can tease out the genetic component of traits.
In this case, though, the paradigm was reversed. Nurture was completely different but nature the same. This was what was so exciting for Segal, whose work is published in the journal Personality and Individual Differences. From their similarities, in all sorts of ways, you can see genetics emerging.
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“The twins just are themselves and they tell you a whole story about human behaviour. They’re so well aligned in so many ways that they don’t even think about it,” Segal said.
The IQ scores of the children were highly correlated and appeared to become more so as they got older, being tested at about 10 and 14, though the small sample size meant they could not be sure about the latter finding.
Either way, said Segal, it fits with a growing body of research into the importance of the genetic component in intelligence — yet that does not make parenting pointless. “Should parents and educators throw their hands up in despair? Absolutely not. Everybody can become smarter. But we’re not going to all be the same.”
