Hair cortisol concentrations in mental disorders: A systematic review

•Hair cortisol was elevated in drug-naïve first-episode patients with psychosis, compared to matched controls; the differences seem to associate with the severity of psychopathology.•Results concerning HCC in people with depression or anxiety disorders are rather inconsistent.•PTSD patients show significantly lower HCC than trauma-exposed controls without PTSD.•The timing of cortisol sampling seems to be vital for explaining the differences of the indicated results.•Associations of hair cortisol concentrations with self-reported stress measures are inconclusive.