Jeroen Saeij

Toxoplasmosis

The brain and behavior: what the evidence shows

This is the claim everyone has heard, a parasite from cats that hijacks the mind. Some of it is real biology, some of it is statistics stretched too far, and telling the two apart is what this page is for.

By Jeroen Saeij · Updated 17 September 2026

The short answer

In rodents, infection genuinely changes behavior, but the change looks less like precise mind control and more like a general loss of caution caused by inflammation in the brain. In people, infection is consistently but modestly associated with schizophrenia and with a few other outcomes; these are correlations, the effects are small, the best population study finds little, and countries where most people are infected do not have more schizophrenia than countries where few are. In people with weakened immunity, the parasite causes real brain disease, and that is not controversial at all.

Where the idea comes from

Rats are born afraid of cat odor. In 2000, Oxford researchers reported that infected rats lost that fear and in some cases were drawn to the smell, a change they called fatal attraction and interpreted as the parasite steering its host toward the animal it needs to complete its life cycle1. Later work found the effect strikingly specific: infected rodents lost their fear of cat odor while other kinds of fear and learning seemed intact2. It is a wonderful story, and it launched a thousand headlines.

What the newer rodent work says

A more careful look has complicated the story without killing it. In 2020 a Geneva group tested infected mice on a battery of behaviors and found that they were less anxious and more exploratory in general, and less afraid of predators in general, not specifically of cats. The size of the change tracked the number of cysts in the brain, and with it the amount of inflammation3. The parasite does change behavior; it seems to do so by inflaming the brain rather than by targeting a fear of cats. My own lab has found a related thread: which parasite strain a mouse is infected with determines whether it develops seizures during brain infection, and a single parasite protein, GRA15, that manipulates the host’s inflammatory signaling is part of the reason4. Inflammation, again.

The human studies

Schizophrenia. This is the most studied link and the most consistent. People with schizophrenia are more likely to have antibodies to the parasite. Across 38 studies the odds were about 2.7 times higher5. A later, more conservative analysis of 50 studies put the figure at 1.8, and found associations of similar size with bipolar disorder and addiction, a larger one with obsessive-compulsive disorder, and none with depression6. For scale, having a parent with schizophrenia raises the risk seven to nine times5. The association is real; whether the parasite causes any of it is unknown. People with schizophrenia may be more exposed, more often institutionalized, or different in ways that also affect infection.

Suicide and self-harm. A Danish study followed nearly 46,000 mothers whose antibodies were measured at childbirth. Infected mothers had about a 50 percent higher rate of self-directed violence over the following years, and the risk rose with antibody levels7. It is a well-designed study with a modest effect.

Traffic accidents, personality, entrepreneurship. A Czech study found infection more common among people involved in traffic accidents8. An American study found infected students more likely to major in business and infected professionals more likely to have started their own business9. These make headlines, and they are the kind of result that needs replication in a population sample before it means much.

The population test. The most important human study, to my mind, is the least dramatic. A New Zealand birth cohort followed since the early 1970s tested 837 people for antibodies and looked at psychiatric disorder, impulse control, personality and cognitive performance. It found little: no association with schizophrenia or depression, none with personality, a marginal excess of suicide attempts, and a worse score on one of fourteen cognitive tests10. When you test everyone rather than patients and volunteers, most of the effect disappears.

The country test

There is a simpler test than any case-control study, and it is the one my colleagues outside the field ask about first. Infection rates differ enormously between countries. If the parasite caused a meaningful share of schizophrenia, countries where most people are infected should have more of it than countries where few are.

The differences in infection are large. In a compilation of surveys from 88 countries, the share of women of childbearing age carrying the parasite ranged from 4 percent in South Korea and 9 percent in the United Kingdom to more than 75 percent in Nigeria, Cameroon and Madagascar, with a median of 39 percent11. Pooled estimates in pregnant women tell the same story: around 56 percent in South America against about 12 percent in the Western Pacific, with the highest rates in low-income countries1213.

What would a real effect predict? Take the published individual-level odds ratios at face value, 1.8 from the more conservative meta-analysis and 2.7 from the more generous one, and treat them as causal. A country where 60 percent of people are infected should then have between 1.4 and 1.7 times as much schizophrenia as a country where 10 percent are, and the most infected countries in that 88-country set should have between 1.6 and 2.3 times as much as the least infected65. The arithmetic is in a short script published with this site.

That is not what the data show. The Global Burden of Disease study, which models schizophrenia prevalence for 195 countries from 129 data sources, estimates a global prevalence of 0.28 percent and reports that prevalence did not vary widely across countries or regions14. Direct survey estimates do vary, by about fivefold between sites, but not in the direction the hypothesis needs: the systematic review that collected them found the lowest prevalence in the least developed countries15, which are precisely the countries with the highest infection rates12. Migration and city living, not infection, are the exposures that track with schizophrenia across sites16.

The one study that screened infection rates against the burden of 128 diseases across those 88 countries found correlations with 23 of them after adjusting for wealth, latitude and humidity. Its authors, who are the leading proponents of the parasite’s behavioral effects, discuss cardiovascular disease, epilepsy, obsessive-compulsive disorder, suicide and traffic accidents in detail. Schizophrenia is on the list of conditions they screened and does not appear among the results they report11. An earlier ecological study did find that suicide rates in older women tracked infection rates across 20 European countries17, so the approach can find associations when they exist.

Two caveats cut in opposite directions. Country comparisons are crude: diagnosis, reporting and survival differ between countries, and a modest real effect could hide inside that noise. But the same crudeness cannot explain away a missing effect of the size the case-control literature implies, because an effect that size should show above the noise. My reading is that the country test argues against the parasite being a major cause of schizophrenia, and leaves room for a small one.

What I make of it

The rodent biology is genuine and is about inflammation. The human associations are mostly small, inconsistent across study designs, unable to separate cause from consequence, and absent at the level of whole countries. If the parasite affects the behavior of healthy people, the effect is at the edge of what our studies can detect. That is a very different claim from the one on the magazine covers.

What is not in doubt is what the parasite does to a brain that can no longer contain it. In advanced HIV, after transplantation, or under immunosuppressive treatment, dormant cysts reactivate and cause toxoplasmic encephalitis, with confusion, seizures and weakness, and it is fatal without treatment18. That, not mind control, is the brain disease clinicians worry about.

Sources

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