The current understanding of testosterone is not so much that it causes aggression, but that aggression (and other circumstances) raise testosterone. Check out this terrific essay (or the book that it references- Testosterone Rex) for more on that.
The truth is, as Cordelia Fine puts it in her book Testosterone Rex: Myths of Sex, Science, and Society, “Although we’re used to thinking of certain kinds of behaviors as ‘testosterone fueled,’ in many cases it would make more sense to instead think of actions and situations as being ‘testosterone fueling.’”
To understand what Fine is saying, just look at the research. It has consistently shown that testosterone cannot predict competitive aggression unless the levels are extremely high or low.
As Brown University anthropologist Matthew Gutmann puts it, one would have to be either castrated or “a gym rat on steroids” before testosterone levels began to correlate with the inclination to pick a fight (Gutmann). In other words, there’s no good boys-will-be-boys biological explanation for playground bullying.
