A study by the Institute de Veille Sanitaire in France, released on Thursday, found that for the years 1989 through 2005, the sperm count in one millilitre of an average 35-year-old Frenchman dropped to 50 million from 74 million.
The 32 per cent decline in sperm count is attributed to various factors, including the wearing of tight underwear.
Joelle Le Moal, who led the research, based the team's findings on 26,000 semen samples from French males within the 16-year period. These samples were from men with infertile female partners, which reduces the risk that fertility problem is on the male side. It is also probably the largest studied sample size in the world.
The study, published in the Human Reproduction journal in Europe, also pointed to environmental pollution and lifestyle as other factors behind the declining male fertility among Frenchmen.
The 50 million average is slightly below the 55 million sperm per milliliter considered the threshold of sperm concentration likely to have influence on the time it takes for a woman to conceive. At 15 million, it is considered a threshold of infertility, according to World Health Organisation benchmark.
While the 50 million is still within the normal range, if the decline continues, French males are in danger of moving down to the infertile range, pointed out Grace Centola, president of the Society for Male Reproduction and Urology in Birmingham, Alabama.
The study said the phenomenon is not unique to France but also is happening to men in Israel, India, New Zealand and Tunisia, Ms Le Moal said, citing other recent researches.
Ms Centola said similar results were discovered in a study involving young sperm donors in Boston.
Besides the decline in sperm count, the same French study discovered an increase in number of abnormally shaped sperm, which is also a factor in fertility.
The study will likely revive the old debate among the preferred underwear of males - the brief or the boxer.