Sperm counts among American men have dropped nearly 60% in the past 40 years, according to a new report in Human Reproduction Update.

In the study, researchers looked at the concentration of guys’ sperm in Western countries from 1973 to 2011. Among North America, Europe, Australia, and New Zealand, sperm concentration has dropped by an average of 1.4% each year since the '70s, the researchers found.

Of course, a low sperm count doesn't necessarily mean you're infertile, but it does decrease your chances of conceiving—and, the researchers note, these numbers could significantly impact fertility rates in the future.

Here’s the thing: The researchers aren't sure exactly why guys have less swimmers than ever before. Factors like improved sperm measurements and undocumented pre-existing conditions may come into play with such a dramatic drop, they suggest. And a number of natural, biological factors can affect male fertility, like blockages in the male reproductive tract, says Daniel Williams, M.D., associate professor of urology at the University of Wisconsin.

But even then, lifestyle changes among American men over the past four decades might be the cause of lower sperm numbers.

Here are five possible reasons why guys are less fertile than ever before.