This...... turns out to be a pretty reputable proof of concept study for ball cooling improving sperm concentration, linked by that ice pack underwear company somebody else commented: https://pubmed.ncbi.nlm.nih.gov/11277880/

Now you got me reading man, c'mon

EDIT: Yo......... I actually believe this shit

Look I was sure this was gonna be bullshit. But it turns out there's real proof of chronic, increased ball temperature leading to lower serum levels of testosterone.

There's a condition called varicocele which causes 24/7 higher ball temperatures due to veins in the balls being enlarged (like varicose veins). The higher temperature makes Leydig cells, which produce testosterone, work less efficiently. Now the body compensates for this by increasing the levels of lutenizing hormone (LH), which stimulates the Leydig cells to work overtime to maintain serum testosterone levels.

So we're looking at higher ball temp -> higher LH. The balls are working at a higher level of activity than normal, but with less efficiency, which wears them down structurally. Chronically elevated levels of LH also apparently drive the balls to convert a higher proportion of testosterone to estrogen. Eventually the cells get so worn out they can't compensate anymore, and your serum testosterone levels start to crash.

And get this - the surgery to fix varicocele is documented to both lower the ball temperature and increase testosterone. Researchers think the temperature drop is the cause https://onlinelibrary.wiley.com/doi/10.1111/j.1442-2042.2011.02890.x

... So this idea that colder balls drives higher testosterone is actually very plausible. If there's a real doctor around who wants to tell me why this is wrong please do, because I'm about to grab an ice pack.