This review highlights the utility of using concepts of consistent behavioural patterns (CBPs) (temperament in healthy individuals; and psychopathology of mental illness) in conjunction with situational contextuality. We also point to multiple roles of central serotonin (5-hydroxytryptamine, 5-HT) as a biomarker of CBPs. We use empirical demonstrations of the complex and trait-dependent effects of manipulating 5-HT via acute dietary tryptophan depletion (ATD) to highlight the relevance of these constructs in the neurochemical control of behaviour. Neurochemical and temperament trait interactions, for example, 5-HT and empathy, psychopathy, neuroticism, impulsivity, and hyperthymia are postulated to underlie this complexity.