A 2017 report published in the journal Human Reproduction Update reviewed the results of over 2000 papers on sperm counts previously published (in English) between 1981 and 2013.

This comprehensive meta-regression analysis reports a significant decline in sperm counts (as measured by SC and TSC) between 1973 and 2011, driven by a 50–60% decline among men unselected by fertility from North America, Europe, Australia and New Zealand. Because of the significant public health implications of these results, research on the causes of this continuing decline is urgently needed."

See: Temporal trends in sperm count: a systematic review and meta-regression analysis.

Since then, several other studies have confirmed the decline.

As stated above, the causes for the apparent dramatic decline are unknown - theories include links to body weight, a lack of physical activity, and smoking etc.

Also under review are possible effects of a diet with high percentages of ultra-processed foods (ref.) and persistent low-level chemical contaminants in the environment (ref.)