Hair cortisol concentration and adolescent mental health: Insight from the Queensland Twin Adolescent Brain Project

INTRODUCTION: Adolescence is a stress-sensitive period for neurodevelopment and mental health, with chronic stress implicated in the onset of psychological disorders. Hair cortisol concentration (HCC) serves as a non-invasive biomarker of long-term hypothalamic-pituitary-adrenal (HPA) axis activity, yet its relevance to adolescent mental health remains inconsistently characterised. METHODS: This longitudinal study examined HCC in 302 community-dwelling adolescent twins from Brisbane, Australia, with data collected at two sessions approximately two years apart, following a standardised assessment protocol. Three cm long hair samples were analysed to quantify cumulative stress exposure over three months, and participants completed self-reported measures of depression, anxiety, daily stress, social support, and adverse childhood experiences (ACEs). Linear mixed-effects models and quantile regression were used to examine mean-level and distributional associations between HCC and psychological and environmental variables. RESULTS: Average HCC decreased significantly between sessions, with no main effect of sex, twin zygosity, or pubertal stage. In males, a higher average HCC at the second session was associated with elevated general anxiety, whereas in females, a higher average HCC was linked to higher exposure to severe lifetime stress. No associations were found between average HCC and ACEs. CONCLUSION: These findings suggest that average HCC, reflecting cumulative cortisol secretion over the three months before each assessment, provides a stable measure of long-term cortisol in adolescents, although its associations with psychosocial stressors were limited in this cohort. Rather than functioning as a broadly sensitive biomarker of chronic stress, HCC may capture specific stress-related processes in certain subgroups, and its utility may depend on the type, timing, and chronicity of stress exposure.