He was thirty-four years old, training for his first marathon, and he asked me a question I have thought about ever since.
I had assessed him twice, about eighteen months apart, for reasons unrelated to fitness. Between the two evaluations, he had transformed his physical life. He had gone from sedentary to running five days a week, working up to a marathon he would complete two months later. When I reviewed his second cognitive assessment against the first, certain scores had moved upward in a way that is unusual for an adult tested twice in adulthood, particularly on measures of processing speed and working memory.
"Did the running do something?" he asked me, half joking. "Did getting in shape actually make me smarter?"
The honest answer surprised him, and it is the reason I decided to write this article. The research does not support most of what you have read about boosting your brain. Brain-training apps, the ones that promised to raise your intelligence through daily puzzle games, have largely failed to deliver on their claims under rigorous testing. Nootropic supplements have thin evidence at best. But there is exactly one lifestyle intervention with genuine randomized-controlled-trial evidence behind it, and it is not the one the supplement industry wants you to buy.
That intervention is physical exercise. And the evidence linking it to intelligence is stronger, larger, and more causal than almost anyone realizes.
This article walks through what the science actually shows about whether exercise increases IQ, how the effect works in the brain, how large the effect really is, and what an evidence-based approach would look like if you wanted to use your body to support your mind.
The Study of 1.2 Million Brains
The single most impressive piece of evidence in this field comes from Sweden, and its scale is almost hard to comprehend.
In 2009, a research team led by Maria Åberg published a study in the Proceedings of the National Academy of Sciences that examined the relationship between cardiovascular fitness and cognitive ability in 1,221,727 Swedish men. These were conscripts, young men entering mandatory military service at age 18, who underwent both standardized cognitive testing and objective measurement of their cardiovascular fitness as part of their induction.
This is not a survey of a few hundred people. It is more than a million young men, tested with objective physiological measures and validated cognitive assessments, in a single national dataset. Studies of this size are extraordinarily rare in any field of science, and they allow researchers to detect relationships with a precision that smaller studies cannot approach.
The findings were clear. Cardiovascular fitness was positively associated with cognitive ability. Young men who were more aerobically fit scored higher on intelligence measures, with the association strongest for logical reasoning and verbal comprehension. The relationship was statistically robust and remarkably consistent across the enormous sample.
One detail is particularly important. Muscular strength, measured separately, showed no comparable association with intelligence. It was specifically cardiovascular fitness, the kind built by running, cycling, swimming, and other sustained aerobic activity, that tracked with cognitive ability. This specificity matters, because it points toward a particular biological mechanism rather than a general "healthy people are smarter" explanation.
But the most compelling part of the Åberg study was not the basic correlation. It was what the researchers did to test whether the relationship was causal.
How the Swedish Study Ruled Out Genetics
Whenever researchers find that two things are associated, fitness and intelligence in this case, the immediate question is whether one causes the other or whether some third factor causes both. Perhaps genetically advantaged individuals simply happen to be both smarter and more athletic, with no causal link between the two traits themselves.
The Åberg team had a powerful tool for addressing this. Among their more than a million participants were 268,496 pairs of full siblings, 3,147 pairs of twins, and 1,432 pairs of identical twins. This allowed them to examine the fitness-intelligence relationship within families, comparing siblings to each other.
Siblings share substantial genetic material and the same childhood environment. Identical twins share essentially all of their genes. If the fitness-intelligence association were driven by genetics, it would largely disappear when comparing siblings to each other, because siblings are genetically similar. If the association persisted even within sibling pairs, that would point toward environmental causation, meaning that the fitness itself, rather than shared genes, was driving the cognitive difference.
The association persisted within sibling pairs. When one brother became more cardiovascularly fit than his sibling between the ages of 15 and 18, he also tended to score higher on cognitive testing at 18. The twin analyses attributed the great majority of the shared variation between fitness and intelligence, more than 80 percent, to environmental factors that were not shared between siblings, rather than to genetics.
Even more tellingly, the researchers found that changes in fitness over time predicted cognitive outcomes. Young men who improved their cardiovascular fitness between ages 15 and 18 showed higher cognitive scores at 18 than would be expected. Fitness gains preceded cognitive gains, which is exactly the temporal pattern you would expect if fitness were causally supporting cognition.
This does not amount to absolute proof of causation, because the study remained observational rather than a true experiment. But it is about as strong as observational evidence gets. A relationship that persists within identical twin pairs and follows the correct temporal sequence is very difficult to explain away as mere correlation.
The Randomized Trials: The Strongest Evidence
For genuine proof of causation, science relies on randomized controlled trials, where participants are randomly assigned to either an intervention or a control condition. Random assignment eliminates the confounding factors that plague observational studies, because the only systematic difference between the groups is the intervention itself.
