Lack of an out group, social media, and declining testosterone among men are other candidate explanations that I find plausible
Even then, it’s not clear that testosterone levels are correlated with any specific behaviors or that the correlation goes the way we intuitively think it does. The anger and aggression can actually cause people to be more sensitive and react disproportionately to any perceived harm. See also roid rage, the prevalence of steroid use among police officers, and their propensity to provoke confrontation.
"because men (and women, for that matter) are doing less exercise period."
"Even then, it’s not clear that testosterone levels are correlated with any specific behaviors or that the correlation goes the way we intuitively think it does. The anger and aggression can actually cause people to be more sensitive and react disproportionately to any perceived harm."