Men’s spunk may be getting noticeably less spunky in some high-income countries, according to a meta-analysis of international swimmers.

Skimming and re-examining sperm data from 185 past independent studies, researchers estimated that sperm counts of men from select high-income regions—North America, Australia, New Zealand, and Europe—dropped about 52 percent between 1973 to 2011, from 99 million sperm per milliliter to about 47 million per milliliter. Likewise, estimates of total sperm count per batch dropped 59 percent, from 337.5 million in 1973 to 137.5 million in 2011.

The researchers, led by Hagai Levine of Hebrew University, also looked at data from what they referred to as “other” countries, including some in South America, Asia and Africa. They saw no trends in these places, but they also had relatively little data from them.