People who are delusional engage in behaviors based on the type of delusion from which they are suffering, such as repeatedly contacting law enforcement because they believe someone is going to harm them or stalking people they believe are in love with them, notes WebMD. People with delusional disorders might also be irritable or angry and experience hallucinations related to their delusions.
A person with erotomanic delusions believes that someone, often someone famous, is in love with him and attempts to make contact with that person, states WebMD. Someone with grandiose delusions has an overdeveloped sense of self and may believe he has made important discoveries or feel more important or knowledgeable than those around him. People with jealous delusions commonly believe their partners are cheating on them, while people with somatic delusions are convinced they suffer from some type of defect or medical issue.
Persecutory delusions may cause people to believe that they, or people close to them, are at risk of harm or are being spied upon, according to WebMD. Some people suffer from mixed delusional disorders, in which they engage in behaviors typical of people with two or more different types of delusions. Doctors do not know what causes delusions, as of 2015, but believe they are related to genetic, biological, environmental and psychological factors.