Neuroticism and cortisol: Pinning down an expected effect

• High Neuroticism (N) scorers should have higher prevailing daily cortisol (C) levels.
• Inconsistent evidence and methodological limitations have vitiated firm conclusions.
• In this rigorous study, high N group had ~ 20% higher daily cortisol levels than low N.
• Interestingly no group differences were evident in the first 45 min post-awakening.

BackgroundThere are strong theoretical arguments that those high on Neuroticism (N) should normally exhibit higher prevailing levels of the stress-linked hormone cortisol (C), but findings are inconsistent, probably reflecting methodological weaknesses especially in taking account of C's diurnal cycle.MethodsHigh and low N students [Total N = 118; mean age = 20.99 years] were recruited and their salivary cortisol measured, ensuring that saliva samples were numerically adequate to assess C's diurnal cycle over two days with objective verification of sample timing.ResultsCortisol secretion was approximately 20% higher in High N than low N participants in the period of 12 h after awakening (p < .008), but no differences in secretion were evident during the first 0.75 of this period, when typically the Cortisol Awakening Response (CAR) rapidly takes cortisol to its daily peak. N effects were thus confined to the 0.75 h–12 h period (p < .007). Males had approximately 25% higher cortisol secretion levels than females, also confined to the 0.75 h–12 h period (p < .003). No significant differences between N groups were evident for dynamic measures of cortisol change, viz. the magnitude of CAR rise and subsequent diurnal fall. All effects were controlled for cohort date of study entry, age, smoking status, study day and time of awakening.DiscussionWith careful control, it appears that an important theoretically predicted effect exists, and is replicated in different student intake cohorts recruited in different years. Most importantly, findings support several lines of evidence that the period of massive rise in the brief 0–0.75 h CAR period should be seen as quite separate from the rest of the diurnal cycle, underpinned by different control mechanisms, and with potentially different correlates.