The age-old nature versus nurture debate—whether our genetics or our environment plays a greater 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 of young monozygotic twins raised apart, found an increase in IQ similarity as the twins aged. These findings suggest that genetic factors become more influential in shaping our intellectual abilities as we age, while environmental factors are more important in our early years.
Twin studies are a cornerstone of behavioral genetics, offering a unique perspective on 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, like normal siblings. By examining these two types of twins, researchers can distinguish the effects of genetics (nature) and environment (nurture) on various traits and behaviors.
The new study aimed to address several limitations of 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, while many studies have looked at twins raised together, fewer have looked at twins raised apart, making it easier to isolate genetic influences from environmental ones.
“Twin studies are a simple and elegant way to examine genetic and environmental influences on behavior. No one has ever conducted a real-time longitudinal study of young, separated twins. I was able to do this using twins, most of whom were adopted separately because of China’s former one-child policy,” said study author Nancy L. Segal, professor of psychology and director of the Twin Studies Center at California State University, Fullerton.
The study involved three separate groups: young Chinese monozygotic twins raised apart, adult Danish monozygotic twins raised apart, and “virtual twins” (same-age, unrelated siblings raised together).
The first group included 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 led to the abandonment of children, including twins, who were then adopted by different families. These twins were identified through various means such as media coverage, reports, and self-reports.
The study measured the twins’ intelligence at two time points using the age-appropriate Wechsler IQ test. At the first test (Time 1), the twins had an average age of about 10.69 years. The second test (Time 2) took place an average of 4.18 years later, when the twins’ average age was 13.93 years. The twins were tested by different trained examiners to avoid bias, and the scores were carefully reviewed for consistency.
The second group consisted of 12 pairs of adult monozygotic twins from Denmark, 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 Intelligence Test.
The Danish twins were on average 51.42 years old at the first test (time 1). The interval between the two testing sessions was on average about 11.17 months. Like the Chinese twins, the Danish twins were tested independently to avoid any exchange of information that could bias 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 were raised together. These siblings were either both adopted or one biological and the other adopted. Virtual twins were defined as unrelated siblings who were born nine months apart, entered the adoptive home at age one, and enrolled in the same school class.
The intelligence of the virtual twins was assessed twice using the Wechsler IQ test. The first test (time 1) took place when the twins were on average 5.11 years old, and the second test (time 2) took place when they were approximately 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 twins raised apart, 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), indicating a strong genetic influence. Additionally, 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 aged, their shared genetic makeup played a larger role in determining their intelligence, despite being raised in different environments.
Similarly, Danish twins raised apart 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 reinforces the idea that genetic factors become more influential in shaping intelligence as individuals age. Danish twins also showed a small but significant increase in their mean IQ scores from Time 1 to Time 2, highlighting the stability of intelligence over time in adults.
Unlike twins raised apart, virtual twins showed a decrease in IQ similarity over time. The intraclass correlation for their IQ scores decreased from 0.30 at Time 1 to 0.11 at Time 2, indicating that the influence of the shared environment on intelligence diminishes as children get 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 do not share a 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 Chinese and Danish twins raised apart, the increasing IQ similarity and stability of their IQ scores over time highlighted the growing importance of genetics. On the other hand, the decreasing IQ similarity of virtual twins highlighted the diminishing impact of shared environmental factors as they age.
“As we become more orderly, genetic factors become more important in shaping our overall abilities,” Segal told PsyPost. “Family factors are more important when we are younger, but as we get older, we choose opportunities and events that fit our genetic potential. Of course, extreme environments can overwhelm genetic potential, as I described in my 2018 book Accidental Brothers. And we can all improve our skills through hard work: genes don’t set abilities in stone.
Despite these compelling results, the study has some limitations. Due to the rarity and difficulty of identifying and following monozygotic twins raised apart, sample sizes were relatively small, which could affect the generalizability of the results.
“Nevertheless, the confirmation of the hypotheses initially formulated is encouraging,” the researchers said. “It is fortunate that additional IQ analyses are planned using participants from the Minnesota Twins Raised Apart Study who were tested twice (Segal, 2012). Further analyses of Chinese twins raised apart are also planned and will hopefully identify additional study participants.”
The study, “Developmental Trends in Intelligence Revisited with New Kindreds: Monozygotic Twins Reared Apart versus Unrelated Same-Age Siblings Reared Together,” was authored by Nancy L. Segal and Elizabeth Pratt-Thompson.