Abstract

Background: Evidence from animal and human studies suggests that early adverse experiences such as maltreatment and bullying victimization have long-lasting effects on the hypothalamic-pituitary-adrenal (HPA) axis. However, uncertainty remains about the causal effect of these experiences in humans as no previous investigations controlled for genetic and shared-environmental influences. We tested whether bullying victimization affected cortisol reactivity using a discordant monozygotic (MZ) twin design and its impact on development.

Methods: Thirty pairs (43.3% males) of 12-year-old twins discordant for bullying victimization were identified in the E-Risk Study, a nationally-representative cohort of families with twins. We ascertained bullying using mothers’ and children’s reports, maltreatment using mothers’ reports and children’s behavioral problems using mothers’ and teachers’ reports.

Results: Bullied and non-bullied MZ twins showed distinct patterns of cortisol reactivity. Specifically, bullied twins exhibited blunted responses compared to their non-bullied MZ co-twins who showed the expected cortisol increase. This difference could not be attributed to children's genetic makeup, familial environments or pre-existing and concomitant individual factors. We subsequently showed in a larger sample (50.5% males; including discordant and non-discordant pairs) that maltreated/bullied children (n=64) also had lower cortisol responses in comparison to controls (n=126). Importantly, blunted responses were associated with more social and behavioral problems among maltreated/bullied children.

Conclusion: Results from this natural experiment support the enduring effects of early-life stress on cortisol reactivity. Moreover, our follow-up study showed that blunted responses may signal social and behavioral problems in maltreated/bullied children. The underlying role of DNA methylation will be discussed.