No one knows exactly what causes paranoia. There are lots of theories and different people will have different explanations for their own experiences. It's likely to be a combination of things.

Researchers have identified some general risk factors – these are things that could make paranoid thoughts more likely.

• Having confusing or unsettling experiences or feelings that you can't easily explain.
• The way you feel – if you are anxious or worried a lot or have low self-esteem and expect others to criticise or reject you.
• The way you think – if you tend to come to conclusions quickly, believe things very strongly and don't easily change your mind.
• If you are isolated.
• If you have experienced trauma in the past.

There are lots of more specific things that may play a role in causing paranoid thoughts. Sometimes this could be because they make you more likely to experience the risk factors above. For example, taking drugs or lack of sleep might make you have more confusing or unsettling experiences.

• Life experiences. You are more likely to experience paranoid thoughts when you are in vulnerable, isolated or stressful situations that could lead to you feeling negative about yourself. If you are bullied at work, or your home is burgled, this could give you suspicious thoughts which could develop into paranoia.
• Experiences in your childhood may lead you to believe that the world is unsafe or make you mistrustful and suspicious of others. They may also affect your self-esteem and the way you think as an adult. (See National Association for People Abused in Childhood (NAPAC) if you need support for dealing with childhood abuse.)
• External environment. Some research has suggested that paranoid thoughts are more common if you live in an urban environment or community where you feel isolated from the people around you rather than connected to them. Media reports of crime, terrorism and violence may also play a role in triggering paranoid feelings.
• Mental health. If you experience anxiety, depression or low self-esteem, you may be more likely to experience paranoid thoughts – or be more upset by them. This may be because you are more on edge, worry a lot or are more likely to interpret things in a negative way. Paranoia is a symptom of some mental health problems. Many people experience paranoid delusions as part of an episode of psychosis.
• Physical illness. Paranoia is sometimes a symptom of certain physical illnesses such as Huntington's disease, Parkinson's disease, strokes, Alzheimer's disease and other forms of dementia. Hearing loss can also trigger paranoid thoughts in some people.
• Lack of sleep. Lack of sleep can trigger feelings of insecurity and even unsettling feelings and hallucinations. Fears and worries may develop late at night.
• The effects of drugs and alcohol. Drugs such as cocaine, cannabis, alcohol, ecstasy, LSD and amphetamines can all trigger paranoia. Certain steroids taken by athletes and weightlifters can also lead to symptoms of paranoia. Some insecticides, fuel and paint have also been associated with paranoia.
• Genetics. Research has suggested that your genes may affect whether you are more likely to develop paranoia – but we don't know which ones.

This information was published in November 2016. We will revise it in 2019.