Daytime basal cortisol and common childhood psychopathology³

Meike Bartels¹, E.J.C. de Geus¹, C. Kirschbaum², M.J.H. Rietveld¹, and D.I. Boomsma<sup>1

1</sup>Department of Biological Psychology, Vrije Universiteit, Amsterdam, The Netherlands
²Institute of Physiological Psychology II, University of Düsseldorf, Düsseldorf, Germany

Individual variation in cortisol levels plays a prominent role as a possible explanatory variable in studies on childhood psychopathology. For instance, high and low basal cortisol levels have been associated with externalizing and internalizing problem behavior, respectively, suggesting a disturbed functioning of the hypothalamic-pituitary adrenocortico (HPA)-axis in the pathogenesis of these behavioral disorders. The aim of this study is to determine whether variation in cortisol and problem behavior share the same genetic source. Toward this end, basal cortisol levels (4 samples per day on two consecutive days) and common childhood psychopathology (as assessed by the Child Behavior Checklist; CBCL) were assessed in a large sample of twelve-year-old children. The results show that heritabilities of daytime cortisol vary from weak right after awakening (h²=. 32), to moderate (h²=. 43) during the afternoon to strong (h²=. 71) during the morning peak (within an hour after awakening). Further, we observed a trend towards a negative association between cortisol and externalizing problem behavior. Whether problem behavior is a consequence of HPA-axis disregulation or a causing factor, or whether an underlying genetic defect independently gives rise to both problem behavior as well as HPA-axis disregulation, remains to be investigated. Bivariate modeling of cortisol and problem behavior could resolve this issue of the etiology causality.