Poor interpersonal relationships in childhood have long been seen as a risk factor for adult depression. However, most of the research has been done on children's relationships with their parents not their relationships with their brothers and sisters. A long-term study of 229 men in the U.S. looked at the quality of their relationships with their siblings and parents and their family history of depression and examined the links between these factors and their likelihood of developing depression, alcoholism and drug abuse later in life. The researchers found that poorer relationships with siblings before the age of 20 and a family history of depression independently predicted both the occurence of depression and the frequency of use of mood-altering drugs by the age of 50, even after allowing for the quality of childhood relationships with parents. However, poor relationships with parents in childhood did not predict the occurence of depression by age 50 once the family history of depression and the quality of sibling relationships were taken into account. The quality of sibling relationships and a family history of depression did not predict later alcohol abuse or dependence.
Waldinger, Robert J., Vaillant, George E. and Orav, E. John - Childhood sibling relationships as a predictor of major depression in adulthood : a 30-year prospective study American Journal of Psychiatry 164(6), 949-954