IQ is shaped by a mix of genetics, environment, education, nutrition, and daily habits, with the relative weight of each factor shifting dramatically across a person’s lifetime. Genes account for roughly 20% of IQ variation in infancy but up to 80% in later adulthood, meaning the environmental factors that matter most are concentrated in childhood and adolescence.
Genetics and How Its Influence Grows With Age
The heritability of intelligence follows a surprising pattern. In infancy, genetic differences explain only about 20% of the variation in cognitive ability between people. By adolescence that figure climbs to around 50%, and by adulthood it reaches 60% or higher. Some research places the number as high as 80% in later adulthood, though it appears to dip back to about 60% after age 80.
This doesn’t mean your genes “activate” later in life. What happens is subtler: as people age, they gain more freedom to choose environments that match their genetic tendencies. A child with a genetic predisposition for curiosity might not have access to books, but an adult with the same predisposition can seek out intellectually stimulating work, hobbies, and social circles. Over time, these self-selected environments amplify genetic differences rather than dampening them. The flip side is that in early childhood, environmental interventions carry outsized weight precisely because genetic influence is at its lowest.
Education’s Measurable Effect
Each additional year of formal schooling raises IQ by roughly 1 to 5 points, with the best overall estimate landing around 3 points per year. That finding comes from a large meta-analysis covering more than 600,000 participants across 42 data sets, using several research designs to isolate education’s effect from the simple fact that higher-IQ children tend to stay in school longer.
The gains aren’t just about learning test-taking strategies. Education appears to strengthen the underlying cognitive skills that IQ tests measure, including reasoning, pattern recognition, and verbal comprehension. This is one of the most actionable levers available: keeping children in school, reducing dropout rates, and improving school quality all translate into measurable cognitive benefits at the population level.
Socioeconomic Status and Chronic Stress
Children from lower-income families consistently score lower on IQ tests, and the reasons go well beyond access to tutoring or test prep. Poverty affects cognition through several overlapping channels. Children in economically deprived areas tend to have less exposure to enriching language at home, fewer books, and lower-quality schools. But one of the most damaging pathways is chronic stress itself.
Living with financial instability, family disruption, or unsafe housing creates a sustained stress response that directly impairs working memory, the mental workspace you use to hold and manipulate information in real time. Working memory is central to problem-solving and learning, so when it’s compromised by ongoing stress, cognitive development slows. Research shows this isn’t a vague association: stress induces psychological exhaustion that measurably degrades working memory performance. The effect is cumulative, meaning years of exposure matter more than any single stressful event.
Lead, Air Pollution, and Environmental Toxins
Lead exposure remains one of the best-documented environmental threats to cognitive development. A meta-analysis found that an increase in blood lead levels from 10 to 20 micrograms per deciliter is associated with a loss of 2.6 IQ points, and researchers consider this relationship causal, not merely correlational. There is no known safe threshold for lead exposure in children, which is why even low-level contamination from old paint, pipes, or contaminated soil remains a public health concern.
Air pollution tells a similar story. Prenatal exposure to particulate matter (tiny airborne particles from traffic, industry, and burning fuel) is linked to lower IQ in children. One study of over 1,000 children found that higher exposure during pregnancy was associated with IQ scores about 2.5 points lower. The effect was especially severe in mothers with low folate levels, a B vitamin important for fetal brain development. In that group, the deficit reached nearly 7 IQ points. Verbal skills appeared more vulnerable than nonverbal reasoning, suggesting air pollution may particularly affect the brain systems involved in language development.
Nutrition in the Womb and Early Life
Iodine deficiency during pregnancy is one of the most potent nutritional threats to a child’s intelligence. A meta-analysis of studies conducted in China found that children born to mothers with severe iodine deficiency scored about 12.5 IQ points lower than children from iodine-sufficient areas. When iodine supplementation was introduced before or during pregnancy, children recovered an average of 8.7 of those lost points. In communities where supplementation programs had been running for more than 3.5 years, children scored 12 to 17 points higher than those from areas still affected by deficiency.
Breastfeeding also contributes. A meta-analysis published in The American Journal of Clinical Nutrition found that breastfed children scored about 3 IQ points higher than formula-fed children after adjusting for factors like maternal education and income. The cognitive benefits appeared as early as 6 months of age, remained stable through later childhood, and increased with longer breastfeeding duration. Three points may sound modest, but at the population level, that shift is meaningful.
Sleep Duration in Adolescence
Even small differences in sleep add up. A large study of adolescents from the University of Cambridge grouped participants by their sleep patterns and found that those who slept an average of 7 hours and 25 minutes per night outperformed those sleeping 7 hours and 10 minutes on cognitive tests measuring vocabulary, reading, problem-solving, and focus. That’s a difference of just 15 minutes of sleep, yet the longer-sleeping group also had larger brain volumes and better brain function on imaging scans.
Interestingly, the groups did not differ significantly in school achievement, suggesting that grades may not capture the cognitive advantages that show up on standardized tests. Earlier bedtimes, not just longer total sleep, were also associated with better performance, possibly because earlier sleep aligns better with the body’s natural hormonal rhythms during puberty.
Brain Structure and Intelligence
Brain imaging studies show that cortical thickness, the depth of the brain’s outer layer where most complex thinking happens, is positively associated with general intelligence. The relationship is widespread: thicker cortex in frontal regions (involved in planning and reasoning), parietal areas (spatial processing), and temporal regions (language and memory) all correlate with higher IQ scores in children, adolescents, and young adults. The frontal and parietal associations hold up even after accounting for age and sex differences, suggesting they reflect something fundamental about how brain structure supports cognitive ability.
This doesn’t mean a bigger brain is automatically a smarter brain. Cortical thickness is itself shaped by many of the factors already discussed: nutrition, stress, sleep, education, and genetics all influence how the brain develops physically. It’s better understood as a biological snapshot of all these influences converging on the organ that ultimately produces intelligence.
The Flynn Effect and Its Reversal
Throughout the 20th century, average IQ scores rose steadily in developed nations, gaining roughly 3 points per decade. This trend, known as the Flynn effect, was driven by improvements in nutrition, education, healthcare, and living standards. But the trend has stalled or reversed in several Western countries for people born after the mid-1970s. Data from Norway, for example, shows a clear decline in scores among more recent birth cohorts.
The reversal appears to be environmentally caused rather than genetic, since it occurs within families (younger siblings scoring lower than older siblings born to the same parents). The specific environmental factors behind the decline are still debated, but candidates include changes in educational practices, increased screen time, shifting nutritional patterns, and the possibility that the low-hanging fruit of environmental improvement (eliminating severe malnutrition, expanding basic schooling) has already been picked in wealthy nations.