Heritability is a statistic that describes how much of the variation in a trait across a population can be traced to genetic differences between people. It’s represented as h² and ranges from 0 to 1 (or 0% to 100%). A heritability of 0.60 for a trait means that 60% of the differences between people in that trait are associated with genetic differences. It does not mean the trait is 60% genetic for any single person.
That distinction is the single most important thing to understand about heritability, and it’s the one most commonly confused. Heritability is always about a population, never about an individual.
What Heritability Actually Measures
Imagine measuring a psychological trait, like extraversion, across thousands of people. You’ll find a spread of scores. Some people are highly extraverted, some are not, and most fall somewhere in between. Heritability asks: of all that spread, how much is explained by genetic variation versus environmental variation?
So when researchers say the heritability of extraversion is about 53%, they mean that roughly half the reason people in a studied population differ from one another on extraversion comes down to genetic differences. The other half comes from environmental factors: upbringing, life experiences, culture, random events. This tells you nothing about how much of your personal level of extraversion was “built in.” It only describes the sources of variation in a group.
Heritability can also shift depending on the population being studied. If everyone in a group grows up in nearly identical environments, environmental variation shrinks, and heritability goes up, not because genes became more powerful but because there’s less environmental noise to compete with. The number is always relative to a specific population in a specific context.
How Researchers Estimate Heritability
The classic method uses twin studies. Identical (monozygotic) twins share virtually 100% of their DNA, while fraternal (dizygotic) twins share about 50%, just like any siblings. Both types of twins typically grow up in the same household, so researchers assume they share similar environments. If identical twins are more alike on a trait than fraternal twins, the extra similarity is attributed to the extra genetic overlap.
The math behind this is called Falconer’s formula: you take the correlation for the trait in identical twins, subtract the correlation in fraternal twins, and multiply by two. If identical twins correlate at 0.85 on IQ scores and fraternal twins correlate at 0.55, the heritability estimate is 2 × (0.85 − 0.55) = 0.60.
Beyond twin studies, researchers also use adoption studies (comparing adopted children to both their biological and adoptive parents) and family studies that track traits across generations. More recently, genome-wide association studies scan hundreds of thousands of DNA markers directly to estimate how much of a trait’s variation can be linked to known genetic variants.
Heritability of Intelligence Changes With Age
One of the most striking findings in behavioral genetics is that the heritability of intelligence increases across the lifespan. In infancy, genetic differences account for roughly 20% of the variation in cognitive ability. By adolescence, that rises to around 40–50%. By late adolescence and adulthood, heritability reaches about 80%, where it stays well into old age.
This pattern, sometimes called the Wilson effect after the researcher who first documented it, seems counterintuitive. You might expect genes to matter most early on, before the environment has had time to shape you. But the opposite happens. One explanation is that as people gain more autonomy, they increasingly seek out environments that match their genetic tendencies. A child with a genetic predisposition toward curiosity, once free to choose, gravitates toward books, stimulating jobs, and intellectually engaged friends, amplifying the genetic effect over time. Meanwhile, the influence of the shared family environment (the home you grew up in, your parents’ income, the schools they chose) drops to about 10% by age 18.
Heritability of Personality Traits
The five broad personality dimensions that psychologists most commonly study, neuroticism, extraversion, openness, agreeableness, and conscientiousness, all show moderate heritability. Twin research estimates them at roughly:
- Openness: 61%
- Extraversion: 53%
- Conscientiousness: 44%
- Neuroticism: 41%
- Agreeableness: 41%
These numbers mean that genetic variation explains about 40–60% of the differences between people on each dimension, with the rest attributable to environmental factors and measurement error. None of these traits is “genetically determined.” They are all substantially influenced by both genes and experience.
Heritability of Mental Health Conditions
Psychiatric conditions tend to run in families, and heritability estimates help quantify how much of that clustering is genetic. Twin and family studies put the total heritability of schizophrenia at about 81%, bipolar disorder at around 75%, ADHD at roughly 75%, and autism at approximately 80%. Depression has a notably lower heritability, around 37%, meaning environmental factors play a larger relative role.
These conditions also share genetic overlap with one another. Schizophrenia and bipolar disorder share about 15–16% of the same common genetic variation, while bipolar disorder and depression overlap by about 10%. This helps explain why these conditions sometimes co-occur in families, even when different family members receive different diagnoses.
The Missing Heritability Problem
When researchers scan people’s DNA directly using modern genomic tools, they consistently find fewer genetic variants than twin studies would predict. Height is a useful example because it’s easy to measure precisely. Twin studies estimate the heritability of height at 73–81%. But by 2010, the specific genetic variants identified through genome scans collectively explained only about 5% of the variation. Even with improved methods that capture the combined effect of all common DNA markers (not just individually significant ones), estimates for height reach about 45%, still well below the twin-study figure.
This gap, known as the “missing heritability” problem, has been debated for over a decade. Several explanations exist. Some genetic effects may come from rare variants that current DNA chips don’t detect. Some may involve interactions between genes, or between genes and environments, that simple models miss. And twin-study estimates may be somewhat inflated because the assumption that identical and fraternal twins share environments equally doesn’t always hold. Identical twins may be treated more similarly, inflating the apparent genetic effect.
Why Heritability Is Easily Misunderstood
The most common mistake is treating a heritability number as a verdict on how fixed a trait is. A trait can be highly heritable and still be changed by the environment. Height has a heritability near 80% in well-nourished populations, yet average height increased dramatically over the 20th century due to improvements in nutrition and healthcare. The genes didn’t change; the environment did. The same principle applies to psychological traits. High heritability does not mean a trait is immutable.
A second misunderstanding is using heritability to explain differences between groups. Heritability is calculated within a specific population. It says nothing about why two different populations might differ on average. Two fields of corn could have high heritability for plant height within each field (the taller plants genuinely have “taller” genes), while the difference in average height between the fields is entirely due to one field getting better soil. The within-group genetic explanation doesn’t transfer to the between-group difference.
Finally, heritability doesn’t mean specific genes are being passed down in a simple way. Parents don’t pass on a genotype; they pass on individual gene variants that recombine unpredictably. Each child receives a different mix. This is why siblings raised in the same home can differ substantially on heritable traits. Half of the genetic variation in a population exists within families, not just between them.