Does cortisol moderate the environmental association between peer victimization and depression symptoms? A genetically informed twin study.

Many youths who are victimized by peers suffer from depression symptoms. However, not all bullying victims show depression symptoms and individuals' biological sensitivity may play an important moderating role in this regard. In line with this notion, peer victimization has been associated with increased depressive symptoms in youth with higher basal cortisol secretion. It is unclear, however, whether this moderating effect of cortisol really concerns the environmental effect of peer victimization on depression. Indeed, genetic factors can also influence individuals' environmental experiences, including peer victimization, and part of these genetic factors may be those associated with depression. Using a genetically informed design based on 159 monozygotic and 120 dizygotic twin pairs (52% girls) assessed at age 14 years, this study examined whether cortisol secretion moderates the environmental or the genetic association between peer victimization and depression symptoms. Salivary cortisol at awakening was obtained with buccal swabs during four school week days. Peer victimization and depression were assessed via self-reports. Cholesky modeling revealed that peer victimization was associated with depression symptoms via both genetic and environmental pathways. Moreover, the environmental association between peer victimization and depression symptoms steadily increased with increasing levels of morning cortisol. The genetic association between peer victimization and depression symptoms also varied, albeit less, as a function of individuals' cortisol secretion. These findings support the hypothesis that peer victimization increases internalizing psychopathology mainly in youth with heightened biological reactivity to environmental conditions.