The analysis of cortisol in human hair constitutes a promising method for the retrospective assessment of cumulative cortisol secretion over extended periods of time. An implicit assumption underlying the use of this method is that in the absence of major life changes hair cortisol concentrations show a high level of intraindividual stability, i.e. single hair cortisol assessments exhibit considerable trait-specificity and are only to a smaller extent influenced by state-dependent factors. Here, we present data from two independent studies examining patterns of intraindividual stability in hair cortisol levels. In study I, 45 participants were examined at two sampling points carried out one year apart from each other. In study II, 64 individuals provided data at three sampling points which occurred at two-month intervals. In both studies, at each time point hair was sampled and relevant psychosocial and hair-related variables were assessed. Results of both studies consistently revealed strong test-retest associations for repeated hair cortisol measurements ('r's between 0.68 and 0.79, 'p's <0.0001). Findings of structural equation modelling applied to data of study II showed that single hair cortisol assessments comprise a strong trait component, explaining between 59 and 82% of variance, and are only to a lesser extent influenced by state-related factors. Only inconsistent evidence for covariation of changes in hair cortisol concentrations and simultaneous changes in perceived stress or other relevant variables was seen across the two studies. The current findings suggest a considerable degree of intraindividual stability in hair cortisol levels which highlights the utility of this method for obtaining trait estimates of long-term cortisol secretion in psychoneuroendocrinological research.
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