A delusion is a fixed, false belief that a person holds with complete conviction despite clear evidence that it isn’t true. What separates a delusion from a simple mistake or a strongly held opinion is that it resists correction. You can present overwhelming proof that the belief is wrong, and the person remains entirely unshaken. Delusions are a core feature of several psychiatric conditions, most notably schizophrenia and delusional disorder, but they can also arise from brain injuries, infections, substance use, and a range of other medical causes.
How Delusions Differ From Strong Beliefs
Everyone holds beliefs that others might consider wrong or irrational. What makes a delusion clinically distinct is a combination of features that go beyond simple stubbornness. Delusions tend to have an abrupt onset rather than developing gradually over time. Their content is often implausible, sometimes dramatically so. And perhaps most telling, people experiencing delusions show a relative indifference to the opinions of others. They aren’t arguing a point or trying to convince anyone. The belief simply is, as far as they’re concerned.
This matters because delusions can look superficially similar to what clinicians call “overvalued ideas,” which are strongly held beliefs that dominate a person’s thinking but develop more gradually and have more plausible content. Research comparing the two found that people with delusions were less likely to identify anything that might change their mind, less preoccupied with defending the belief, and less concerned about how others reacted to it. Interestingly, the level of conviction was similar in both groups. That means you can’t distinguish a delusion from an overvalued idea just by asking how certain someone is. The better markers are how suddenly the belief appeared, how plausible its content is, and whether the person engages with counterarguments at all.
Delusions also differ from hallucinations, though the two often occur together. A hallucination is a sensory experience: hearing, seeing, or feeling something that isn’t there. A delusion is a belief. Someone might hallucinate a voice (sensory) and then develop the delusional belief that the government is transmitting messages to them (interpretation). One is perception, the other is conviction.
Bizarre vs. Non-Bizarre Delusions
Clinicians divide delusions into two broad categories based on their content. Non-bizarre delusions describe situations that could theoretically happen in real life but simply aren’t true. Believing your spouse is having an affair when they aren’t, or that a celebrity is secretly in love with you, or that coworkers are conspiring to get you fired. These scenarios are possible, just false.
Bizarre delusions, by contrast, involve things that are physically impossible. Believing that someone has removed your internal organs and replaced them with someone else’s, or that an outside force is inserting thoughts directly into your mind, or that your thoughts are being broadcast so that everyone around you can hear them. These beliefs violate the basic physical rules of how the world works. Bizarre delusions are more closely associated with schizophrenia, while non-bizarre delusions are the hallmark of delusional disorder, a separate diagnosis that requires one or more non-bizarre delusions lasting at least a month.
Common Types of Delusions
Delusions are also categorized by their theme, and certain themes come up repeatedly across patients and cultures:
- Persecutory delusions are the most common type. The person believes they are being followed, spied on, poisoned, or targeted for harm by an individual, group, or organization.
- Grandiose delusions involve an inflated sense of identity or importance. The person might believe they have extraordinary abilities, secret knowledge, or a special relationship with a powerful figure.
- Erotomanic delusions center on the belief that another person, often someone famous or of higher social status, is in love with them.
- Jealous delusions involve the unshakable conviction that a partner is being unfaithful, despite no credible evidence.
- Somatic delusions focus on the body. The person might believe they have a serious disease, that their body is infested with parasites, or that they emit a foul odor.
A diagnosis doesn’t require that these beliefs be obviously outlandish. Someone with a jealous delusion may seem like any suspicious partner until you notice that no amount of reassurance or evidence makes any difference, and that the belief appeared suddenly without any precipitating event.
Rare Misidentification Syndromes
Some of the most striking delusions involve failures of identification. In Capgras syndrome, a person becomes convinced that someone close to them, a spouse, parent, or friend, has been replaced by an identical-looking impostor. The person can see that the “replacement” looks exactly like their loved one but insists it isn’t really them. This syndrome appears in roughly 1% to 4% of psychiatric populations and is seen in about one in ten patients hospitalized for a first psychotic episode. It occurs most often alongside schizophrenia but also shows up in dementia, mood disorders, and after brain injuries.
The neuroscience behind Capgras syndrome is revealing. It appears to involve a disconnection between the brain’s facial recognition area and the emotional circuits that generate the feeling of familiarity. The person can process a face normally (they see it looks like their mother) but the emotional signal that says “this is someone I know” doesn’t fire. Faced with that mismatch, the brain generates an explanation: this must be an impostor. A related but rarer condition, Fregoli delusion, works in reverse. The person believes that different people they encounter are actually the same person in disguise.
