Most people get anxious in their daily lives, but where does the line between sanity and madness lie and is it easy to recognise?
Do you ever worry about leaving the gas on, or arrive at work and worry that you've not locked your front door? Perhaps you get anxious before having to give a talk or wake up in the morning and are barely able to drag yourself out of bed?
These are normal reactions to the twists and turns of modern life. But all these actions can also be seen as symptoms of mental illness.
Those with Obsessive Compulsive Disorder (OCD) know this only too well. Classic symptoms of the illness include repeated checking of things like the gas and door locks.
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Horizon: How mad are you? Part Two is broadcast on BBC Two on Tuesday, 18 November at 2100BST
It is motivated by a genuine and deep-rooted fear that the person or their loved ones will be in grave danger if they don't check. It's an emotion that everyone can relate to, the idea of the house burning down is probably enough to get anyone out of bed just to check "one more time".
But checking and rituals can quickly come to dominate a person's life. Hours every day are taken up with compulsions which have a major impact on work and relationships.
Distinguishing between reactions that are generally considered healthy and those that are signs of a full-blown mental disorder has always been a challenge for psychiatry, the science of the mind.
The 10 volunteers are challenged to perform a stand-up comic routine.
Most disorders have symptoms that in a milder form can just seem like character traits. But how pessimistic do you have to be before a psychiatrist would say you had depression? How much insecurity makes you schizophrenic? Just how hard is it to tell the difference between "illness" and "health"?
The BBC science series, Horizon challenged three mental health experts to see if they could make that distinction.
It selected 10 volunteers of all ages and backgrounds. Half the group had a history of various mental disorders, the other half didn't. The question for the panel was simple - who is who?
They were filmed during a week of activities, ranging from group bonding exercises to specific psychological tests. All were designed to explore some of the classic symptoms and traits of some major psychiatric disorders.
A distorted body image is a key symptom of eating disorders like Anorexia Nervosa and Bulimia Nervosa. People living with these illnesses tend to see themselves differently to the way they actually are, both in terms of body shape and weight. But what complicates matters is that almost all of us do that to some extent.
Those with eating disorders have a distorted body image
The volunteers took photos of each other wearing white lycra suits and the images were stretched out of shape. They were then asked to shrink them back to the way they saw themselves.
Yasmin, 36, is an amateur rugby player and physically she is one of the larger members of the group. Like all the other volunteers she was faced with an unflattering image of her headless body stretched. She exaggerated her own size by only 8%, which was less than anyone else in the group.
But what interested the panel was how she really agonised over it. She said it was her least favourite task of the week by a very long way and she took twice as long to do the test as any of the others.
Even the panel felt uncomfortable watching her, including Ian Hulatt, a psychiatric nurse and mental health advisor to the Royal Collage of Nursing. But he felt her reaction was character, not illness.
"That was probably in response to how she feels about social messages about her being overweight in our culture," he says. "That wasn't sufficiently out of context or unusual to make us feel… it was pathological."
The volunteers are asked to clean out a cowshed.
The results of the group as a whole mimicked larger research studies showing that most people think they're bigger than they are. Effectively when we look in the mirror we don't see a true picture of ourselves reflected back. Those with eating disorders tend to be on the extreme end of this scale.
Millions in Britain worry about the way they look, count calories and diet. Like many of the disorders explored by Horizon, the facts are shocking: 10% of adult women will be affected by Bulimia, one in 10 of those with Anorexia are men.
The test the volunteers found most challenging was performing a five minute stand-up comedy routine on stage in a local pub. Even at the first rehearsal there were no volunteers to go first and almost everyone in the group found the idea daunting.
Several exhibited signs of social anxiety. One actually admited feeling sick at the very thought of public speaking, but ended up with a strong performance. Another who appeared more relaxed beforehand ducked out early.
"From what I saw there is no difference at all between people who have had mental health problems and have recovered and those that never have," says Yasmin, after the performances.
But the panel picked up several indications. One volunteer displayed pessimism, another had disjointed speech and another showed risk-taking behaviour.
The public need to realise that you cannot just look at someone and make assumptions
Ian Hulatt Psychiatric nurse
But for the panel the biggest clue was not a true symptom. One of the volunteers used a rationalisation technique to cope with the situation, imagining the worst that could happen and realising it isn't that bad after all. The panellists identified this as Cognitive Behavioural Therapy, a sign she had undergone counselling.
The programmes explored major disorders including Depression, Schizophrenia, Bi-Polar Disorder, Obsessive Compulsive Disorder and Eating Disorders. All of them are serious illnesses that have the power to devastate the lives of those affected by them.
But despite the seriousness of these illnesses, identifying who has them was a real challenge for the panel of experts.
"It makes a point the public need to realise that you cannot just look at someone and make assumptions," says Mr Hulatt.
"When someone has been labelled with a disorder or episode of mental illness it's very unhelpful to interpret everything they do through a poorly understood label."
The programme found that it is wrong to assume they cannot lead normal lives. This is coupled with the fact that mental illness is a problem that affects as many as one in four of the UK population, yet it is often misunderstood, shrouded in stereotype and stigma.
Making the two documentaries was a real eye opener. In the course of the production we met some amazing people who have struggled with disorders and overcome them. We met brave people with no history of mental illness who, to explore the line between mental health and mental illness, were willing to risk being labelled with an illness they do not have.
They all tell a powerful story, that having a mental illness doesn't have to become your defining characteristic and that it shouldn't set you apart in society.
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