AS HEALTH scares go, the idea that sperm counts are plummeting across the industrialised world, probably as a result of chemical pollution that has an adverse hormonal effect, takes some beating. In 1992 a meta-analysis of 61 papers, published in the British Medical Journal, suggested they had fallen by half in the preceding half-century, from 113m per millilitre of semen to 66m. Since then, the decline has apparently continued. The most recent paper, just published in Human Reproduction, by Joëlle le Moal, Matthieu Rolland and their colleagues at France’s Institute for Public Health Surveillance, is also one of the most comprehensive yet.

Its conclusions are stark. The sperm count of the average Frenchman, say the researchers, fell by 32.2% between 1989 and 2005. At the same time, the proportion of properly formed sperm also fell, from 60.9% to 52.8%.

This paper is an important contribution to a lively debate. For although the idea of falling sperm counts has entered the public mind as an established fact, fertility experts remain divided about just how big the effect really is. Not all studies have found drops. Though one of Parisians in 1995 suggested that counts were indeed falling, by about 2.1% a year, another, carried out in Toulouse, suggested that they weren’t.

The new study is bigger than most (it involved more than 26,000 men, drawn from 126 fertility clinics all over France), and is notable for its attention to detail. The researchers examined only men whose partners were being treated for infertility—a cunning way around one problem with sperm studies, which is that, because it is difficult to recruit from the general population, researchers often have to rely on couples attending fertility clinics, and are thus dealing with men who, by definition, are much more likely to have low sperm counts.

Even so, not everyone is convinced. Allan Pacey, an andrologist at the University of Sheffield, in England, thinks much of the effect could be caused by changes in how sperm quality is measured. He points out that the World Health Organisation’s manual on the subject has been revised four times since it was first published in 1980, and that quality control at fertility clinics has improved dramatically over the same period.

There is support for his position from other studies. One, a continuing investigation of Danish conscripts, is notable for having been established with a consistent method of measurement from the start. Its data show no changes over the years. And Dr Rolland and Dr le Moal admit that, despite the apparent drop in sperm counts they found, there was no increase in the number of infertile men during the period of their study.

All this is not to deny that there may be a problem. Even if methodological changes account for some of the observed drop in semen quality, they may not account for all of it. Dr Pacey points out that there is separate evidence suggesting the incidences of testicular cancer, and of genital abnormalities in boys, are both rising. That is consistent with the idea that something out there actually is monkeying with masculinity.