I linked that study because it is particularly interesting because they run the mass spec test on archived blood samples.

But there are other studies and meta analyses which cover other countries and come to the same conclusion.

I think many of the studies claiming to find significant population level decline are older and overstate the issue due to the methodological errors outlined in my previous post. If you are thinking of a particular one, please share a link.

I am not familiar with the research on testosterone levels in animals. In humans, while not conclusory, I do think the evidence suggests that increasing rates of obesity and metabolic syndrome are the proximate cause of testosterone decline.