Your IQ is shaped by a combination of your genes, your environment, and the interaction between the two. Neither nature nor nurture alone tells the full story, and the balance between them shifts dramatically depending on your age and your circumstances. In childhood, environment plays an outsized role. By adulthood, genetics dominates, accounting for roughly 60 to 80 percent of the variation between people.
Genetics Sets a Wide Range, Not a Fixed Number
Intelligence is what geneticists call “highly polygenic,” meaning thousands of tiny genetic differences each contribute a small amount. A major genome-wide study of over 78,000 people identified 336 specific genetic variants across 18 regions of the genome linked to intelligence, yet all of those variants combined explained only about 4.8 percent of the variation in IQ scores. That’s not because genes don’t matter. It’s because each individual variant has an almost negligible effect, and researchers have only identified a fraction of them so far.
The heritability of intelligence, meaning how much of the difference between people’s scores can be attributed to genetic differences, increases steadily across the lifespan. It starts at around 20 percent in infancy, climbs to about 40 percent in adolescence, and reaches 60 to 80 percent in later adulthood. This seems counterintuitive. You might expect environment to accumulate influence over time, but the opposite happens. As people gain more freedom to choose their own surroundings, they tend to gravitate toward environments that match their genetic predispositions, amplifying genetic effects.
Your Childhood Environment Matters Enormously
One of the most striking findings in intelligence research comes from studying how socioeconomic status interacts with heritability. In children from impoverished families, about 60 percent of the variation in IQ is explained by the shared home environment, and the contribution of genes is close to zero. In affluent families, the pattern flips almost exactly: genes explain most of the variation, and shared environment contributes very little.
This doesn’t mean poor children lack “intelligence genes.” It means that poverty creates such powerful environmental constraints, limited nutrition, less cognitive stimulation, higher stress, greater exposure to toxins, that genetic potential is effectively suppressed. When basic needs are met, genes have room to express themselves. When they aren’t, environment becomes the bottleneck.
Education Raises IQ by Several Points Per Year
Formal schooling has a measurable effect on IQ that goes beyond just teaching facts. A meta-analysis of 42 data sets covering more than 600,000 people found that each additional year of education raises IQ by approximately 1 to 5 points, with the overall average landing around 3.4 points per year. This effect holds across different study designs, including natural experiments where policy changes forced some children to stay in school longer than others.
The gains aren’t limited to the specific skills schools teach. Education appears to improve general cognitive ability, the kind of abstract reasoning and problem-solving that IQ tests are designed to measure. This is one reason researchers believe IQ is not purely a reflection of innate capacity. It responds to sustained mental challenge.
Nutrition and Toxins During Development
The developing brain is especially vulnerable to nutritional deficiencies and environmental toxins, and the damage often shows up directly in IQ scores.
Iodine deficiency is one of the most well-documented examples. A systematic review estimated that children with iodine deficiency lose an average of 7.4 IQ points compared to children with adequate iodine levels. Severe deficiency during pregnancy can cause even larger deficits. This is why iodized salt programs have been one of the most cost-effective public health interventions worldwide.
Lead exposure tells a similar story. A long-running study that followed people from childhood to age 38 found that for every 5-microgram increase in blood lead levels per deciliter, a person lost about 1.5 IQ points. Children with blood lead levels above 10 micrograms per deciliter at age 11 scored an average of 4.25 points lower on IQ tests as adults compared to peers with lower exposure. These losses persisted decades after the initial exposure, suggesting the damage is largely permanent.
Breastfeeding has also been studied extensively. When researchers control for the mother’s own IQ, which is important because more educated mothers are more likely to breastfeed, the benefit of breastfeeding on a child’s IQ is about 2.6 points. That’s a real but modest effect, likely driven by nutrients that support early brain development.
Epigenetics: How Environment Rewires Gene Activity
Between pure genetics and pure environment sits epigenetics, the set of chemical modifications that turn genes up or down without changing the DNA sequence itself. These modifications respond to environmental signals like nutrition, stress, and toxin exposure, offering one explanation for how environment gets “under the skin” to affect cognition.
A study of nearly 1,500 adolescents found that chemical tagging on a gene involved in dopamine signaling (a brain chemical tied to motivation and learning) was significantly associated with general IQ, explaining about 2.7 percent of the variation. Higher levels of this chemical tagging, which effectively dials down the gene’s activity, were linked to lower IQ scores. The same study found that the best model for predicting IQ combined genetic risk scores, this epigenetic marker, and brain activity during reward processing. In other words, IQ emerges from layers of influence: your DNA sequence, how actively those genes are being read, and how your brain functions in real time.
IQ Trends Are Shifting Globally
Throughout the 20th century, IQ scores rose dramatically worldwide, a phenomenon known as the Flynn effect. Average scores increased by roughly 3 points per decade in many countries, driven by improvements in nutrition, healthcare, education, and overall living standards.
That trend is no longer uniform. In economically developing countries, IQ scores are still rising as living conditions improve. But in many wealthy Western nations, the gains have stalled or even reversed slightly. The gap between countries remains large, around 19 IQ points between the highest-scoring and lowest-scoring world regions based on international student assessments, but it is narrowing.
This global pattern reinforces the central point: IQ is not a fixed biological trait handed down at conception. It responds to the conditions people live in. When those conditions improve across a population, scores rise. When they plateau, so do scores. The fact that entire nations can gain or lose IQ points over a few decades tells you that environment, broadly defined, is doing a lot of heavy lifting alongside genetics.