What Happens in the Brain
The brain constantly makes predictions about what will happen next and then checks those predictions against what actually occurs. When reality doesn’t match expectations, a “prediction error” signal fires, telling the brain to update its model of the world. This system depends heavily on dopamine signaling in a circuit connecting the midbrain, a region deep in the brain called the striatum, and the prefrontal cortex (the area behind your forehead responsible for reasoning and judgment).
In people who develop delusions, this system appears to malfunction. The prediction error signal fires when it shouldn’t, making random coincidences feel intensely meaningful. A stranger glances at you on the street and instead of your brain filing it away as irrelevant, it flags the event as deeply significant, demanding an explanation. Over time, the brain stitches these false signals together into a coherent narrative: that stranger was watching you, which connects to the car you saw parked outside yesterday, which connects to the feeling that your phone sounds different. The result is a delusional belief built from a chain of spurious associations, each one feeling genuinely important in the moment.
Brain imaging studies in patients with schizophrenia have recorded these abnormal prediction error signals during learning tasks, and the strength of those aberrant signals correlates with the severity of delusions. The dysfunction appears concentrated in the “associative” part of the striatum, a region connected to the prefrontal cortex and involved in abstract thinking rather than basic reward processing. This helps explain why delusions involve complex, narrative beliefs rather than simple emotional reactions.
Conditions That Cause Delusions
Delusions are not a disorder on their own but a symptom that arises across a wide range of conditions. The most commonly associated psychiatric diagnoses are schizophrenia, delusional disorder, schizoaffective disorder, and severe episodes of bipolar disorder or major depression with psychotic features.
But virtually any condition affecting brain function can produce delusions. Neurological causes include Parkinson’s disease, Lewy body dementia, Huntington’s disease, multiple sclerosis, traumatic brain injuries, strokes, and brain tumors. Infections like HIV, neurosyphilis, and certain forms of encephalitis (including a type caused by the immune system attacking brain receptors) can trigger them. Endocrine problems like thyroid disorders and metabolic conditions like vitamin B12 deficiency or Wilson’s disease are also documented causes. Substances ranging from alcohol and cannabis to LSD, PCP, and MDMA can induce delusions, as can certain prescribed medications including steroids and some antimalarial drugs.
This long list matters because it means delusions appearing for the first time, especially in someone without a psychiatric history, often prompt a medical workup to rule out an underlying physical cause. The delusions themselves may look identical regardless of the cause, but treatment depends entirely on what’s driving them.
How Delusions Are Treated
Antipsychotic medications are the primary treatment. Reviews of outcomes over several decades show that roughly one-third to one-half of people with delusional disorder respond to antipsychotic treatment, though “respond” can mean anything from a significant reduction in conviction to full remission. There is no single best medication for delusions. All antipsychotics appear to work, but people tolerate them differently, and poor tolerability is one of the main reasons treatment fails. If someone stops taking a medication because of side effects like weight gain, sedation, or restlessness, the delusion typically returns.
A specialized form of talk therapy called cognitive behavioral therapy for psychosis takes a different approach. Rather than directly challenging the delusion (which rarely works and can damage the therapeutic relationship), it helps the person examine the evidence for their belief, consider alternative explanations, and reduce the distress and behavioral disruption the delusion causes. A meta-analysis of 13 studies found this approach produced a small to moderate benefit compared to standard treatment alone at the end of therapy. However, those gains faded over the following year, with the difference no longer statistically significant after an average follow-up of about 47 weeks. Newer versions of the therapy that specifically target the psychological mechanisms behind delusions, such as jumping to conclusions or intolerance of uncertainty, have shown stronger effects, with effect sizes roughly double those of earlier approaches.
For delusions caused by an underlying medical condition, treating that condition often resolves the delusions. Correcting a thyroid imbalance, treating an infection, or discontinuing a medication that triggered psychosis can eliminate delusional thinking without any psychiatric treatment at all.
Living With Delusions
One of the most challenging aspects of delusions is that the person experiencing them rarely recognizes them as false. This isn’t stubbornness or denial. The brain circuits responsible for evaluating beliefs and flagging errors are the same ones malfunctioning. Asking someone to “see reason” about a delusion is like asking someone with a broken thermometer to get an accurate temperature reading. The tool they would use to check is the tool that’s broken.
For family members, this means that arguing, presenting evidence, or expressing frustration almost never helps and can make the person more guarded and isolated. Maintaining the relationship while gently encouraging treatment tends to be more effective. In conditions like Capgras syndrome, direct confrontation of the belief is specifically discouraged because it can escalate distress or even provoke aggression. The person’s experience of the impostor feels completely real to them, and dismissing it feels like a betrayal.