Siblings' similarity in neural responses to loss reflects mechanisms of familial transmission for depression.

Having a depressed first-degree relative is one of the most replicated risk factors for depression. Research on the familial transmission of depression, however, has largely ignored siblings, even though sibling relationships are commonplace and characterized by frequent and intense emotions. It has been suggested that frequent contacts in close relationships lead to similarities in emotions and cognitions over time, a process underpinned by biobehavioral synchrony. Consequently, to shed light on the neural mechanism underlying familial transmission of depression, the present study tested whether neural similarity in response to reward and loss, indexed by inter-subject correlation (ISC), was associated with major depressive disorder (MDD) diagnosis, depression symptom severity, and relationship quality in sibling pairs. Same-sex, full-sibling pairs (N = 108 pairs) with a wide range of depression severity separately completed a monetary reward task during electroencephalography acquisition. The ISC in response to reward and loss feedback was calculated using circular correlation between siblings' phase angles in delta and theta frequency bands, respectively. Significant sibling ISC to reward and loss was observed, with activity maximal at frontocentral sites. Loss-related theta, but not reward-related delta, ISC was associated with: (a) greater depression risk (both lifetime MDD diagnosis and self-reported symptom severity), but not anxiety, and (b) worse sibling relationship quality during childhood in each sibling. Findings provide initial evidence that similarities in neural responses to loss may be a result of disturbed childhood sibling relationships, which may specifically increase risk for depression during adulthood.