01/07/2014 13:15 BST | Updated 31/08/2014 06:59 BST

'What's the Problem?'

I was much too far out all my life And not waving but drowning -

Stevie Smith, Not Waving But Drowning

Dennis Stevenson is a well-known British businessman, and has occasional bouts of depression. This is how he describes them. 'I once broke my leg in ten places. As I was taken to hospital, someone shut the door on my leg. You can imagine the pain. But I can tell you the pain of depression is many times worse: it is excruciating.'

Mental pain is as real as physical pain. It is experienced in the same areas of the brain as physical pain and is often more disabling. Yet these two types of pain are not treated equally. While nearly everyone who is physically ill gets treatment, two in three of those who are mentally ill do not. If your bone is broken you are treated automatically, but if your spirit is broken you are not.

This is a shocking form of discrimination, which occurs in every healthcare system in the world. It is particularly shocking because we have very good treatments for the most common mental health problems, which are depression and crippling anxiety disorders. The treatments - modern psychological therapy and drugs when appropriate - are not expensive. And the economics are striking.

Treating mental health problems produces extraordinary savings - fewer people on welfare benefits, and fewer people being treated for physical illnesses made worse by mental illness. So on any reasonable estimates the treatments pay for themselves. They cost society nothing. And yet they are provided to under a third of those who need them. That is a great injustice and a gross inefficiency. And it is the main reason we have written this book - the pain of untreated mental health problems, and the fact that they can be treated at little or no cost.

There is also a wider issue. The last fifty years have seen enormous progress in advanced societies - less absolute poverty, better physical health, more education, and better housing. And yet in the USA, Britain, and many other countries, there is almost as much misery as there was fifty years ago - at least as many social problems and more family conflict, more crime, and more anti-social behaviour. Dealing with the externals of income, education, physical illness, and housing has not been enough to produce happier or more orderly lives. We have left something out - the inner person. Mental health is something that requires deliberate cultivation and expert help when it goes wrong. If our society had better mental health, we should all gain. That is the second reason we have written this book: the huge social cost of mental illness.

The facts we lay out are in many cases quite remarkable - indeed after many years in the field some of them still amaze even us. So here are the main questions we investigate.

How many suffer?

Mental illness is the great hidden problem in our societies, so most people are amazed when they hear the scale of it. In Britain today one in six of all adults suffers from depression or a crippling anxiety disorder. The same is true in the USA and in continental Europe.

Roughly a third of families currently include someone who is mentally ill. So when people ask us what we work on and we say mental health, the reply is almost always, 'Oh, my son..', or 'my mother...', or sometimes, 'I have to admit that I...' but then usually, 'And please don't tell anyone.' (This is particularly true when it's a politician.) In America, more people commit suicide than are killed in road accidents.

Mental illness is not only common, but it can be truly disabling through its impact on people's ability to care for themselves, to function socially, to get around, and to avoid physical and mental pain. In that sense, depression is on average 50% more disabling than angina, asthma, arthritis or diabetes.

So here is an extraordinary fact. When the World Health Organization measure the scale of illness and allow for its severity, they find that in rich countries mental illness accounts for nearly 40% of all illness. By contrast, stroke, cancer, heart disease, lung disease, and diabetes account for under 20%.

Mental illness is extremely difficult to adapt to - much more so than most physical illness except for unremitting pain. So it is terrible for those who experience it. But it is also bad for business, since it gives rise to nearly half of all days off sick. And it is bad for taxpayers, since mental illness accounts for nearly half of all the people who live on disability benefits.

Given all this, you would think that mental illness would be high on the priorities of every government's department of health. But not so. In 2007 we met with Britain's new secretary of state for health after he had been in his job for three weeks. At the end of our meeting he said, 'Something has struck me. I've been in post for three weeks and gone to about forty meetings, but I have not so far heard the phrase "mental health".'

The situation is similar with employers. In January 2012 the World Economic Forum was having its usual snow-bound con- ference in Davos. This included a meeting of the Workplace Wellness Alliance - a group of sixty of the world's most enlightened employers. The meeting was about the health challenges facing employers, and there were detailed presenta- tions on cardiovascular disease, diabetes, lung problems, cancer, and musculoskeletal issues - but there was nothing on mental illness, even though it causes so much sickness absence. People just don't want to talk about it.

Thrive: The Power of Evidence-Based Psychological Therapies, by Richard Layard and David M Clark, is published by Allen Lane on 3rd July 2014, priced: £20. Available from Amazon

Richard Layard is one of the world's leading labour economists, and in 2008 received the International Prize for Labour Economics. A member of the House of Lords, he has done much to raise the public profile of mental health. His 2005 book Happiness has been translated into 20 languages.

David M Clark, Professor of Psychology at Oxford, is one of the world's leading experts on CBT, responsible for much progress in treatment methods. With Richard Layard, he was the main driver behind the UK's Improving Access to Psychological Therapies programme.