Nevertheless, for some reason, human male sperm counts have been declining for decades, at least in developed nations. Concurrently, the testicular cancer rate has doubled. The exact cause is unknown, but environmental toxins seems to make the the most sense.
Actually I've seen research that suggests (but hasn't proven) the real culprit for sperm count issues, decreased motility (sperm propulsion or "swimming"), and testicular cancer may be modern male clothing styles. In the form of tight underwear and snug pants. As well as a sedentary sitting lifestyle, things that raise the scrotal temperature, compared to free-balling as Nature intended. Or these may be contributing factors, that could indeed include environmental causes, as well.
So up boxers, and down balls!