The researchers emphasize that their findings do not mean that a poor childhood relationship with a sibling causes depression, but they say the two are strongly associated. Moreover, whether the men’s parents did a good or a poor job of raising them seemed to have little effect on their risk of depression.
“Poor parenting may be reflected in poor sibling relationships,” said Dr. Robert J. Waldinger, the lead author of the study and an associate professor of psychiatry at Harvard. “But once you’ve taken account of the quality of sibling relationships, knowing about the quality of parenting doesn’t add much information.”
The findings, published in the June issue of The American Journal of Psychiatry, are based on an analysis of data from 229 men who were followed for more than 30 years beginning at age 18 or 19. They were first assessed in the period 1939-42 by internists, psychiatrists, psychologists and anthropologists, and then they completed questionnaires every other year. Researchers also interviewed their parents.
The men were reinterviewed when they were about 25, 30 and 50, and the biennial questionnaires have continued to the present. This prospective design and long follow-up are major strengths of the study.
Using information from these questionnaires and interviews, experts rated the men’s relationships with siblings during childhood and adolescence, and how well they were raised by each parent. They also recorded whether a parent had died before the child was 18 and whether there was a family history of depression. The 26 only children in the sample were excluded from the rating of relationships with siblings.
None of the 21 men who had a parent die in childhood became depressed. The 15 percent who had a poor relationship with their mothers and the 16 percent who had a family history of depression suffered depression later in life. But among those who had poor or destructive relationships with siblings, 26 percent had episodes of major depression as adults.
The significantly increased rate for depression among this last group remained even after controlling for family history of the illness, which suggests that the poor relationship with brothers and sisters did not grow out of a genetic tendency for depression but was an independent predictor.
“This is a really important study,” said Myrna M. Weissman, a professor of psychiatry at Columbia University who was not involved in the work. “The one caveat is that these were all men, and sibling relationships may pertain more to men than to women.”
The researchers offered some other examples of how the risks interact. A man whose parents did an average job and who had no family history of depression, but who had a poor relationship with a sibling, would have a 9.9 percent chance of developing depression. But a similar man with a good relationship with at least one sibling would have only a 2.3 percent chance of becoming depressed. In a man with an average parental experience, a family history of depression and poor relationships with siblings, the risk for depression rises to 30 percent.
The authors acknowledged that they collected diagnostic information on depression before the appearance of the Diagnostic and Statistical Manual of Mental Disorders, which established strict psychiatric diagnostic criteria. Also, the small sample consisted only of white male college students who came of age in the World War II era, and who were selected specifically for excellent mental health.
The study offers no conclusions about whether depression causes destructive interactions between siblings, whether the destructive relationships are an early indication of depression, or whether the two factors act on each other to increase the risk.
“This is just one study, and we need more to see whether this finding will be confirmed,” Dr. Waldinger said. “Sibling relationships have been underemphasized in learning about child development.”