Sibling similarity for MDD: evidence for shared familial factors.

BACKGROUND: Sibling designs have the potential to identify familial factors that are shared and unshared between siblings. Unfortunately, this methodology has rarely been applied to mood disorders.
METHODS: The current study uses the original participants from the Oregon Adolescent Depression Project (OADP) and their closest-aged sibling to investigate familial influences on sibling similarity and differences for psychopathology. Diagnostic interviews were conducted with the original participants on three occasions and with full siblings and biological parents at the time of the third assessment.
RESULTS: Maternal major depressive disorder (MDD) was associated with sibling similarity for MDD and anxiety disorders, and maternal anxiety disorder was associated with sibling similarity for MDD. A number of maternal and paternal disorders were associated with sibling differences for psychopathology. The data suggest that maternal MDD and anxiety disorder have a greater influence than paternal MDD and anxiety disorder in influencing sibling similarity for MDD. LIMITATIONS: Original participants were assessed on up to three occasions while their siblings were assessed only once.
CONCLUSIONS: Maternal MDD and anxiety disorder appear to act as shared familial risk factors with respect to sibling MDD. In addition, parental psychopathology has non-shared influences on sib-pairs. Future research should assess differential parental treatment and individual sibling characteristics as additional factors that may influence sibling similarity and differences in psychopathology.