THERE IS a sinister link between the American presidential candidates and 25 per cent of the world's top tennis players: they are all left-handed. Is it a help or a handicap to be among the 10 per cent of the population that is left- handed? Scientists can't agree.

Left-handers certainly suffer a level of insult, built into the language, that if they were a racial minority would overwhelm any equality commission. 'Sinister' (Latin for left-handed) is just one of the words with a negative spin; 'lefties' also get a bad press. The prejudice is not confined to the English language: in Russia, lyeva means black marketeer; the Maoris of New Zealand believe death and misery come from the left side.

Living with the language is one thing; having its negative connotations lent credibility by the majesty of science must make left- handers feel as cheerful as a hypochondriac reading a medical dictionary. But that is just what Stanley Coren, Professor of Psychology at the University of British Columbia, has done in a new book, The Left-Hander Syndrome.

According to Professor Coren, a proportion of left-handers are brain damaged. He is undecided whether it is about half or virtually all of them, but for these unfortunates the news is not good. They are more likely to be criminals, schizophrenics and alcoholics, and may have learning difficulties. They are more vulnerable to hay fever and asthma and to allergies in general (11 1/2 times more likely to be sufferers, according to one study). They are more likely to suffer from eczema and diabetes. They are also more likely to be men. They are more prone to depression and suicide, and are anyway likely to die sooner than their right-handed fellows - 10 years sooner in the case of men and four for women.

The finding that many groups of people with problems have more than the expected proportion of left-handers has been known for a long time: a classic study in 1921 showed that while 7 per cent of children in normal schools were left-handed, in schools for mentally retarded children left-handers made up 17 per cent. What is new is classifying left-handers as suffering from a definite syndrome with a clear cause and a devastating range of effects.

Professor Coren's theory begins with the observation that the powerful bias to the right is unique to humans. Among all other animals, including chimpanzees, individuals favouring either the left or the right are split about 50-50. This right-side preference may be connected with something else unique to humans: language, controlled by the left side of the brain - which runs the right side of the body.

Professor Coren discounts the view that handedness is simply genetic. He says his studies of several thousand families showed no clear genetic base. Identical twins, for instance, can have different handedness. Instead, he suggests it is brain damage at birth that turns people who would normally be right-handed into left-handers.

The switch happens because there is a design flaw in the left side of the brain that makes it more vulnerable to damage if the blood supply to it is temporarily interrupted. Stroke victims, for instance, are four times more likely to suffer paralysis on the right side of the body (controlled by the left brain). When this happens in the womb or during a difficult labour, he argues, the right side of the brain becomes the dominant one and you get a left-handed baby. Premature babies are five times more likely to become left-handers, while mothers aged more than 40 are 120 per cent more likely to have a left-handed child.

Professor Coren says left-handedness is associated with a range of problems, and using our hands involves at least 23 different centres in the brain. Damage to any one of them can cause a switch from left- to right-side control.

Once born, brain-damaged left- handers move clumsily into a right-biased world where corkscrews, scissors and potato peelers do not work and where they are much more likely than right-handers to suffer a range of problems: insomnia (more likely by a ratio of 2:1), cross-eyedness (2:1), deafness (5:2), alcoholism (4:1), dyslexia (11:1) and depression and suicide (3:1).

When asked if, in view of this unflattering portrait, there were any plans for a Left-Handers' Liberation Front , a spokesperson for the Left-Handed Society replied that its latest big event was a party to which the former boxer Henry Cooper was invited.

He pointed to famous and successful left-handers such as Picasso, Charlie Chaplin, John McEnroe and Jimmy Connors, and even wheeled out Gerald Ford and Ronald Reagan. But he was more assertive about the likelihood of his dying earlier than a right-hander, quoting an official of the American Academy of Actuaries, which calculates insurance risks, as saying: 'This research would never have passed the muster of an actuary.'

A more feisty champion in the left corner is Dr Marion Annett, lecturer in clinical psychology at Leicester University, who has been working in the field for 30 years. When some of the evidence, such as the very close connection with dyslexia, was put to her, she simply replied: 'I don't believe it.'

In a recent study she found that far from being disadvantaged, left-handers are more likely to turn in outstanding performances in their chosen fields. Her findings suggest that there is a greater than normal proportion of left-handers among very bright children and that left-handers are likely to have better all-round ability. She suggests that children who are strongly right-handed have the greatest learning problems.

Dr Annett is scathingly dismissive of Professor Coren's elaborately constructed portrait of the brain-damaged left-hander. 'The evidence for a connection between all these problem groups and left-handers is very weak. The trouble with these studies is that one person finds a big connection and two find nothing, but it's the big one that is remembered.'

Despite her findings that left- handers may be more able than right-handers, Dr Annett's overall view is that if you look at normal school populations the most remarkable finding is that there are no significant differences between left- and right-handers. 'What differences there are, are quite hard to tease out,' she says.

Also contrary to Professor Coren, she says there is a genetic element to left-handedness. She believes that while most people inherit a bias to the left side of the brain, controlling both speech and hand movements, 20 per cent do not. They may end up either left- or right-handed.

Support for the superior-lefty view also comes from research published by the Centre for Left- Handed Studies two months ago. In a survey of more than 1,800 Manchester pupils, it found a small number of very bright left- handers and a greater connection between dyslexia and right- handers.

Diane Paul, director of the centre and herself a left-hander, says she has found another difference between left- and right-handers: left- handers relate more to concrete than abstract concepts, and are likely to be more visually than verbally orientated: 'They work problems out via a different route, going all round the houses to arrive at the correct answer.'

Right-handers seem to have the edge, but the matter is far from resolved. When President George Bush stood against Michael Dukakis, psychologists guessed - rightly - that the taller man would win. The left-handedness of both candidates this time allows no such predictions.

'The Left-Hander Syndrome; The Causes and Consequences of Left-Handedness', by Stanley Coren, is published by John Murray, pounds 16.95.