I know they're out to get me ...

Your boss cold-shoulders you, your friends don't phone. Just another bad day, or has paranoia got you in its grip?; 'Paranoia is quite a sophisticated mental process ... it is not a sign of stupidity'

When a usually phone-friendly friend of mine didn't call for weeks, I knew what the problem was: she'd heard something horrible I'd supposedly said about her. When she did finally ring, I detected a hint of coldness in her voice. Actually, it turned out that her mother had died and she'd suddenly had to fly to the States for the funeral. The hint of coldness was entirely in my mind. I was looking for it, to endorse my paranoia; I found it.

When he was made redundant from work, Rick became convinced that a colleague had connived to get rid of him. He became reclusive and started plaguing his former colleague with silent phone calls at her home. It was six months before he discovered that she had actually been made redundant only hours after himself.

When Sandra arrived late for a dinner party, she hurried into the living room where the guests were enjoying pre-dinner drinks. The laughter and lively exchanges she'd heard as she approached stopped the moment she walked in. They had obviously been talking about her; the stretched smiles and over-hearty handshakes said it all.

Paranoia. Today people suffer an increasing sense of alienation and isolation; not surprisingly, a tenth of hospital admissions for mental disorder now involve paranoia in some way. With higher street crime and increased random terrorist attacks, the rise of uncontrolled communications systems such as the Internet, mysterious viruses from Aids to Ebola (punishment for bad behaviour?), deadly diseases caused by invisible dust and the fear that the millennium heralds the end of the world, we live in a paranoid age.

In his new book, Understanding Paranoia (Thorsons pounds 6.99), published this week, Dr Peter Chadwick, a lecturer in psychology at Birkbeck College, points out that everybody suffers varying degrees of paranoia. The thinking of mad people isn't really different from that of normal people, he argues; what the officially paranoid, the officially mad - the paranoid psychotics and schizophrenics among us - feel is merely an exaggeration of what many of us suffer in daily life

At one end of the spectrum, there is Timothy McVeigh, the man charged with the Oklahoma bombing, who was convinced the authorities had planted a microchip in his buttocks. Christopher Clunis, the paranoid schizophrenic who murdered Jon Zito on the Tube, was haunted by voices that persuaded him the world was against him. Then there are the less threatening sufferers of paranoia, the drug-users and old people who believe that the neighbours are drilling through their walls to spy on them or, like one elderly relative of mine, those who can't watch television because they're sure the people on it can see them. (She knows that when the newsreaders turn and talk to each other at the end, they're whispering about her.)

At the other end of the spectrum, there are the pretty much normal people who turn everyday situations into painful anxieties. According to the ''paranoia scales'' used by psychologists, few people ever score zero, if only because our fears are based on what's called "rational paranoia". After all, it would be a fool who wouldn't be frightened to walk the streets of Toxteth estate alone at midnight.

Who are the most vulnerable? People living and working alone may become more paranoid because they've got no one around to laugh them out of their fears. Some paranoics compound this loneliness by alienating people who pooh-pooh their fears, often then including them in the paranoid plot. Then there are those whose parents never made them feel safe and who live their lives as if they were in a glasshouse, constantly on the alert for slights or cunning plots against them.

What is it that paranoics fear? Some have irrational fears about guilty secrets being exposed - like excessive masturbation or the fact that once, years ago, they were caught shop-lifting. Other common fears are forces outside the paranoic's control. MI5 is a common focus, along with the KGB, television, computers or radios. Paranoics often believe spies have planted listening devices in their flats or that they've been singled out by agents of a foreign power. "Anything that is strangely powerful of a very transcending kind tends to attract their interest." says Dr Chadwick. "So a lot of paranoid people are extremely superstitious, they often have interests in all kinds of occult ideas, ouija boards, pendulums and so on."

Paranoics are on the constant lookout for confirmatory evidence of their fears. Headlines in newspapers speak directly to them; the labels of cans in supermarkets spell out special messages; anyone staring at them, even absent-mindedly, becomes the agent of a malign force. "Random pieces of information coalesce in your mind into a different model of reality," says Dr Chadwick.

He himself suffered a paranoid breakdown. It started with a few people finding out about a part of his life that caused him excessive and needless shame, and from the knowledge that a few people were talking about him, he came to believe there was an organisation out to discredit, destroy and humiliate him. Everything around him confirmed his worst fears. He even misheard someone saying "pieces of chewing gum" as "Peter Chadwick". "It was like a gauntlet of knives every day as I walked the streets," he says.

While paranoia, even of the mildest kind, can cause torture for the people suffering it, paranoics, amazingly, have their plus points ."Paranoia is quite a sophisticated mental process, maybe unique to humans," explains Dr Chadwick. "It is not at all a sign of stupidity. It requires anticipation, imagination, empathy and often quite sophisticated inferences and second- guessing. Paranoid people are often highly intelligent and can rise to high places in our society."

Private detectives and social security fraud investigators use their cunning and empathy to make a decent living. Many novelists channel their fantasies into elaborate plots of fear and intrigue, rather than sit at home paralysed by those fears. But the activities of such gifted paranoics become truly frightening when their paranoia invades a whole society. Hitler managed to tap into the paranoia of a whole generation, as did McCarthy with his witch-hunts, and the murderously paranoid Stalin who induced paranoia into the whole of Russia. Indeed, the Cold War is considered by some as the politics of paranoia.

I once spent an evening in the company of a dissident refugee from a country in the eastern bloc. He talked persistently about people following him, about contracts out for his death, about mysterious phone-calls. "Oh dear me," I said, kindly. "How very worrying!" - though inside I was wondering when the men in white coats were coming to get him.

To my horror, he was killed by a hired assassin the following month.

This is a rare case; all too often the fantasies of paranoics are the result of their own tormented reality. Paranoia affects not just the patients of Broadmoor; those of us who feel our office colleagues are out to get us or that our darkest secrets are about to be revealed can be victims too.