In 1992, a controversial paper in the British Medical Journal suggested that sperm counts were decreasing worldwide. Since then, the issue has been hotly debated, and recent research has pointed to geographic differences rather than a uniform, global decline. However, critics say this work has been based on unrepresentative subgroups, such as prevasectomy patients, from urban centers.

To investigate sperm quality in a broader sample of men, epidemiologist Shanna Swan of the University of Missouri, Columbia, and colleagues examined semen from 512 men from New York, Los Angeles, Minneapolis, and Columbia. All of the men were fertile--as evidenced by their ability to conceive a pregnancy within the preceding 34 weeks. But men from the three cities had higher sperm counts--ranging from 38% to 75% higher--than men from Columbia, the team reports in the 11 November online edition of Environmental Health Perspectives. Moreover, the total number of motile sperm was lower in Missouri compared to the urban areas--almost 50% lower than in Los Angeles participants. The researchers found that only 1.1% of mid-Missouri men met the minimum criteria--based on sperm count, shape, and motility--to be considered definitely "fertile." In contrast, 8.5% of urban men passed muster. Swan says this indicates reduced fertility in Missouri but not infertility.

"It may be harder for people at the low end of the spectrum to get [someone] pregnant," she says. The researchers say that their experiments rule out many possible causes--such as age, smoking, body weight, sexually transmitted diseases, history of infertility, recent fevers, and education--and propose that exposure to agricultural chemicals might be to blame.

That's a reasonable hypothesis, says Harry Fisch, a fertility researcher at Columbia Presbyterian Medical Center in New York City, but other factors should be considered as well--including local genetic differences. He adds that the research supports his previous findings that semen quality varies with geography and argues against a uniform drop-off: "The concept of a worldwide decline in sperm counts must be reevaluated."

--VIRGINIA GEWIN