The age-old debate over nature versus nurture (whether our genes or environment play a more important role in shaping our intelligence) has long intrigued scientists, educators, and the public. A recent groundbreaking study, published in the journal Personality and individual differencessheds new light on this discussion.
The longitudinal study, the first of its kind using young monozygotic twins raised apart, reveals an increase in IQ similarity as the twins grow older. These findings suggest that genetic factors become more influential in shaping our intellectual abilities as we get older, while environmental factors are more important during our early years.
Twin studies are a cornerstone of behavioral genetics and provide a unique window into the ongoing nature versus nurture debate. These studies typically involve comparing identical (monozygotic) twins, who share 100% of their genetic material, with fraternal (dizygotic) twins, who share about 50% of their genetic material, similar to regular siblings. By examining these two types of twins, researchers can disentangle the effects of genetics (nature) and environment (nurture) on various traits and behaviors.
The new study aimed to address several limitations in previous twin research, particularly the need for longitudinal data on twins raised apart. Most twin studies are cross-sectional, meaning they capture only a single snapshot in time. This approach can miss important developmental changes and trends. Additionally, many studies have examined twins raised together, but fewer have looked at twins raised apart, which helps isolate genetic influences from environmental ones.
“Twin studies are a simple and elegant way to examine the genetic and environmental influences on behavior. No one had ever done a longitudinal study of young separated twins in real time. I was able to do this with twins who were largely adopted separately because of China’s former one-child policy,” said study author Nancy L. Segal, a professor of psychology and director of the Twin Studies Center at California State University, Fullerton.
The study included three different groups: young Chinese monozygotic twins raised apart, Danish adult monozygotic twins raised apart, and “virtual twins” – siblings of the same age who are not related and grew up together.
The first group consisted of 15 pairs of young monozygotic twins from China, who were separated due to the country’s one-child policy. This policy, implemented between 1980 and 2016, often resulted in the abandonment of children, including twins, who were subsequently adopted by different families. These twins were identified through various means, including media coverage, referrals, and self-referrals.
The study measured the intelligence of these twins at two time points using the age-appropriate Wechsler IQ test. At the first test (Time 1), the twins had an average age of approximately 10.69 years. The second test (Time 2) occurred an average of 4.18 years later, when the average age of the twins was 13.93 years. The twins were tested by different trained examiners to avoid bias, and the scores were carefully checked for consistency.
The second group consisted of 12 pairs of adult monozygotic twins from Denmark, who were identified between 1954 and 1957. These twins were separated early in life and followed longitudinally. The intelligence of these twins was assessed using the Wechsler-Bellevue test for intelligence.
The Danish twins had a mean age of 51.42 years at the first test (time 1). The interval between the two test sessions averaged about 11.17 months. Like the Chinese twins, the Danish twins were tested independently to avoid sharing information that could influence the results. In six pairs, the same examiner performed both tests, while a different examiner tested the remaining pairs.
The third group, virtual twins, consisted of 43 pairs of unrelated siblings of the same age who had grown up together. These siblings were either both adopted or one biological and one adopted. Virtual twins were defined as unrelated siblings who were born within nine months of each other, entered the adoptive family at one year of age, and were in the same grade at school.
The virtual twins’ intelligence was assessed twice using the Wechsler IQ test. The first test (time 1) occurred when the twins were on average about 5.11 years old, and the second test (time 2) occurred when they were about 10.77 years old, with an average interval of 5.65 years between tests. To avoid potential bias, different examiners tested each sibling pair on the same day.
For the Chinese separated twins, the study found that their IQ scores became more similar over time. The intraclass correlation for their IQ scores increased from 0.51 at the first test (Time 1) to 0.81 at the second test (Time 2), suggesting a strong genetic influence. Furthermore, the within-pair difference in IQ scores decreased from an average of 11.93 points at Time 1 to 7.93 points at Time 2. This convergence suggests that as these twins grew older, their shared genetic makeup played a more prominent role in determining their intelligence, despite growing up in different environments.
Similarly, the Danish adult separated twins also showed an increase in IQ similarity over time, with their intraclass correlation increasing from 0.64 at Time 1 to 0.74 at Time 2. This pattern further supports the idea that genetic factors become more influential in shaping intelligence as individuals age. The Danish twins also showed a slight but significant increase in their mean IQ scores from Time 1 to Time 2, highlighting the stability of intelligence over time in adults.
In contrast to the apart-reared twins, the virtual twins showed a decrease in IQ similarity over time. The intraclass correlation for their IQ scores decreased from .30 at Time 1 to .11 at Time 2, indicating that the influence of the shared environment on intelligence decreases as children grow older.
The within-pair difference in IQ scores increased from an average of 10.74 points at Time 1 to 14.12 points at Time 2. This divergence suggests that nongenetic factors, such as unique experiences and individual choices, become more important in shaping intelligence over time for siblings who share no genetic link.
Overall, the study found that while intelligence levels can change over time, the direction and magnitude of these changes are influenced by genetic factors. For the Chinese and Danish twins raised separately, the increase in IQ similarity and the stability of their IQ scores over time emphasized the growing importance of genetics. On the other hand, the virtual twins’ decreasing IQ similarity emphasized the diminishing impact of shared environmental factors as they grew older.
“As we get organized, genetic factors become more important in shaping our overall potential,” Segal told PsyPost. “Home factors are more important when we’re younger, but as we age, we select opportunities and events that align with our genetic potential. Extreme environments can, of course, overwhelm genetic potential, as I described in my 2018 book Coincidental brothers. And we can all improve our skills through hard work — genes don’t set skills in stone.”
Despite these compelling findings, the study has some limitations. Due to the rarity and difficulty of identifying and following apart-reared monozygotic twins, sample sizes were relatively small, which could affect the generalizability of the results.
“Nevertheless, confirmation of the hypotheses specified at the outset is encouraging,” the researchers said. “It is fortunate that ongoing IQ analyses are planned with participants in the Minnesota Study of Twins Reared Apart who were tested for IQ twice (Segal, 2012). Further analyses of the [Chinese reared-apart twins] are also being planned and will hopefully provide additional participants for research.”
The study, “Developmental Trends in Intelligence Revisited with New Relationships: Monozygotic Twins Reared Apart Versus Unrelated Same-Age Siblings Reared Together,” was authored by Nancy L. Segal and Elizabeth Pratt-Thompson.