What Affects IQ: Genes, Environment, and What You Can Change
Genes explain 50 to 80% of individual differences in IQ — the most reproducible figure in behavioral genetics. The rest comes down to environment: childhood nutrition, quality of education, health, stress, and even air pollution. Some of these factors are outside our control; others aren't. Here's what science knows for certain, and where the debate is still ongoing.
Genetics: the foundation, not the ceiling
Twin studies show that the older a person gets, the stronger the genetic contribution to IQ becomes. In children it sits around 40–50%; in adults it rises to 60–80%. This doesn't mean IQ is hardwired from birth — it means that as people age, they increasingly choose environments that match their natural inclinations, which in turn amplifies the genetic effect.
That said, no single gene "controls" intelligence on its own. Genome-wide association studies (GWAS) have identified thousands of variants, each contributing a fraction of a percent to explained variance. The genetics of intelligence isn't one switch — it's millions of weak signals.
Early childhood: the window of greatest vulnerability
A child's brain develops at a remarkable pace in the first three years of life, making it especially sensitive to external influences during this period.
Nutrition
Iodine deficiency during pregnancy and early childhood is one of the most well-studied and preventable causes of IQ loss. The WHO estimates that iodine deficiency remains the leading preventable cause of intellectual impairment worldwide. Iron deficiency in the first two years of life is also linked to lasting cognitive losses — neurons myelinate less effectively, and the effects aren't always reversible even after nutritional correction.
Breastfeeding is consistently associated with slightly higher IQ scores — on average 3–5 points in large studies — though separating the effect of breast milk from that of the broader social environment is methodologically tricky.
Lead and other neurotoxins
Lead is one of the few factors for which there is no safe threshold of exposure for the developing brain. Epidemiological data show IQ reductions even at blood concentrations previously considered acceptable. Organophosphate pesticides, mercury, and polychlorinated biphenyls work in similar ways. These risks aren't limited to industrial areas: old water pipes, lead paint in older homes, and high-mercury fish are all real-world sources of exposure.
Stress and the home environment
Chronic stress in early childhood raises cortisol levels, which in high doses are toxic to the hippocampus — a structure critical for learning and memory. Children raised in predictable, emotionally safe environments tend to show higher cognitive outcomes on average, even after controlling for socioeconomic status.
Education: the most testable lever
Each additional year of schooling is consistently associated with an IQ gain of roughly 1–5 points. This is one of the most robust findings in psychometrics — it holds up in Mendelian randomization (a method that helps separate causation from correlation) and in natural experiments, such as when compulsory schooling laws changed at different times in different states or countries.
Quality of education matters too: structured instruction, rich vocabulary exposure, and tasks requiring logical reasoning all train the very skills that IQ tests measure.
Health and lifestyle
Sleep
Chronic sleep deprivation slows information processing and impairs working memory — both of which directly affect IQ test performance. Even a single bad night's sleep can temporarily push scores down by several points.
Physical activity
Aerobic exercise stimulates production of brain-derived neurotrophic factor (BDNF), which supports neuroplasticity. Several meta-analyses report a modest but consistent effect of regular exercise on cognitive performance — particularly in children and older adults.
Alcohol and psychoactive substances
Alcohol during pregnancy causes fetal alcohol syndrome, one of the leading preventable causes of intellectual disability. In adolescents, heavy alcohol and cannabis use is associated with lower cognitive scores, though whether these changes are fully reversible remains an open question.
Factors that are often overstated
- Mozart for babies. The "Mozart effect" — a popular idea from the 1990s — has not held up in reproducible experiments as a method for raising IQ.
- Brain training games. Most commercial programs improve performance on the specific tasks they practice, but transfer to general intelligence remains weak and inconsistent.
- Vitamin-enriched foods in healthy children. Supplementation helps when there's a deficiency, but in children without one, extra vitamins generally produce no measurable cognitive benefit.
Factors and their relative impact: summary table
| Factor | Impact on IQ | Controllability |
|---|---|---|
| Genetics | 50–80% of variance in adults | None |
| Iodine deficiency (prenatal) | Up to −15 points in severe cases | High (iodized salt) |
| Lead exposure (early childhood) | −1–5 points per µg/dL of blood | Moderate (removing sources) |
| Education (+1 year) | +1–5 points | High |
| Chronic childhood stress | Significant, not precisely quantified | Moderate |
| Sleep | Temporary, reversible effect | High |
| Aerobic exercise | Modest, consistent | High |
What this means in practice
IQ isn't a fixed mark on a ruler. In children it's more fluid; in adults it's more stable — but even in adults, scores can shift up or down depending on health, cognitive load, and lifestyle. If you're curious where you stand right now, you can take the test on IQMeter — it takes about 30 minutes and gives you a standardized result.
The core takeaway from the science is straightforward: genetics sets the range, and environment determines where within that range any given person ends up. And that range is wider than most people assume.
Frequently asked questions
Can IQ increase in adulthood?
Yes, but modestly, and mostly by addressing temporary factors — fixing sleep deprivation, reducing stress, or correcting nutritional deficiencies. No scientifically validated method has yet been shown to produce dramatic IQ gains in a healthy adult through training alone.
Does being bilingual affect IQ?
No direct effect on overall IQ has been demonstrated. Bilingualism does improve certain executive functions — like attention switching and inhibition — but it doesn't raise standard IQ scores.
Are children from wealthier families actually smarter?
Higher family socioeconomic status consistently correlates with higher IQ in children. But the causes are intertwined: genetics (parents with higher IQs tend to earn more), better nutrition, fewer environmental neurotoxins, and higher-quality education all operate simultaneously.
Does IQ decline with age?
Processing speed and working memory begin to decline slowly after age 30. Vocabulary and accumulated knowledge, by contrast, can continue growing into a person's 60s and 70s. Standard IQ tests are age-normed, so a score of 100 at 25 and at 60 means the same thing relative to one's peers.
Does air pollution affect intelligence?
Yes. Several large epidemiological studies have found an association between childhood exposure to fine particulate matter (PM2.5) and lower cognitive scores. The likely mechanisms are neuroinflammation and oxidative stress. This is an active area of research, and the details are still being worked out.