In 2024, a research team led by Javier Morales published a meta-analysis in the journal Pediatrics that pooled exactly this kind of evidence. They gathered 14 randomized controlled trials involving 3,203 children and adolescents between the ages of 5 and 14. In each trial, young people had been randomly assigned to either a structured exercise intervention or a control group, and their intelligence had been measured with validated tests before and after.
The results provided the causal evidence that observational studies cannot. Children assigned to exercise interventions showed significant improvements in general intelligence compared to control groups, with a standardized mean difference of 0.54. They also showed improvements in fluid intelligence, the capacity for novel reasoning and problem-solving, with a standardized mean difference of 0.20.
In an accompanying editorial in the same issue, the physician Michele LaBotz translated these statistical effect sizes into a figure ordinary readers can grasp. Children in the exercise groups showed an overall improvement of approximately 4 IQ points compared to children in the control groups. To provide a sense of scale, LaBotz noted that this is roughly the same magnitude of IQ improvement that has been associated with twelve months of breastfeeding in infancy.
Four IQ points from an exercise program is not a transformation from average to genius. But it is a real, measurable, causally established improvement in intelligence produced by a behavioral intervention available to essentially everyone. In a field crowded with false promises about boosting intelligence, this is one of the very few claims that survives rigorous experimental testing.
What Exercise Does Inside the Brain
The causal evidence naturally raises a question: what is exercise actually doing to the brain to produce these cognitive effects? The neuroscience here is well developed and genuinely fascinating.
The clearest demonstration comes from a 2011 randomized controlled trial led by Kirk Erickson, published in the Proceedings of the National Academy of Sciences. The researchers randomly assigned 120 older adults to either an aerobic walking program or a stretching control group, and they measured the size of the hippocampus, a brain structure central to memory and learning, using MRI before and after the intervention.
The hippocampus normally shrinks with age, losing roughly 1 to 2 percent of its volume per year in late adulthood. This atrophy is part of why memory tends to decline as people get older. The researchers found that in the stretching control group, the anterior hippocampus shrank by about 1.4 percent over the year, as expected. But in the aerobic exercise group, the anterior hippocampus actually grew, increasing by about 2 percent.
Read that carefully. Aerobic exercise did not merely slow the age-related shrinkage of a critical brain structure. It reversed it, effectively turning back one to two years of hippocampal aging in a single year of walking. This is one of the most striking demonstrations in all of neuroscience that the adult brain remains physically responsive to how we use our bodies.
Several biological mechanisms underlie these effects. Aerobic exercise increases blood flow to the brain, delivering more oxygen and nutrients to neural tissue. It stimulates the production of a molecule called brain-derived neurotrophic factor, often described as fertilizer for neurons, which supports the growth, survival, and connection of brain cells. It promotes the formation of new blood vessels in the brain and, in animal studies, the birth of new neurons in the hippocampus. And it reduces the chronic inflammation and improves the metabolic health that otherwise degrade cognitive function over time.
The cardiovascular specificity of the Åberg findings makes sense in light of these mechanisms. It is the sustained elevation of heart rate and blood flow characteristic of aerobic exercise, rather than the brief maximal efforts of strength training, that most powerfully drives these brain-supporting processes. This is why the million-man Swedish study found cognitive associations with cardiovascular fitness but not with muscular strength.
Does This Work for Adults, Not Just Children?
The strongest causal evidence, the randomized trials in the Morales meta-analysis, came from children and adolescents. This raises a fair question about whether adults can expect similar cognitive benefits from exercise, or whether the developing brains of the young are uniquely responsive.
The evidence for adults is somewhat less direct but still substantial. The Erickson hippocampus study was conducted in older adults, demonstrating that even aging brains physically respond to aerobic exercise. And a large 2018 meta-analysis by Joseph Northey and colleagues, published in the British Journal of Sports Medicine, examined 39 studies of exercise interventions in adults over 50 and found a significant overall improvement in cognitive function, with a standardized mean difference of 0.29. Both aerobic exercise and resistance training produced benefits when performed at moderate intensity.
The honest summary is this. The evidence that structured exercise produces measurable improvements in intelligence test scores is strongest in children and adolescents, where randomized trials have directly demonstrated the effect. In adults, the evidence more consistently shows improvements in cognitive function, including memory, processing speed, and executive function, along with physical changes in the brain that support cognition. Whether these adult improvements translate into gains on a full-scale IQ test of the same magnitude seen in children is less firmly established.
But the direction of the evidence is entirely consistent across the lifespan. Exercise supports the brain. It does so through biological mechanisms that operate at every age. And the cognitive benefits, while modest in magnitude, are real, measurable, and causally established in a way that no supplement, brain-training app, or other purported intelligence booster can match.
Important Caveats
As always, I want to represent the science honestly, including its limitations.
