The Heritability of Different Indices of the Diurnal Cortisol Rhythm in Middle Childhood and Phenotypic Associations with the Home Environment

Research has shown that environmental stressors that occur during childhood and early adolescence are associated with multiple deficits in physiological and psychological functioning later in life. The hypothalamic-pituitary-adrenal (HPA) axis has been proposed as a potential biological mechanism through which these phenotypic alterations occur as studies have shown a link between early life adversity and altered diurnal cortisol patterns (Goldman-Mellor, Hamer, & Steptoe, 2012; Gunnar & Quevedo, 2008). Given research has shown that diurnal cortisol levels are influenced by genetic factors (Veen et al., 2011), but that a majority of differences across subjects can be attributed to the environment (Schreiber et al., 2006), phenotypic associations were explored between the quality of the home environment and children's diurnal cortisol patterns. The first aim of this study was to determine the level of genetic and environmental contributions to different parameters of diurnal cortisol rhythm. The second aim of this study was to examine whether the quality of the home environment, particularly indicators involving parenting and the physical environment, was associated with these same diurnal cortisol measures. A diverse sample of 320 twin children were assessed at 8 years using gold standard home environment interviews and a measure of diurnal cortisol rhythm across three days with three samples taken from each twin every day. Twin intraclass correlations indicated high levels of heritability for the morning to afternoon diurnal cortisol slope as well as the afternoon to evening slope, while measures of cortisol in the afternoon, evening, and across the day showed low levels of heritability, which suggested that differences in the environment were a more influential factor. Multilevel regression analyses showed that the overall quality of the home environment was found to be significantly negatively associated with cortisol levels at bedtime and negatively associated across the morning to afternoon slope at a trend level. The physical environment and emotional climate of the home were not significantly associated with any indicators of the diurnal cortisol pattern. A unique seasonality effect was noted as cortisol measurements taken from participants during the summer were significantly increased when compared to participants throughout the rest of the year. Overall, these findings showed a unique association between the quality of the home environment and diurnal cortisol levels at bedtime and perhaps the change in cortisol levels across the morning to afternoon, as well as a possible seasonal covariate which may affect diurnal cortisol measurements and one which often goes overlooked in cortisol research.