Serotonin is a neurotransmitter conserved through at least 500 million years of nervous system evolution. Serotonin orchestrates adaptive responses to aversive stimuli in invertebrates and an analogous role can be discerned in the more complex behavioural repertoire displayed by mammals to adversity. However, this formulation fails to capture the range of human social behaviours influenced by serotonin, for example, affiliation, empathy and cooperation. In a psychopharmacology experiment I received paroxetine for three weeks. This boost in brain serotonin levels failed to alter my subjective ratings of mood and anxiety. My wife felt differently; ‘Can't you stay on it?’, she said.