Intelligence is shaped by a combination of genetics, brain structure, environmental conditions, and early life exposures, with no single factor acting alone. Genes account for roughly 60 to 80 percent of the variation in brain measures linked to cognitive ability, but that genetic influence plays out differently depending on the environment a person grows up in. Understanding what determines intelligence means looking at how all these factors interact across a lifetime.
How Much Comes From Genetics
Twin studies consistently show that genetics explains a large share of the differences in cognitive ability between people, with heritability estimates for brain measures typically falling between 60 and 80 percent. That percentage isn’t fixed, though. It tends to increase from childhood into adulthood, meaning genes play a progressively larger role in shaping your cognitive profile as you age. This happens partly because adults have more freedom to seek out environments that match their genetic tendencies, reinforcing those traits over time.
But heritability is not destiny. A large UK study tracking over 8,700 twin pairs from age 2 through 14 found that in lower-income families, shared environmental influences on intelligence were significantly greater than in higher-income families, while the genetic contribution stayed roughly the same across income levels. In other words, growing up in a disadvantaged environment doesn’t change your genetic potential, but it does mean environmental factors like household stress, nutrition, and educational access exert a stronger pull on where you end up. Family-based interventions appear to have more impact in these settings precisely because there’s more environmental variance to work with.
Brain Structure and Efficiency
Your brain’s physical architecture matters, but not in the simple “bigger is better” way people often assume. When researchers use brain imaging to predict intelligence from gray matter volume, two networks stand out: the fronto-parietal network (involved in reasoning and attention) and the cerebellum (which coordinates not just movement but also cognitive processing). The cerebellum’s gray matter volume predicted intelligence scores with a correlation of 0.27, while the fronto-parietal network came in at 0.18. These are modest correlations, which tells you that no single brain region is the “seat” of intelligence.
Cortical thickness, the depth of the brain’s outer layer, has a more nuanced relationship with intelligence than you might expect. At age 9, there’s essentially no correlation between how thick a child’s cortex is and their IQ scores. By age 12, a clear pattern emerges: children with higher intelligence scores tend to have a thinner cortex, particularly in the left frontal region. This thinning happens at the onset of puberty and appears to be driven by genetic influences. The relationship is strongest for verbal intelligence, while nonverbal IQ shows no significant correlation with cortical thickness at either age.
This points to something called neural efficiency. When researchers measure brain energy consumption during cognitive tasks, people with higher intelligence scores show lower brain activation, not higher. The original studies in this area found that intelligence scores and regional metabolic rates were negatively correlated, with values between -0.48 and -0.84 across different brain areas. Smarter brains aren’t working harder; they’re working more efficiently, using less energy to accomplish the same task. Interestingly, when tasks are calibrated to be equally difficult for each individual (rather than giving everyone the same problem), the activation differences between higher and lower intelligence groups disappear. The efficiency advantage shows up specifically when brighter people find a task easier than their peers do.
Working Memory as a Bottleneck
If there’s one cognitive mechanism that best predicts fluid intelligence (your ability to reason through novel problems), it’s working memory capacity. Working memory is the mental workspace where you hold and manipulate information in real time: keeping a phone number in mind while you search for a pen, or tracking multiple variables in a logic puzzle. Working memory capacity shares about 50 percent of its variance with fluid intelligence, and the correlation between the two ranges from about 0.31 to 0.64 depending on the study and the specific tasks used.
The connection grows stronger as problems get harder. On easier reasoning items, working memory capacity matters less because most people can handle the load. On the most difficult items, the correlation between working memory and correct answers climbs steadily. This suggests that working memory acts as a bottleneck: it doesn’t limit you on simple tasks, but it increasingly determines your performance as cognitive demands rise.
Nutrition and Toxic Exposures
Some of the most dramatic effects on intelligence come from nutritional deficiencies and environmental toxins during critical periods of brain development. Iodine deficiency is the clearest example. A meta-analysis of 18 studies found that children growing up in iodine-deficient areas scored an average of 13.5 IQ points lower than children in iodine-sufficient areas. A later analysis of 37 studies in China put the gap at 12.5 points. Even mild iodine deficiency can cost 8 to 15 IQ points. Since iodine is essential for thyroid hormones that drive brain development, the damage occurs primarily during pregnancy and early childhood, and it’s largely preventable through iodized salt.
Lead exposure tells a similar story. Data from the National Institute of Environmental Health Sciences shows that every increase of 10 micrograms per deciliter in a child’s lifetime average blood lead concentration is associated with a 4.6-point decrease in IQ. Unlike iodine deficiency, lead exposure is cumulative and harder to reverse. The effects persist even at levels once considered “safe,” which is why acceptable blood lead thresholds have been revised downward repeatedly over the past few decades.
The Flynn Effect and Its Reversal
For most of the 20th century, IQ scores rose steadily across developed nations, climbing three to five points per decade. This trend, known as the Flynn effect, was likely driven by improvements in nutrition, education, healthcare, and cognitive stimulation rather than any genetic change. It served as powerful evidence that environment shapes intelligence at the population level.
That trend has now reversed in some areas. A Northwestern University study examining a large U.S. sample between 2006 and 2018 found declining scores in verbal reasoning, visual problem solving, and mathematical ability. Only spatial reasoning scores continued to rise. The declines held regardless of age, education level, or gender, ruling out simple demographic shifts as the explanation. The causes remain debated, but the pattern suggests that whatever environmental improvements drove a century of gains may have plateaued or that new environmental pressures are pulling scores in the opposite direction.
How These Factors Interact
No single factor determines intelligence in isolation. Genes set a wide range of potential, but where you land within that range depends heavily on your environment, particularly early in life. A child with strong genetic potential for cognitive ability who grows up with severe iodine deficiency or chronic lead exposure will not reach that potential. A child in an enriched, well-nourished environment has the best chance of expressing whatever genetic advantages they carry.
The interaction between genes and environment also means that population-level changes in intelligence, like the Flynn effect or its reversal, happen far too quickly to reflect genetic shifts. They reflect changes in the conditions under which brains develop: better nutrition, less toxic exposure, more years of schooling, or conversely, new environmental pressures that haven’t yet been identified. Intelligence is best understood not as a fixed trait you’re born with, but as a developmental outcome shaped by biology and experience acting on each other continuously from conception through adulthood.