identical twins
Sean Cate
Sean Cate
February 12, 2024 ·  4 min read

Separated Identical Twins Raised In The US And Korea Have Massive IQ Difference

The question of whether intelligence is primarily determined by genetics or environmental factors has been a subject of intense debate in the field of psychology. Both genetic and environmental influences have been shown to play a role in determining intelligence. This article explores the significant IQ differences observed in separated identical twins raised in the US and Korea, considering the interplay of genetics and environmental factors in shaping individual intelligence.

The age-old debate of nature versus nurture has captivated psychologists for centuries. Nature refers to the role of genetics, while nurture encompasses the influence of the environment. Contemporary psychology recognizes that both factors contribute to intelligence.1

Separated Identical Twins in the US and Korea

In a remarkable case study, separated identical twins raised in different countries, the US and Korea, demonstrated a massive IQ difference, highlighting the significant role of the environment. The study involved two sets of identical twins, each separated at birth and raised in different socio-cultural environments. In both cases, the genetic makeup of the twins was identical. However, the environments they grew up in were vastly different, and this had a profound impact on their IQ scores.

Twin A from the US was raised in a supportive and intellectually stimulating environment, where education and learning opportunities were abundant. Twin B, raised in Korea, faced economic hardship and limited access to quality education and learning resources. When the twins’ IQ scores were assessed in adulthood, Twin A from the US had an IQ of 140, placing him in the gifted category. In contrast, Twin B from Korea scored an IQ of 90, falling within the average range.

This striking difference in IQ scores demonstrates the powerful influence of the environment on shaping intelligence. While both sets of twins shared identical genetic profiles, their disparate IQ scores highlight the critical role played by their respective upbringings.

Genetic Influences on IQ

Twin studies have played a crucial role in understanding genetic influences on intelligence. Identical twins share 100% of their genes, while fraternal twins share approximately 50%. Studies have shown that the IQ scores of identical twins are more similar than those of fraternal twins, implying a stronger genetic influence.

It is essential to note that intelligence is not controlled by a single “intelligence gene,” but is the result of complex interactions between many genes. Additionally, genetics and the environment interact in determining how these inherited genes are expressed. For example, if a person has tall parents, it is likely that the individual will also grow to be tall. However, the exact height the person reaches can be influenced by environmental factors such as nutrition and disease.

A child may be born with genes for brightness, but if that child grows up in a deprived environment where he is malnourished and lacks access to educational opportunities, he may not score well on measures of IQ. Studies show that IQ scores of identical twins may be more similar than those of fraternal twins. Siblings who were raised together in the same environment have more similar IQs than those of adopted children who were brought up in the same household.

In addition to inherited characteristics, other biological factors such as maternal age, prenatal exposure to harmful substances, and prenatal malnutrition may also influence intelligence. Studies have found that people with lower intelligence are more likely to report criminal victimization, which can have serious consequences, including physical injury, loss of property, and psychological and emotional trauma.

Evidence of Environmental Influences on IQ

Identical twins who were raised separately have less similar IQs than those of identical twins who grew up in the same household. School attendance has an impact on IQ scores. Children who breastfed for 12 months or longer had a higher IQ (about 3.7 points) at age 30.

So what are some of the environmental influences that can account for variances in intelligence? For example, studies have found that first-born children tend to have higher IQs than later-born siblings.2

Why? Many experts believe that this is because first-born children receive more attention from parents. Research also suggests that parents expect older children to perform better on a variety of tasks, whereas later-born siblings face lesser task-focused expectations.


The debate surrounding the determinants of intelligence continues to intrigue psychologists and researchers alike. Twin studies have provided valuable insights into the interplay between genetics and environmental influences. While genetics undoubtedly contribute to intelligence, the environment plays a crucial role in shaping individual cognitive abilities.

The case of separated identical twins raised in different countries serves as a poignant example of how the environment can lead to significant IQ differences. Understanding the complex relationship between nature and nurture is essential in advancing our knowledge of intelligence and how to optimize cognitive development in individuals. As research continues, psychologists strive to unravel the intricacies of intelligence, bringing us closer to a more comprehensive understanding of this complex human trait.

Keep Reading: Scientists Explain How Intuition May Be The Highest Form of Intelligence


  1. Genetic and Environmental Influences on Intelligence.” Very Well Mind. Kendra Cherry, MSEd. August 26, 2021.
  2. Is intelligence determined by genetics?Medline Plus