English News


Why identical twins are not equally bright despite common genes

Scientists got a rare opportunity to study how identical twins differ and are similar, when brought up in different environments (Pic. Courtesy Twitter/@CristinaDragani)

It was one of the rare chances which the scientists could not dream of letting go. It was a golden opportunity, described a report in sciencealert.com, to study monozygotic or identical twins in common parlance, who in their childhood got separated and were raised in completely dissimilar environments – different countries and families.

The findings of this research — published in Personality and Individual Differences – were both interesting and amazing.

Twins generally have similar scores in cognitive tests thereby pointing that intelligent quotient is heritable to a tune of 80 per cent but in this particular case, there was a 16-point difference between the twins.

While there were several similarities too, the divergences were sharp and stood out. The findings stress on the need to relook at the perception about how much of intelligence is due to our genes and how much comes about because of the environment one is brought up in.

The scientists in their paper wrote: "Similarities were evident in personality, self-esteem, mental health, job satisfaction, and medical life history. In contrast with previous research, the twins' general intelligence and non-verbal reasoning scores showed some marked differences."

Born in 1974 in South Korea, the twins got separated at the age of two when one got lost in a market. The lost child found herself in a hospital located miles from her home and even as her biological parents and family desperately tried to locate her, she landed in the United States after being adopted.

It was after more than four decades, in 2020, they were reunited. This happened when the sibling in the US gave her DNA sample as part of a project to trace children lost to families in South Korea. Then the two were met by scientists who made them go through a series of interviews and tests.

Likeness in many spheres including that of job satisfaction and mental health were observed yet the child brought up in South Korea had scored highly in terms of processing speed and perceptual reasoning.

Though the scores were evident what caused them could not be fathomed.

The researchers also mentioned in their paper that the US twin as an adult suffered three concussions. This made her feel like a "different person", however, it was not possible to say categorically that if this had affected the cognitive tests scores.

It is vital to take note of the homes in which these siblings stayed and were brought up in. The US home of one twin had more conflict and less freedom when juxtaposed with the Korean home and family. The scientists mentioned this aspect in their paper. "The twins were raised in very different environments, aside from their different countries and cultures.”

On the issue of nature versus nurture, this study noted that certain attributes of behaviour can be the same even when the environments are diverse. The twins had high scores in terms of their levels of conscientiousness and self-esteem.

The US twin is more individualistic and less collectivist in terms of national culture in contrast to the South Korean one. Cultural differences like these, the scientists feel, could have impacted some of the personality scores.

Researchers agree that one case study like this is not enough to surmise about twins but with easy and accessible DNA testing available, they hope that more long-lost twins will be found and thus helping them to enlarge the scope and scale of their study.

Talking about this aspect, the study’s first author Nancy Segal, told PsyPost: "We need to identify more such cases if they exist. And we still do not understand all the mechanisms involved from the genes at the molecular level to the behaviours we observe every day."

Segal is from California State University.

Also read: Why Amazon tribes suffer less from Alzheimer’s disease and dementia