Identical twins separated as babies, raised by different families, sometimes in different cities, meet for the first time as adults. Their IQ scores turn out to be remarkably close, often closer than those of ordinary siblings who grew up in the same house.
And yet, over the twentieth century, average IQ scores rose so quickly that genes can't possibly explain the change. Both findings are true, and understanding how they fit together is the key to the nature-versus-nurture question. Here's what twin studies actually show, and what they don't.
How twin studies work
Twin studies rely on a neat natural experiment. Identical twins share essentially all of their genes. Fraternal twins share, on average, about half of the genetic variation that differs between people, just like ordinary siblings. Both kinds of twins usually grow up together, in the same home and at the same time.
So if identical twins are much more alike in IQ than fraternal twins, that extra similarity points to genetic influence. Researchers use the comparison to estimate heritability: the proportion of differences in a trait, within a particular group of people, that can be linked to genetic differences.
Two other types of study support the picture:
- Twins raised apart. The Minnesota Study of Twins Reared Apart, led by Thomas Bouchard, found that identical twins raised in different homes were still strikingly similar in IQ as adults.
- Adoption studies. These compare adopted children with both their biological and adoptive parents, helping separate the effects of genes from the effects of the home.
What the numbers say
Across many studies, heritability estimates for intelligence typically land somewhere around 50%, with a wide range depending on age and sample. That makes intelligence one of the more heritable psychological traits.
The most important detail, though, is how this changes with age. A 2010 study led by Claire Haworth combined data on more than 11,000 pairs of twins from several countries:
| Age group | Estimated heritability of intelligence |
|---|---|
| Childhood (around 9) | About 41% |
| Adolescence (around 12) | About 55% |
| Young adulthood (around 17) | About 66% |
Studies of adults, including the Minnesota study, have often reported even higher figures.
The twist: genes matter more as you get older
You might expect the influence of genes to be strongest at birth and fade as life experience piles up. The research suggests the opposite. This pattern, sometimes called the Wilson effect, is one of the most consistent findings in the field.
Meanwhile, the influence of the shared family environment, the things siblings have in common such as their home and parents, tends to shrink to very little by adulthood, at least within the range of ordinary families studied.
The leading explanation is that genes and environments work together. As children grow up, they gain more control over their own surroundings. A child who enjoys reading seeks out more books; a child who loves puzzles finds more puzzles. Small genetic differences lead people to choose experiences that amplify them. Psychologists call this gene-environment correlation, and it means that "genetic" influence often works partly through the environment.
What heritability does not mean
Heritability is one of the most misunderstood ideas in psychology. A figure of 50% does not mean:
- That half of your IQ comes from your genes. Heritability describes differences across a group, not the make-up of one person's ability.
- That intelligence is fixed. Height is highly heritable, yet average height rose a lot over the twentieth century thanks to better nutrition and health.
- That environment doesn't matter. A heritability estimate only applies to the range of environments in the group studied. Change the environments, and the figure can change.
- That there is an "intelligence gene." There isn't one, as the next section explains.
The Flynn effect is the clearest demonstration. Average scores rose by roughly three points per decade across much of the twentieth century, far too fast for any genetic change. Environments, including schooling, nutrition and the kinds of thinking modern life demands, shifted the average while genes stayed the same.
Finding the actual genes
For decades, twin studies could show that genes mattered without identifying any of them. Modern genome-wide studies, which scan DNA from hundreds of thousands of people, have started to fill that gap.
What they've found is that intelligence is highly polygenic. Hundreds, probably thousands, of genetic variants are involved, and each one has a tiny effect on its own. Researchers can combine them into a polygenic score, but these scores currently predict only a modest share of the differences between people, much less than twin studies would suggest. Closing that "missing heritability" gap is an active area of research.
Environment still matters a lot
None of this means upbringing and experience are irrelevant. Some of the clearest evidence concerns education. A 2018 meta-analysis by Stuart Ritchie and Elliot Tucker-Drob found that an extra year of schooling was associated with a gain of roughly one to five IQ points, with natural experiments suggesting much of that is a genuine causal effect.
Other environmental factors with good evidence include:
- Nutrition and health in early life, especially avoiding serious deficiencies.
- A stimulating home, with books, conversation and play.
- Stable, low-stress conditions, which support learning and attention.
Some research, including a well-known 2003 study by Eric Turkheimer and colleagues, has suggested that genetic influence on IQ may be smaller among children growing up in poorer circumstances, where environmental differences can be larger. Later studies have produced mixed results, so this remains debated.
What this means for you
Your current IQ reflects a mix of genetic tendencies and a lifetime of experiences, and the two are tangled together in ways that can't be neatly separated for any one person. Genes may shape your starting point and the experiences you're drawn to, but they don't set a ceiling you can read off a chart.
If you'd like to see where your reasoning skills are today, take our free IQ test. Treat it as a snapshot, not a genetic verdict. And if you want to know what you can realistically change, our guide to whether you can increase your IQ covers the habits with real evidence behind them. For more on what IQ tests are measuring in the first place, see the g factor.
The bottom line
Twin studies show that intelligence is substantially heritable, around 50% on average and rising to more than 60% by adulthood. But heritability describes differences within a group, not a fixed limit for any individual. Intelligence is shaped by thousands of genes with tiny effects working alongside education, health and the environments people choose, and the Flynn effect shows how much environments can move the average.
FAQ
Is intelligence inherited from the mother?
There's no good evidence that intelligence comes mainly from one parent. It is influenced by many genes spread across the genome, inherited from both parents.
What percentage of intelligence is genetic?
Twin studies estimate heritability at around 50% overall, rising from roughly 40% in childhood to 60% or more in adulthood. This describes differences across a group, not a percentage of one person's ability.
If intelligence is heritable, can you still improve it?
Yes, within limits. Heritable traits can still be shaped by environment, and education in particular is linked to real gains in IQ scores.
Is there a single gene for intelligence?
No. Intelligence is influenced by thousands of genetic variants, each with a very small effect.
