Horizon – the BBC’s flagship science strand – continues its exploration into the fine line between mental illness and sanity, in the concluding episode of this two-part special.

There are 10 volunteers, half have psychiatric disorders, the other half doesn’t – but who is who?

In the first programme, three leading experts in mental health successfully identified volunteer Dan as having Obsessive Compulsive Disorder (OCD). But they mistakenly chose Yasmin as the person they thought was least likely to have a psychiatric disorder. The process of observation turns out to be far harder than any of them expected.

The volunteers have already spent three days at Hever Castle facing a series of challenges. In this episode they turn their hand to orienteering and take part in a virtual reality experiment at the London Institute of Psychiatry. The events are designed to explore the character traits of mental illness and ask whether the symptoms might be within all of us.

The programme explores some of the ideas behind a seminal Seventies experiment by Dr David Rosenhan – where sane patients faked symptoms to gain admission to psychiatric hospitals. It was an experiment which transformed the way in which mental illness was diagnosed.

Investigating the labels associated with psychiatric disorders, the programme questions people’s perceptions and preconceptions of what mental illness is.

Today, around one in four people in the UK has a diagnosis of mental illness at some point in their lives. For many, simply being called “mentally ill” is a heavy burden as it can bring profound social stigma, leaving some patients outcast all their lives.

Tuesday 18 November
9.00-10.00pm BBC TWO