Causes
The cause of delusional disorder is unknown, but genetic, biochemical, and environmental factors may play a significant role in its development. Some people with delusional disorders may have an imbalance in neurotransmitters, the chemicals that send and receive messages to the brain. There does seem to be some familial component, and immigration (generally for persecutory reasons), drug abuse, excessive stress, being married, being employed, low socioeconomic status, celibacy among men, and widowhood among women may also be risk factors. Delusional disorder is currently thought to be on the same spectrum or dimension as schizophrenia, but people with delusional disorder, in general, may have less symptomatology and functional disability. Diagnosis
Differential diagnosis includes ruling out other causes such as drug-induced conditions, dementia, infections, metabolic disorders, and endocrine disorders. Other psychiatric disorders must then be ruled out. In delusional disorder, mood symptoms tend to be brief or absent, and unlike schizophrenia, delusions are non-bizarre and hallucinations are minimal or absent.Interviews are important tools to obtain information about the patients life situation and history to help make a diagnosis. Clinicians generally review earlier medical records to gather a full history. Clinicians also try to interview the patients immediate family, as this can be helpful in determining the presence of delusions. The mental status examination is used to assess the patients current mental condition. A psychological questionnaire used in the diagnosis of the delusional disorder is the Peters Delusion Inventory (PDI) which focuses on identifying and understanding delusional thinking.
The Diagnostic and Statistical Manual of Mental Disorders (DSM) defines six subtypes of the disorder characterized as erotomanic (belief that someone is in love with one), grandiose (belief that one is the greatest, strongest, fastest, richest, or most intelligent person ever), jealous (belief that one is being cheated on), persecutory (delusions that the person or someone to whom the person is close is being malevolently treated in some way), somatic (belief that one has a disease or medical condition), and mixed, i.e., having features of more than one subtype.Delusions also occur as symptoms of many other mental disorders, especially the other psychotic disorders. The DSM-IV and psychologists agree that personal beliefs should be evaluated with great respect to cultural and religious differences, since some cultures have widely accepted beliefs that may be considered delusional in other cultures.An earlier, now obsolete, nosological name for delusional disorder was "paranoia". Classification
The International Classification of Diseases classifies delusional disorder as a mental and behavioural disorder. Diagnosis of a specific type of delusional disorder can sometimes be made based on the content of the delusions, to wit,  the Diagnostic and Statistical Manual of Mental Disorders (DSM) enumerates seven types:

Erotomanic type (erotomania): delusion that another person, often a prominent figure, is in love with the individual. The individual may breach the law as they try to obsessively make contact with the desired person. Grandiose type (megalomania): delusion of inflated worth, power, knowledge, identity or believing oneself to be a famous person, claiming the actual person is an impostor or an impersonator.