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A hormone is a chemical messenger released by secretory cells (including neurons) in various glands.
Once released, it enters the bloodstream, which can affect all cells in the body that have receptors for it.
Hormones do not determine, command, cause or invent behavior.
Instead, they make us more sensitive to the social triggers of emotionally charged behaviors and reinforce our pre-existing tendencies in these areas.
Neurotransmitters directly affect only the neurons on the other side of the synapses, while a hormone can potentially affect any of the trillions of cells in the body.
Neurotransmitters that signal hormonal effects occur over hours to days and can last forever (because synapses are bridged in milliseconds. In contrast, puberty often disappears after some time).
Testosterone has far less to do with aggression than most assume
Within the normal range, individual differences in testosterone levels do not predict who will be aggressive.
The more an organism has been aggressive, the less testosterone is needed for future aggression.
If testosterone plays a role, it is a facilitating one — testosterone does not “invent” aggression.
It makes us more sensitive to aggression triggers, especially those most prone to aggression.
