Instead of asking "What do individual levels of testosterone have to do with aggression?", he posits a better question: "Does difference in testosterone levels among individuals predict who will be aggressive?"
The answer: Among birds, fish, mammals, and especially other primates, "generally no". As for humans, it has been extensively studied, and the jury is in (Sapolsky is quoting a British endocrinologist, John Archer): "There is a weak and inconsistent association between testosterone levels and aggression in [human adults], and ... administration of testosterone to volunteers typically does not increase their aggression."
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The executive summary being testosterone's effects are hugely context dependent—this in turn means "rather than causing X, testosterone amplifies the power of something else to cause X."