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THE PROCESS OF HUMAN CONCEPTION is almost absurdly inefficient. During copulation, a man expels tens of millions of sperm, with considerable force, into his partner's vaginal canal. Despite the head start, most of the tiny, tadpole-shape, self-propelled cells never come within shouting distance of the woman's egg, floating deep inside the Fallopian tube. And if one does finally complete the journey, it may or may not have the energy left for fertilization.

With these abysmal odds, a man clearly needs every last sperm cell he's got. Any fewer than 20 million or so per ml of semen--40 million to 120 million in a typical ejaculation--and his chances of fathering a child begin to plummet. That's why doctors are so concerned about a trend they have noticed over the past few years. In study after study, sperm counts in men the world over seem to be dropping precipitously.

The latest appeared last month in the British Medical Journal. Researchers in Edinburgh, Scotland, reported that men born after 1970 had a sperm count 25% lower than those born before l959--an average decline of 2.1% a year. A 1995 study of Parisians also found a 2.1% annual decline over the past 20 years. And in the most comprehensive analysis of all, covering nearly 15,000 men from 21 countries, Danish scientists discovered an alarming plunge of nearly 50% in average sperm counts over the past half-century.

None of these studies are without their critics, and a handful of others show either no decline or some localized increase. There are further indications, however, that something disturbing is going on. Not only do sperm counts seem to be dropping, but the quality of sperm--the percentage of healthy, vigorous cells versus malformed, sluggish ones--appears to be in serious decline as well. Doctors have also noted an increase in the incidence of testicular cancer and undescended testicles.

Together, these factors add up to a significant drop in male fertility. In the 1960s, says Dr. Masood Khatamee of New York City's Fertility Research Foundation, only about 8% of the men who came for consultation had a fertility problem. Today that number is up to 40%. "This concerns us a great deal," he says, "and that's why we're so adamant about finding the causes."

Just what these causes might be is still largely a mystery. Stress, smoking and drug use are all known to be involved. So is the fact that men are having children later in life, when sperm counts naturally fall off, as well as the increase in sexually transmitted diseases. Even the shift in underwear fashion from boxers to briefs has been offered as an explanation.

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