Paranoid personality disorder typically manifests as an irrational fear or paranoia that someone is planning to harm you. The condition is diagnosed when the person fears this to such an extent that it has started to impact on their work, personal or social life.

Such a pervasive mistrust of others can also be seen in schizophrenia and the two conditions share similar symptoms such as withdrawing from others and preferring isolation. However, people with paranoid personality disorder do not usually suffer from hallucinations, a key feature of schizophrenia.

Causes

There is no single known cause of paranoid personality disorder. Genetic traits, familial factors and traumatic life events, however, may play a role in development of the condition. People with the condition are more likely to have a relative with schizophrenia, suggesting there could be a link between the two conditions.

Symptoms

Symptoms that may be seen in a person with paranoid personality disorder include:

• An irrational fear that others will harm them
• Persistent searching for evidence that others are out to harm them
• Repeated questioning of hidden motives and people's trustworthiness
• Hostility and aggression
• Aloofness and social withdrawal
• Lack of humour
• Misinterpretation of compliments as criticisms
• Other mental health problems such as anxiety and depression.

At least four of the following features are present in cases of paranoid personality disorder:

• Excessive reactions and agitation in response to perceived criticism
• Tendency to bear grudges
• Suspiciousness
• Combative and disproportionate regard for personal rights
• Excessive self importance
• Preoccupancy with suspected conspirators

One of the main differences between paranoid personality disorder and schizophrenia is that hallucination is not a typical feature of paranoid personality disorder. The hallucinations experienced in cases of schizophrenia may involve seeing, hearing, smelling, feeling or even tasting something that is not actually present.

The most common type of hallucination in schizophrenia is auditory, with the sufferer hearing voices in their head.