Why are male testosterone levels falling?
As someone who's studied this quite a bit, I believe the answer is actually rather counterintuitive, and it is: changing culture.While most people assume the direction of causality goes from testosterone to personality (higher testosterone leads to higher aggression), plenty of evidence shows it also goes in the opposite direction -- adopting aggressive/dominant attitudes, strength training and boxing, etc. leads to higher testosterone.For my grandfather's generation, getting into fights (as adult men) was a thing that happened not infrequently. Having your manhood questioned was something you remained vigilant against. And yes, your handshake was firm because it showed dominance, the same way throwing around insults was a lot of the conversation. And it obviously went along with all of the sexism, homophobia, racism, etc.But culture today is different. You work in teams, you get along, you're open to diverse points of view. You're rewarded for collaboration, not dominance. With most people you shake hands with, a firm handshake is going to freak them out, not assert confidence -- "what the hell is his deal?" Throwing around insults as the basis of conversation isn't how conversation works anymore.So I'm actually pretty convinced it's changing culture that's leading to lower testosterone. And I see no problem with this either -- there seems to be a widely shared belief that this is a problem because it will somehow lead to more difficulty conceiving, lower reproductive success, etc. But there's zero evidence that men as a whole are having problems conceiving. And even if it did become a problem (which there are no signs of), it's obviously something evolution would fix real quick.