More than one in 10 of us may have a personality disorder, according to Professor Eddie Kane from the Institute of Mental Health. Kane last month told the BBC that international studies show rates of personality disorder to be between 4% and 13%. Under-diagnosed and undertreated, they can cause havoc in families and increase the risk of people who have them killing themselves or even ending up in prison.

What is a personality disorder?

It's a pattern of behaviour that makes it difficult for the person to live with both themselves and other people. When someone has a personality disorder, they may be impulsive or aggressive, find it difficult to control themselves, and therefore may self- harm, or hurt those around them. They will come across as unpredictable and feel misunderstood and alone. They may find it hard to make and keep relationships.

Are there different types?

There are three clusters of personality disorders, and to some extent they overlap. People who have traits in the suspicious cluster may be emotionally cold, suspicious, sensitive to rejection and tend to hold grudges. They may seem eccentric and believe they can read minds or tell the future. Those with traits in the emotional and impulsive cluster, meanwhile, may seem incredibly selfish, putting their needs above others, even if it hurts other people. They are impulsive, tend to be aggressive and frustrated and often lie. People with anxious traits can be rigid, sensitive to criticism, worry and be tense. These categories are not terribly well defined and we could recognise ourselves in many of these traits. But the defining point with a personality disorder is the problems people have with moderating their behaviour to cope with life, and how difficult other people find them to deal with.

What causes personality disorders?

It's likely to be a mixture of genetics and upbringing. Parents who have serious mental health problems may increase the risk of personality disorders in their children, as may parents who drink heavily, are heavy-handed in their discipline and inconsistent in their own behaviour.

Can they be treated?

Yes although there isn't much research on what works, and personality can be tough to change. Talking therapies such as counselling, cognitive behavioural therapy (which trains people to change behaviour), cognitive analytical therapy (which changes destructive ways of thinking), and other interventions such as anger and stress management can help but may take a long time (more than a year). Some people may benefit from drug treatment for any depression and anxiety they have, which could be making their personality disorder worse.

What can friends and family do?

Focus on what's good about the person and try not to condemn them too much. Encourage them to get enough sleep, to take exercise to reduce stress and to avoid drugs.

• This article was amended on 15 March 2011. The original referred to cognitive behavioural therapy (which changes destructive ways of thinking) and cognitive analytical therapy (which trains people to change behaviour). These have both been corrected.