Theresa Riggi analysis: A personality trait or medical disorder? Sometimes, it's hard to tell

Unlike other types of mental illness, personality disorder can be difficult to diagnose because its features overlap with those of normal personality.

To be a bit obsessional can be part of someone's normal personality. But when does it becomes a disorder? It can be quite hard to judge, as there is no absolute dividing line.

Personality disorder is also something that is essentially lifelong - it is not something that starts when you are 25. To have a personality disorder, it generally has to have manifested itself before you are 18 and been typical of the way you are throughout your life.

But where it becomes a disorder is when aspects of your personality start to seriously affect the way you live your life and how you interact with others. Personality disorders will often respond to what is going on in someone's life. If someone's life is stable and they have good, stable relationships and then that falls apart, someone without a personality disorder may struggle to cope. But someone with a disorder may start to have serious difficulties and, for example, self-harm or demonstrate other types of problem.

There are different types of personality disorder, all defined in slightly different ways. Narcissistic personality disorder can include traits such as obsession with yourself, or vanity, and having a degree of grandiosity or egotism - we all know people like that who do not have a personality disorder, but it's to the degree where it actually becomes a disorder and causes serious problems.

Once you get past diagnosing a personality disorder, you move on to the question of treatment. Personality disorders can be controversial both in terms of diagnosis and treatment. Some psychiatrists will argue this is not a mental illness and is essentially not directly treatable, while others may try treating aggressively, which can cause further problems.

In the middle ground, it can be argued we can probably help to a degree but won't be able to solve all the problems.

In cases where someone has committed an offence, you can ask whether the offending behaviour is because of their personality disorder and whether that means they are not responsible for their behaviours. Most would argue that, in the main, it doesn't - they are still responsible, at least to a degree. Part of treatment could involve helping them to take responsibility for what they have done.

• Gordon Lehany is a forensic psychiatrist at the Lynebank Hospital in Dunfermline.