The observation of strange behavior depends at least in part upon the perceptions of the observer. For most people, behaviors that are observed as out of the ordinary or strange are a feature of psychotic disorders, mood disorders, and other mental-health conditions. Examples of conditions in which psychosis (altered perception of reality) are present include schizophrenia, delusional disorder, and schizoaffective disorder. People suffering from psychosis may have an altered perception of reality that often affects their behavior, leading them to act in ways that may be perceived as strange. Problems controlling one's behavior can also occur as a result of brain damage or disorders. People suffering from dementia may display strange behaviors. Behavioral changes can be accompanied by other psychiatric symptoms or by physical symptoms if they are related to a medical condition. Substance abuse and drug overdoses can also lead to problems with behavior.

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