Showing that testosterone causes aggression requires a “subtraction” plus a “replacement” experiment. Subtraction—castrate a male. Do levels of aggression decrease? Yes (including in humans). This shows that something coming from the testes causes aggression. Is it testosterone? Replacement—give that castrated individual replacement testosterone. Do precastration levels of aggression return? Yes (including in humans). Thus, testosterone causes aggression. Time to see how wrong that is.
