Startling new evidence suggests male infertility may be much worse than it appears. According to Levine and Swan’s work, sperm levels—the most important measurement of male fertility—are declining throughout much of the world, including the U.S. The report, published in late July, reviewed thousands of studies and concluded that sperm concentration had fallen by 52 percent among men in Western countries between 1973 and 2011. Four decades ago, the average Western man had a sperm concentration of 99 million per milliliter. By 2011, that had fallen to 47.1 million. The plummet is alarming because sperm concentrations below 40 million per milliliter are considered below normal and can impair fertility. (The researchers found no significant declines for non-Western men, in part because of a lack of quality data, though other studies have found major drops in countries like China and Japan.) And the decline has grown steeper in recent years, which means that the crisis is deepening. “This is pretty scary,” says Swan, who has long studied reproductive health. “I think we should be very concerned about this trend.”

Although there have reports of declining sperm counts before, they were easy to ignore. Research on sperm levels has been spotty, using different methodologies and drawing from varying groups, making it difficult to know that the declines some scientists observed were real, and not a function of miscounting. Skeptics of the latest conclusions countered that the new report was a study of many studies—it could only be as good as the work from which it drew. And even if the conclusions of the meta-analysis are accurate, the average sperm count still leaves most men on the normal side of fertile. Just barely.