The effect sizes are modest. Four IQ points in children, and a standardized improvement of around 0.29 in older adults, are real but small effects. Exercise will not transform an average intelligence into a genius one. Anyone promising dramatic IQ gains from any intervention, including exercise, is overstating what the evidence supports. What exercise offers is a meaningful, incremental cognitive benefit alongside its many other health advantages, not a radical cognitive transformation.
Much of the adult evidence concerns cognitive function rather than IQ specifically. Memory, attention, processing speed, and executive function are components of cognitive ability, and improving them is genuinely valuable, but they are not identical to a full-scale IQ score. The claim best supported for adults is that exercise improves cognitive function and protects against cognitive decline, which is slightly different from the claim that it raises IQ in the strict psychometric sense.
Individual results vary enormously. These studies report average effects across groups. Any individual might respond more or less strongly than the average, depending on their starting fitness, genetics, age, the type and intensity of exercise, and many other factors. The population-level finding that exercise supports cognition does not guarantee any specific outcome for any specific person.
Exercise is not a substitute for cognitive engagement. The brain benefits from both physical activity and intellectual challenge, and these appear to work through partly different mechanisms. A comprehensive approach to cognitive health involves both moving your body and challenging your mind, not choosing one over the other.
What survives all these caveats is the core finding, which is robust and genuinely important: physical exercise, particularly sustained aerobic activity, produces real, measurable, biologically grounded benefits for the brain and for cognitive ability, established through evidence that ranges from million-person cohort studies to randomized controlled trials.
What This Means for You
If you have ever believed that your intelligence is a fixed quantity, stamped on you in childhood and unchangeable thereafter, the exercise research is part of a larger body of evidence that should revise that belief. Intelligence is more malleable than the fixed-number model suggests, and physical exercise is one of the genuinely evidence-based ways to support it.
This has a practical implication that I find genuinely motivating. The same behavior that strengthens your heart, extends your lifespan, improves your mood, and protects against a long list of diseases also supports your brain and your cognitive ability. There are very few interventions in all of health that offer this breadth of benefit, and exercise is the most accessible of them. It requires no prescription, no purchase, and no special equipment. It requires only sustained aerobic movement, performed consistently over time.
If you are considering using exercise to support your cognitive health, the evidence points toward a straightforward approach. Prioritize sustained aerobic activity, the kind that elevates your heart rate for extended periods, because that is what the strongest evidence supports. Aim for consistency over intensity, since the benefits accrue from regular activity over months and years rather than from occasional maximal efforts. Include some resistance training as well, since the adult evidence supports its cognitive benefits alongside aerobic exercise. And be patient, because the brain changes documented in these studies unfold over weeks and months, not days.
There is also a specific way to make this concrete for yourself. If you want to understand whether a fitness change is affecting your own cognition, the logic of the research suggests establishing a baseline. You would not begin a training program without knowing your starting weight or your current running pace. The same principle applies to cognition. Knowing where your cognitive abilities currently stand gives you a reference point against which any future change can be understood.
A validated cognitive assessment provides that baseline, with a percentile-ranked score and a profile of your relative strengths across the major cognitive domains. Establishing your cognitive baseline before a significant fitness change, and revisiting it after several months of consistent training, is the closest an individual can come to running the kind of study on themselves that the researchers ran on their participants. The population evidence tells you what happens on average. Your own baseline and follow-up tell you what is happening for you.
Final Thoughts
The marathon runner I described at the beginning of this article left our second session with a different understanding of the relationship between his body and his mind than the one he had arrived with.
He had assumed, like most people, that his physical training and his cognitive ability lived in separate compartments, that running was for his heart and his legs while thinking was for his brain, and that the two had nothing to do with each other. The research told him otherwise. The sustained aerobic training he had undertaken for his marathon had been, without his knowing it, one of the most brain-supporting things he could have done. The improvements I observed in his cognitive scores were consistent with exactly what the science would predict.
I want to be careful not to overstate this. His cognitive gains were modest, as the research says they should be. He did not become a fundamentally different intelligence through running. But he did provide his brain with the blood flow, the growth factors, and the metabolic health that the evidence shows support cognition, and his scores reflected it.
The larger lesson is the one I try to convey to every patient who treats their intelligence as a fixed and unchangeable number. The brain is not a static organ that received its final configuration in childhood. It is a living, responsive system that changes in response to how you live, including how you move. Among all the interventions that promise to improve your mind, the great majority of which are unsupported by good evidence, physical exercise stands out as one of the few that genuinely delivers, backed by evidence that spans from a million Swedish soldiers to carefully controlled experimental trials.
Your body and your brain are not separate projects. The research could hardly be clearer on this point. Taking care of one is, in a real and measurable sense, taking care of the other.
If you want to understand your own cognitive abilities, and perhaps to track how they respond to the way you live, the first step is knowing where you currently stand. That baseline is worth having, whatever you decide to do with your body afterward.
