It could just be that a factor which increases testosterone has been steadily disappearing. For example, testosterone is elevated in males after winning a figh

It could just be that a factor which increases testosterone has been steadily disappearing. For example, testosterone is elevated in males after winning a fight. So if physical confrontations among males are getting less frequent for some reason (which seems like a good thing to me), then that could lead to lower testosterone levels. Another possible explanation is that we've had good birth control for a few generations now, which has eliminated the advantage of high sex drive on a males' chances of fathering children. So there is no longer a selection effect for high testosterone males. (Also seems like a good thing.) Anyway, my point is not that the above explanations are correct; just that there might be benign explanations.