Study after study over the past 20 years has indicated that male fertility is declining in industrialized societies, but doctors are still not sure.

The latest study shows that the same phenomenon is occurring among men in Paris. Researchers at the South Paris University discovered that the quality of sperm has declined over a 20 year period until 1992, while testicular cancer has risen over the same time.

As the men grew older, so the quality of their sperm declined, indicating that the cause could be environmental and possibly linked to the water supply, say the French researchers.

Their findings, reported in the New England Journal of Medicine (February 2, 1995), endorse a number of other papers which produced similar results.

Despite this, some doctors remain unconvinced. Writing a peer review paper on the findings, Dr Richard Sherins, from the Genetics and IVF Clinic in Fairfax, Virginia, says the sample for the study was unrepresentative as the participants were sperm donors, attended infertility clinics or were about to undergo a vasectomy.

A 10 year study into the quality of semen among potential sperm donors in Wisconsin showed no significant change, said Dr Sherins, although he does not say if these men were from rural areas.

However, nationwide surveys in the US indicate that rates of infertility have remained constant at around 8 to 11 per cent for the past three decades.

Dr Sherins found his results surprising because he expected an increase in infertility to reflect the recent trend toward later motherhood, when women are thought to be less fecund.