Birth order moderates the association between adverse childhood experiences and externalizing behavior symptoms in adolescence.

Adverse childhood experiences (ACEs) are associated with externalizing behaviors. Whereas some ACEs affect individual children (i.e., child-specific; e.g., failing a grade), others affect the family unit (i.e., family-wide; e.g., parent losing a job); effects of ACEs on externalizing behavior may manifest differently across groupings of ACEs. Moreover, birth order may modify the association between child-specific and family-wide ACEs and externalizing behavior due to differences in the experience of being a younger versus older sibling. This study examined the externalizing behavior of siblings in relation to their experiences of child-specific and family-wide ACEs to test the hypothesis that younger siblings are at greater risk for developing externalizing symptoms following familial ACE exposure. Participants were 61 sibling pairs (younger sibling M = 11.37 years, 44.1% male; older sibling M = 13.1 years, 52.5% male) recruited from six schools in the northeastern United States. Parents rated each child's externalizing behaviors (e.g., bullying, meanness) and retrospectively reported on each child's experience of 34 ACEs; two raters categorized ACEs as child-specific (n = 10) or family-wide (n = 24). Multilevel modeling revealed that both child-specific and family-wide ACEs were associated with increased externalizing behaviors. Birth order moderated the effect of family-wide (but not child-specific) ACEs on externalizing behaviors, independent of sex and age. Externalizing behavior was higher for younger siblings as compared with older siblings, particularly when a high number of ACEs (6+) were reported. This research should prompt future exploration of mechanistic theories of the impact of family-wide and child-specific ACEs and the role of birth order.