Men's sperm counts are sinking fast -- and have decreased inexplicably by a third over the last twenty years, a new study shows. The study, conducted by the Cochin Hospital Group in Paris, will appear in tomorrow's edition of the New England Journal of Medicine. It reinforces a wave of other research suggesting the quality and quantity of semen is declining among men in industrialized countries. The French study shows the concentration of sperm dropped by 2 percent per year -- from 89 million per milliliter of ejaculate in 1973 to 60 million in 1992. A sperm count of under 20 million per milliliter indicates infertility. The researchers couldn't pinpoint a reason for the decline. "That's the real concern," says Time science writer Christine Gorman. "That there may be a continuing decline and nobody knows why."