Steroids are an extensive family of chemical agents distributed widely in the brain. They include the classical stress hormone cortisol, oestradiol, testosterone and progesterone, aldosterone and dehydroepiandrosterone (DHEA). The hypothalamic-pituitary-adrenal (HPA) axis is the neurochemical system through which the release of cortisol is regulated. Prospective studies of human infants can be carried out delineating both the glucocorticoid genetic variation and early rearing environment and examining both the HPA axis stability and behavioural patterns of function. Research has implicated a role for infant exposure to adverse experiences in the formation of HPA axis sensitivity. Animal studies have reported a substantial non-genetic effect of adverse maternal rearing practices on the development of chemical coding systems for behaviour, including HPA axis, hypothalamic and extra-hypothalamic corticotropin-releasing hormone, monoaminergic and gamma-aminobutyric acid (GABA)/benzodiazepine systems. Children, adolescents and young adults with severe conduct disorders have been shown to have remarkably suppressed cortisol levels compared with controls.