Testosterone is an androgen which produces male characteristics, although it is present in both males and females and is linked to aggression. It is suggested that testosterone acts with androgen or oestrogen receptors in the brain, affecting the neural transmission and the amount of neurotransmitters that are released at the serotonergic synapses. It appears to lower the amount of serotonin that is available, thereby leaving people less able to control their aggressive responses to situations and people.