In a chicken shed somewhere near Mannheim, Germany, last month, 3,000 hens were subjected to 14 days of wall-to-wall Mozart. Their eggs were then counted and compared with their usual fortnightly yield to see whether true chicken fulfilment - and hence more eggs - could be found in the Viennese master's music. Now the results have been announced: the fowl labouring to the strains of Mozart's violin concertos produced...exactly the same number of eggs as usual.

An own goal for the Mannheim Mozart festival, which is behind this bizarre experiment? Apparently not: festival-goers were sold the eggs during concert intervals. "And," a festival spokeswoman reported, "people said they definitely tasted better."

The chicken-testing extravaganza is, oddly enough, not uncharacteristic of the quest to find out what miracles Mozart's music can perform. It seems that it is not enough that it can stir the soul - the warmth, wit and power of the symphonies, the aching beauty of the arias, the intimate brilliance of the string quartets. If the many claims flying around are to be believed, this is music that can produce prize marrows, turn babies into boffins, give pets a new personality and cure a nasty bout of flu (all right, I made the last one up).

To start with, there are the assertions that Mozart's work enhances intelligence. The Sonata for Two Pianos in D, K448, may be among Mozart's best-loved pieces for its sparkle and expressive delicacy, but it is also the magic number for the so-called Mozart Effect. K448 was alighted upon by Gordon Shaw and Frances Rauscher, University of California researchers, in the early 1990s for its potential intellect-boosting characteristics (highly organised musical motifs repeated at regular yet lengthy intervals). They played the sonata to a group of college students for 10 minutes and found that after listening, their skills in cutting and folding paper had come on a treat. "Mozart makes you brainier!" boasted the headlines, and a folk belief was born.

Then there is Don Campbell. This self-styled Mozart Effect guru from Colorado spun the findings of the Shaw- Rauscher research into a multi-million-dollar industry of books, recordings and lecture tours, employing an inimitable brand of pop psychology and wide-eyed mysticism. He has even trademarked the phrase "Mozart Effect". "Mozart's music has an exceptional power of organising and clarifying space-time perception," he claims. "But the Mozart Effect is much more than just Mozart. It demands that we look at how auditory stimulation, in the form of music, can improve long-range memory, language development and physical co-ordination as well as reduce stress levels."

Campbell's ideas have been a hit with parents desperate to get their hands on the holy grail of how to increase baby brainpower. His latest book is entitled The Mozart Effect for Children, and sales of his numerous recordings - which include 10 CDs for children, three for babies and one specifically designed for the foetus - have topped the 2m mark. Inevitably, his theories have made their way into educational and health policies. In Tennessee and Georgia, for example, every newborn is given a free Mozart CD.

I asked Campbell why Mozart has the magic touch, but not, say, Haydn, whose music has structural similarities. Campbell puts it all down to image. "He was a child prodigy who died young and yet he wrote a phenomenal amount of music. He has a fascinating story that people can identify with."

Not everyone is convinced. Professor Curtis Price, principal of the Royal Academy of Music in London, is sceptical. "I can't comment on up-to-date research, but a few years ago I attended a conference in Chicago, a large part of which was on the Mozart Effect. And I came away with the distinct impression that the Mozart Effect does not exist. It's a nice idea, but it's a fiction - a product of flawed, pseudo-scientific research."

However, Price is certain that Mozart does have a certain power. "I've just spent two weeks conducting auditions for the Royal Academy, and the music that is the most revealing and the most difficult is Mozart's. It sounds so simple, but it reveals everything about the performer's talent, potential and accomplishment. It's so beautifully crafted that the slightest flaw in performance is noticeable. If you listen to Alfred Brendel, Mitsuko Uchida - the most distinguished performers - this is the music they say they find the most challenging."

Listening to Price enthuse, you would hardly believe that Mozart's music is often relegated to mere background listening as a means of self-improvement. But reports of its uncanny powers just keep cropping up. Welsh science teacher Anne Savan demonstrated in the mid-1990s that when disruptive children were played Mozart symphonies during lessons over a five-month period, their behaviour improved. And last month a South Yorkshire primary school reported that the composer's symphonies had brought about a dramatic improvement when used to accompany maths lessons, leading the head teacher to comment that Mozart reaches parts of the brain that other composers simply don't.

It is not simply schoolchildren who benefit from a spot of Mozart. In 1990, Newsweek reported that officials in the Canadian city of Edmonton piped his string quartets into the city squares to pacify jostling pedestrians; they found that it had the added benefit of deterring drug dealers. In January this year, Britain's First Great Eastern Railways discovered that late-night vandalism and violence at railway stations in Essex was tempered by Mozart. Eine Kleine Nachtmusik, perhaps?

And the Mannheim chicken-testers are not alone. In 1996, a study by a German agricultural federation and Purdue University, Indiana, concluded that cows treated to Mozart melodies produced substantially more milk. Dogs in a Belfast shelter relaxed and barked less when played Mozart, according to research by Queen's University, Belfast, and the National Canine Defence League.

Gratifying though it is to discover that dogs and even livestock have a taste for the finer things in life, someone somewhere is missing the point. This is the composer of whom Schubert wrote in 1816: "O Mozart...How infinitely many inspiring suggestions of a finer, better life have you left in our souls!" Mozart wrote wonderful music: why don't we just sit back and enjoy listening to it?