Identical Twins Separated Intercontinentally

Nancy Segal has a new study of 15 pairs of Chinese identical twins who were separated overseas. What was the impact on their IQs?

Psychologist Nancy L. Segal of Cal State Fullerton has one of the best specialties in all of the social sciences: twins.

Everybody, academic and non-academic, likes talking about twins and their endless variety of human interest stories. But, also, the data derived from the rigorous study of different types of twins — identical, fraternal, separated soon after birth, etc. — are crucial for disentangling some of the great conundrums of science, such as the roles of nature and nurture in human outcomes.

Segal was a co-author on Thomas Bouchard’s famous 1990 Minnesota Twins study that reunited identical twins who had been out of touch. She runs the CS Fullerton Twin Studies Center and often gets called when new examples of twins relevant to nature-nurture questions are discovered. She grew up with a fraternal twin sister, is a nice lady, and is really good at the science, so she’s ideal for overseeing research into these kind of fascinating family dramas.

For example, Professor Segal was called in to do the research into the remarkable two pairs of identical twins who were born in the same hospital in Colombia on the same day, but then, over the protests of the mothers that their babies were identical, the nurses told the mothers that all newborns look alike, and the babies got sent home mixed up into unrelated pairs. More than two decades later, to their great surprise, the four guys ran into each other on the streets of Bogota.

Professor Segal found in this Colombia case that the better educated city kids outscored the country kids on cognitive tests. That’s a fairly unusual outcome.

Now she’s back with a new set of data on 15 Chinese pairs of identical twins (14 of them girls) who were legally split up around 18 months of age on average as part of Mao’s one-child policy. Then, some of the children got adopted overseas, so the new dataset includes very rare examples of separated identical twins growing up speaking different languages.

From The Times of London:

Identical twins raised apart match IQ scores

Study of babies separated during adoption under China’s one-child policy has revealed nature may play greatest role in intelligence

The 15 sets of twins helped to shed light on how intelligence develops

Tom Whipple, Science Editor

Sunday June 30 2024, 10.25pm BST, The Times

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.”

This is known as the Wilson Effect.

… “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.

There are only several hundred well-documented examples of separated pairs of identical twins in the literature, so adding 15 is important by itself, and the intercontinental nature of the separation makes these datapoints particularly interesting.

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.”

For the full paper, see Personality and Individual Differences:

Developmental trends in intelligence revisited with novel kinships: Monozygotic twins reared apart v. same-age unrelated siblings reared together

Nancy L. Segal , Elizabeth Pratt-Thompson