The researchers found that according to three of these criteria the quality of U.S. sperm had fallen significantly. Average concentration, motility, and total motile sperm count all fell over the 16-year study period, apart from in Spokane, which was discounted due to a lack of donors. Indianapolis showed a decrease in concentration and total motile sperm but an increase in motility. Ejaculate volume did not decrease significantly.

The research was presented by co-author Chelsea Canon, a fertility expert at the Icahn School of Medicine at Mount Sinai in New York City, at the American Society for Reproductive Medicine conference in Baltimore last week.

According to the researchers, it’s not clear what caused the sperm quality decline but it’s noted that environmental and lifestyle factors have been implicated in sperm decline before.