Is IQ heritable? What science really says

"Does my IQ depend on genes or upbringing?" — one of the oldest questions in psychology. The scientific answer is clear: both. But the proportions surprise most people.

This text shows what genetic research really reveals — twins, adoptions, the latest GWAS studies. And what it means in practice.

Short answer

The most common surprise: the influence of family (upbringing) on adult IQ is practically zero. What you bring out of home in terms of IQ disappears by around age 20. After that, heritability of IQ is ~80%, and the rest is your individual, non-shared experiences.

This sounds controversial — and it is controversial. But that's what the data shows.

The most surprising finding: heritability rises with age

Logically we would think the opposite — that a small child is "closer to genes", and an adult "further". The data shows the opposite:

• 5-year-old child: IQ heritability ~30%
• 10-year-old child: heritability ~50%
• 16-year-old teenager: heritability ~65%
• 30-year-old adult: heritability ~75-80%

Sounds paradoxical — but it makes sense. With age, people increasingly choose their own environment. A 5-year-old has the environment parents arrange. An adult chooses school, friends, books, work, hobbies. They choose according to their genetic predispositions.

A child with natural intellectual curiosity will choose college, reading, similarly-minded friends. A child without this tendency — something else. In this way, genes manifest more and more strongly in the environment they themselves "build".

This phenomenon is called gene-environment correlation and is one of the strongest effects in developmental psychology.

Four main ways to study heritability

1. Identical vs fraternal twins

The simplest design. Monozygotic (MZ) twins share 100% of genes, dizygotic (DZ) — on average 50%. If genes have an effect, MZ should be more similar to each other than DZ.

Results (Plomin & Deary meta-analysis, 2015):

• IQ correlation in MZ raised together: 0.86
• IQ correlation in DZ raised together: 0.60
• IQ correlation in MZ raised separately: 0.75
• IQ correlation between unrelated siblings raised together: 0.20

This is the fundamental evidence base for the conclusion that genes have a large effect.

2. Adoption studies

The second classic design. An adopted child has the genes of biological parents and the environment of adoptive ones. Years later you can check who they are more similar to in terms of IQ.

Results (Texas Adoption Project, Colorado Adoption Project):

• In childhood: correlation with adoptive siblings ~0.30, with biological parents ~0.30. Roughly equal.
• In adulthood: correlation with biological parents ~0.40, with adoptive ~0.00. The influence of adoptive siblings vanishes.

In other words: differences in upbringing environment have an effect in childhood, but this effect evaporates into adulthood.

3. Family studies

Compares IQ similarities between different types of relatives:

Correlations decrease as genetic relatedness decreases. Exactly what we would expect under strong heritability.

4. GWAS studies (since 2010)

The newest technique. Thanks to DNA sequencing, researchers look for specific genetic variants associated with IQ.

Results (Savage 2018, Plomin 2018):

• About 1,300 genetic variants statistically associated with IQ have been identified
• Together they explain ~10-20% of differences in IQ (much less than the 80% suggested by twin studies)
• Conclusion: IQ is a polygenic trait — depends on thousands of genes, each with minimal effect

The gap between 10-20% (GWAS) and 80% (twins) is called "missing heritability" — there are hypotheses, but a definitive explanation remains open.

What this does NOT mean

Many myths have grown around IQ heritability. Worth dispelling:

"Since IQ is heritable, upbringing doesn't matter"

False. Upbringing matters greatly, but not in terms of differences in IQ between children. It can affect:

• the child's character and motivation
• learning habits
• emotional coping with stress
• quality of relationships later in life

It's simply that with usual variability in upbringing, genes "win" in explaining differences in the IQ indicator itself. Extremely poor conditions (neglect, malnutrition, lack of stimulation) — yes, lower IQ. But the difference between a middle-class family and a wealthy family — minimal effect.

"Black people have lower IQs, so it's genetic"

Not scientifically supported. Differences between ethnic groups in average IQ stem from differences in environment (education, access to food, systemic discrimination, stress), not from differences in genes. This is shown by the Flynn effect — since 1932, average IQ has risen by ~30 points across generations. Genes don't change like that.

"Parents' IQ = children's IQ"

Only partially. The parent-child correlation is ~0.42. That means: parents with IQ 130 more often have children with higher IQ, but the average IQ of their children is ~115, not 130. This happens because of regression to the mean — geniuses have slightly less brilliant children, slightly geniuses have somewhat fewer with regular parents.

What about neuroplasticity and training

A natural question: "If IQ is so heavily heritable, can it be improved at all?"

Short answer: only to a very limited extent.

• Education — each additional year of education = ~1-2 IQ points. Real, but small.
• Early childhood support programs (Head Start, Perry Preschool) — help children from difficult environments, but the IQ effect often disappears by teenage years.
• "Brain training" apps — effect on general IQ practically zero (more in the IQ training article).
• Very intensive n-back and similar — minimal, controversial effects on working memory.

The strongest things that influence IQ as an adult:

• Sleep quality (chronic sleep deprivation lowers IQ by 5-10 points)
• Physical activity
• Treating chronic illnesses (diabetes, depression, hypothyroidism)
• Avoiding substance abuse

This is maintenance, not raising. A genetic "ceiling" exists.

How to interpret your score

If you took an IQ test and got 130 — 50-80% is "your genes", 20-50% is your experiences, education, last night's sleep, diet. The score tells you something about your predispositions, but nothing about your life potential, character, creativity, or resourcefulness.

What an IQ score actually predicts — in short: not as much as people think.

Summary

IQ is largely heritable — in adulthood about 70-80%. This is one of the best-documented facts in psychology. But it doesn't mean "everything is written in advance". It means that within normal variability of upbringing, genes turn out to be a larger differentiating factor than the family you grew up in.

This is information worth accepting with humility — what you have, you have; with openness — genes don't define everything; and with responsibility — whether you use what you have depends on you.