For decades, a rather devastating virus has been quietly tearing out the heart of the middle classes and replacing it with a profound dissatisfaction with life. Despite being surrounded by the trappings of wealth and luxury like never before, as a society we have never been more restless and unhappy.

Television psychologist and author Oliver James believes this so-called virus is responsible for increased mental distress among the population. Its name is affluenza, and it is festering in a home near you.

"Affluenza is placing a high value on money, possessions, physical and social appearances and fame," explains James. "People who do that, according to studies in 14 different nations, are significantly more likely to suffer from depression, anxiety, addictions and personality disorders - the most common mental illnesses." He claims that up to half the population of Britain is suffering from these conditions.

Western society's preoccupation with judging people by material symbols is creating a population focussed on satisfying individual wants, not fundamental needs. Using the envy and approval of others as a gauge for success, instead of nurturing strong family and community networks, we are creating deep insecurities which have moulded us into a nation of shopaholics, workaholics, substance abusers and anxiety sufferers.

"Since the Second World War, this is the first time people have not improved their quality of life, emotionally speaking, compared to the previous generation," says James. "Things were getting better between 1950 and 1975 but from then on, in America, it started to go wrong and the same happened in Britain from around 1980."

He argues that the problem is not an exclusively middle-class one, but says it is to do with attitude.

"It doesn't matter how much you have got. The more that you put possessions before everything else, the more likely you are to become mentally ill."

James embarked on his investigation in 2003, after coming across research carried out in various countries which reported increased levels of mental ill health. "The strange phenomenon of us being much richer and yet a lot more mentally ill has preoccupied me for a long time," admits James.

He interviewed 240 people in six countries including Denmark, America, Britain and China about their values. While affluenza is as old as the hills, James says profound changes in western society in the late-twentieth century escalated the problem.

"It is since the late 1970s onwards that affluenza has really taken off in the English-speaking world. It's much less common in parts of the world where what I call Selfish Capitalism' is absent." Indeed, statistics compiled by the World Health Organisation in 2004 show levels of emotional distress (depression, anxiety, substance abuse and impulsivity-aggression) at 26.4% in America, compared to 4.7% in Nigeria. James identifies key factors as businesses being valued according to their share price, which leads to short-term thinking, a focus on short-term profits and temporary contracts; not taxing the rich properly; suppressing unions; and the subsequent creation of massive inequalities.

"All of these things are characteristic of English-speaking nations, and not Europe. They cause people to become more materialistic and more obsessed with money and possessions, and to stop bothering with their families and the community. They cease to value beauty and the things in life that really make us feel good."

Dr Alex Yellowlees, consultant psychiatrist at the Priory Hospital in Glasgow, agrees that defining your self-worth by something outwith your control - wealth, body shape, the opinions of others - will always bring unhappiness, but he adds: "I think James is using the term mental illness' in too sweeping a way, particularly by including personality disorders, but essentially I believe he is right. When people base their self-worth on things which will not be empowering at an inner level, they become vulnerable to strain and anxiety. Vast numbers of Scottish men who base their self-worth exclusively on whether their football team is doing well can be miserable for days if their team loses a match."

Through his research, James has identified how be to be content.

"In general, I would say be' rather than have'. Meet your true needs rather than the wants created by advertisers. In particular, at the moment when interest rates are rising, don't let property rule your life. Think very hard about whether you actually need a new kitchen or whether you just want it. Do you need a conservatory or extension, or do you just desire it? Do you have to move house because you have had another child, or could two children share one room? Don't live for your mortgage; let your mortgage enable you to live your life in a nice home. To put it very simply, don't always want what you haven't got."

He adds: "If you are a woman, concentrate on being beautiful rather than attractive. When you look in the mirror, don't worry about whether men are going to desire what you are seeing and don't secretly wish that women are going to envy what you are seeing.

Concentrate on what you like."

However, James is not just another prophet of doom, saying: "I am rather optimistic. I think that people are already sick to the back teeth of all this garbage. They are already tired of endlessly relying on consumer goods which don't work, and fed up with not being able to get hold of anybody on the end of a phone when they've got a problem."

The notion of shirking material possessions and helping others, not yourself, is common in religious teaching, and James admits that many of the most contented people he interviewed were those with a strong religious conviction.

Morag Milne, convener of the Church and Society Council of the Church of Scotland, says: "The idea of finding contentment not through material possessions, but through family and community, is one which chimes with the beliefs of the church. It reminds me in particular of the Beatitudes; blessed are the meek, for they shall inherit the earth. It is nothing new - it's a belief which has been around for a long time, and which Christ himself preached. However, in this time of mass media, celebrity and great uncertainty, it is one that is definitely worth stating again."

Aspiring millionaires need not panic, though, as James stresses that becoming rich doesn't necessarily equate to being miserable. A quick glance at the top of Scotland's rich list reveals several individuals who appear to be perfectly content. JK Rowling, for example, continues to maintain a normal family life and regularly gives to charity.

Sir Tom Hunter, one of the country's richest men with a fortune of £678m in 2005, is a well-known philanthropist who set up a £100m charitable foundation.

Sir Tom never seems more happy nowadays than when he is giving away his money. Perhaps he remembers Andrew Carnegie's motto: "The man who dies rich dies in disgrace."