Groundbreaking study reveals the impact of genetics on IQ scores over time

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The age-old debate of nature versus nurture—whether our genetics or environment plays a more critical role in shaping our intelligence—has long intrigued scientists, educators, and the public alike. A recent groundbreaking study, published in the journal Personality and Individual Differences, sheds new light on this discussion.

The longitudinal study, the first of its kind involving young monozygotic twins reared apart, reveals an increase in IQ resemblance as these twins age. These findings suggest that genetic factors become more influential in shaping our intellectual abilities as we grow older, while environmental factors are more significant during our early years.

Twin studies are a cornerstone of behavioral genetics, providing a unique window into the ongoing debate of nature versus nurture. These studies typically involve comparing identical (monozygotic) twins, who share 100% of their genetic material, with fraternal (dizygotic) twins, who share roughly 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 reared apart. Most twin studies have been cross-sectional, meaning they only capture a single snapshot in time. This approach can miss important developmental changes and trends. In addition, while many studies have examined twins raised together, fewer have looked at twins raised apart, which is helps to isolate genetic influences from environmental ones.

“Twin studies are a simple and elegant way of examining the genetic and environmental influences on behavior. No one had ever conducted a longitudinal study of young separated twins in real time. I was able to do this using twins mostly adopted apart due to 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 involved three distinct groups: young Chinese reared-apart monozygotic twins, Danish adult reared-apart monozygotic twins, and “virtual twins” — same-age, unrelated siblings reared together.

The first group involved 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, 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 testing (Time 1), the twins had a mean age of approximately 10.69 years. The second testing (Time 2) occurred on average 4.18 years later, when the twins’ mean age was 13.93 years. The twins were tested by different trained examiners to prevent bias, and the scores were carefully reviewed 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 of intelligence.

The Danish twins had a mean age of 51.42 years at the first testing (Time 1). The interval between the two test sessions averaged about 11.17 months. Similar to 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 conducted both tests, while a different examiner tested the remaining pairs.

The third group, virtual twins, consisted of 43 pairs of same-age unrelated siblings reared together. These siblings were either both adopted or one biological and one adopted. Virtual twins were defined as unrelated siblings born within nine months of each other, entering the adoptive home by age one, and being enrolled in the same school grade.

The intelligence of the virtual twins was assessed twice using the Wechsler IQ test. The first testing (Time 1) occurred when the twins were approximately 5.11 years old on average, and the second testing (Time 2) took place when they were about 10.77 years old, with an average interval of 5.65 years between tests. To avoid any potential bias, different examiners tested each sibling pair on the same day.

For the Chinese reared-apart 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 testing (Time 1) to 0.81 at the second testing (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 more prominent role in determining their intelligence, despite being raised in different environments.

Similarly, the Danish adult reared-apart twins also showed an increase in IQ similarity over time, with their intraclass correlation rising from 0.64 at Time 1 to 0.74 at Time 2. This pattern further supports the notion that genetic factors become more influential in shaping intelligence as individuals grow older. The Danish twins also exhibited 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 reared-apart twins, the virtual twins displayed a decrease in IQ similarity over time. The intraclass correlation for their IQ scores dropped from 0.30 at Time 1 to 0.11 at Time 2, indicating that the shared environment’s influence on intelligence diminishes 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 non-genetic factors, such as unique experiences and individual choices, become more significant 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 extent of these changes are influenced by genetic factors. For the Chinese and Danish reared-apart twins, the increase in IQ similarity and the stability of their IQ scores over time highlighted the growing importance of genetics. On the other hand, the virtual twins’ decreasing IQ similarity highlighted the reduced impact of shared environmental factors as they aged.

“As we get order genetic factors become more important in fashioning our general ability,” Segal told PsyPost. “Home factors are more important when we are younger, but as we age we select opportunities and events that align with our genetic potentials. Of course, extreme environments can overwhelm genetic potentials, as I described in my 2018 book Accidental Brothers. And we can all improve our skills by working hard — genes do no set abilities in stone.​”

Despite these compelling findings, the study has some limitations. Due to the rarity and difficulty of identifying and following reared-apart monozygotic twins, the 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 continued IQ analyses are planned using participants in the Minnesota Study of Twins Reared Apart who have been IQ tested on two occasions (Segal, 2012). Further analyses of the [Chinese reared-apart twins] are also planned and will, hopefully, identify additional participants for study.”

The study, “Developmental trends in intelligence revisited with novel kinships: Monozygotic twins reared apart v. same-age unrelated siblings reared together,” was authored by Nancy L. Segal and Elizabeth Pratt-Thompson.

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