IQ is highly heritable, but heritability is not set in stone. It depends on the environment that a population is in. As a result, many scientists have hypothesized that heritability might be higher in environments that permit people to develop to their full genetic potential.
In this study, the authors examined whether heritability was higher in socioeconomically better environments, a hypothesis called a “Scarr-Rowe interaction” (SRI). Whether SRIs exist has been disputed, and the results of studies are often contradictory. The researchers who produced this study hypothesized that the standard practice of combining different socioeconomic variables may mask the influence that each individual variable could have.
The authors examined data from three cohorts of twin pairs, totaling 5,506 people. These twins were representative of people their age (between 10 and 25) in Germany and span the entire range of socioeconomic status in that country.
The overall results match prior research very well. Heritability of IQ was lowest in the youngest sample (47% in ages 10-12) and higher in older groups (69% at ages 16-18 and 21-25). A shared environment effect was present in children (17%), but not in adolescents or young adults.
An SRI was only found in the youngest cohort with children of parents with higher occupational prestige showing higher heritability. SRI was not found in most cases making it inconsistent across age groups.
Even though the heritability of intelligence was again verified, it was not significantly higher in socioeconomically better environments.
@lonelyboy637 The key finding: income, education, and occupation each affect IQ differently—but previous studies lumped them together into one “social class” score.
That’s why we got contradictory results for decades. It’s like asking “does fruit affect health?” when apples, oranges, and avocados all do different things.
Parents’ job showed an effect. Income didn’t. Education didn’t. When you mix them all together, you see nothing.
Lesson: stop combining different things and expecting simple answers.
Here’s what really matters: genes influence IQ more as you age (47% → 69%), while family environment drops from 17% to 0%.
Why? Kids don’t choose their environment—parents do. But teenagers and adults pick friends, activities, and careers that match their natural tendencies. The bookish kid joins the library club. The athletic kid joins sports.
By adulthood, your “environment” is mostly stuff you chose based on your genes. So it’s nearly impossible to separate “genetic effects” from “environmental effects”—they’re tangled together.
The study couldn’t find consistent effects in older kids because there’s no truly independent environment left to measure.
@CloverL It’s a common finding, but let’s be careful with the “0% shared environment” figure for adolescents/adults. That just means any shared environmental influences left (like how they were parented or their household’s wealth) affect both twins equally, meaning it no longer contributes to the similarity between them beyond their shared genes. It doesn’t mean environment has a zero influence overall; it just means the unique, non-shared environment (e.g., different friends, unique life experiences) has taken over, alongside genetic effects.
@Marcelo Okay, but why would occupation show an effect where income and education didn’t, especially in 10-12 year olds? Occupational prestige often correlates with cognitive demands and social networks/resources accessible to the family (like better schools or enriching activities). Maybe it’s less about the status itself and more about the structure of the family life and resources that a demanding job provides. It’s a key finding, but the mechanism remains muddy.
I appreciate the breakdown, but I think the interpretation here oversimplifies what the study actually shows. You say “lumping variables together” like these factors work independently, but they don’t. Income, education, and occupation are highly correlated and causally related. They typically work together to create cumulative advantage or disadvantage. Yes, they’re distinct, but they’re not as different as “apples, oranges, and avocados.” Composite SES measures exist precisely because the combination of resources, knowledge, and social status creates environments. Separating them risks missing that these advantages compound each other.
Why would parental occupation moderate heritability at age 10-12 but not at 16-18 or 21-25? Children don’t suddenly escape their parents’ socioeconomic environment as teenagers. If anything, genetic influences should strengthen with age as people select into environments matching their potential. The fact that the effect disappears suggests it wasn’t capturing something real about gene-environment interaction.
