Sperm counts among Western men are in sharp decline, and have decreased by more than half since the 1970s, a new study suggests. Although prior research has, variably, both supported and cast doubt on the notion of a global sperm decline, this literature review in Human Reproduction Update is perhaps the largest analysis of the issue to date. The authors conclude that men in North America, Europe, Australia and New Zealand may face a significant problem with their swimmers, whereas men in South America, Asia, and Africa are just fine.

“Our study is the largest, most definitely, and the most rigorous of any study on sperm decline that’s been done,” coauthor on the study Dr. Shanna Swan of the Icahn School of Medicine at Mount Sinai told Fatherly.

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Although men have always worried about their virility, the first modern study to sound the alarm about the global sperm decline was a literature review published in 1992, which analyzed 61 studies. Swan and her colleagues have updated these findings, more than tripling the amount of studies in their analysis. In total, they looked at 185 studies conducted between 1973 and 2011. They found that men in Western countries have experienced a 52.4 percent decline in sperm concentration and a 59.3 percent decline in total sperm count.

As for past studies that indicate sperm quality is not declining, Swan makes a distinction between global versus local sperm decline. It may be the case that individual populations are not experiencing a measurable decline in sperm count or concentration while, globally, sperm counts plummet. “It certainly can be that there’s a global decline and that locally there’s a no decline or an increase,” she says. “That’s not in conflict.”

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While the study didn’t look at why sperm counts and concentrations have taken a nose dive since the 1970’s, Swan suspects that stress, diet, obesity, chemical exposure (like phthalates), or crowding unique to Western men may play a role. “Animals living in closer quarters produce less offspring,” she says.

But it’s hard to make the argument that Western men live in more crowded conditions than men living within major metropolitan areas in China or India. So Leigh Simmons, Editor-in-Chief of the journal Behavioral Ecology (who was not involved in the study) has another theory—namely that monogamy, and lack of the “sperm competition” that has been observed among polygamous animal species, is causing our sperm counts to bottom out (and our testicles to shrivel up). “If you look across species, those that have low levels of sperm competition tend to have smaller testes and lower sperm production capacities than species where sperm competition is much stronger,” Simmons told Fatherly. “That’s a very widespread phenomenon.”

Unfortunately, there weren’t enough studies in Eastern countries to pinpoint how men outside of the Western world are dodging sperm decline—or even if they are indeed immune to the problem. That, combined with the fact that several of the studies in the literature review suggest low sperm count correlates to low life expectancy, explain why it’s crucial that future studies focus on bolstering these datasets. “As a society, we have to make an investment to understand what’s driving these trends so that we can help get healthy kids,” Swan says. “Right now we’re producing a fair amount of children who will not be able to conceive.”

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