Adults with a sibling who’s suffered from a mental disorder may themselves be at risk of depression, a new study suggests.

Researchers found that of more than 800 adults followed over decades, those who had a sibling who’d ever suffered from depression, an anxiety disorder or other mental illness were 63 percent more likely to have ever been diagnosed with depression.

They also tended to have poorer psychological well-being in general - particularly those with a brother who’d been diagnosed with a psychiatric illness.

The findings suggest that a combination of genetics and environment may make these adult siblings more vulnerable to depression, the researchers report in the Journal of Family Psychology.

In some families, the investigators note, these siblings may have a genetic susceptibility to mental health problems - which, coupled with the stress of a brother’s or sister’s symptoms, may contribute to depression.

“Our findings highlight the need for families of the mentally ill, specifically siblings, to be more aware of their own mental health needs throughout their lifetimes,” lead researcher Dr. Julie Lounds Taylor, of the University of Wisconsin-Madison, noted in a written statement.

The researchers based their findings on data from a long-term health study that has followed roughly 5,800 sibling pairs since their graduation from high school in the 1950s. Taylor’s team focused on 83 participants with a sibling who’d ever been diagnosed with a mental health disorder - most commonly, depression or anxiety - as well as

268 whose sibling was mildly intellectually impaired (having an IQ lower than 85).

These men and women were compared with a group of 791 study participants whose siblings were free of mental illness or impairment.

Compared with this latter group, Taylor’s team found, adults with a sibling who’d suffered from a mental illness were at greater risk of ever being diagnosed with depression.

In contrast, this was not true of those whose siblings had a low IQ, the researchers found. These men and women did, however, tend to be less emotionally attached to their sibling than study participants in the comparison group.

This, Taylor and her colleagues note, may be because siblings of mentally impaired adults often feel they have an “obligatory” type of relationship with their brother or sister. That is, they feel a responsibility to support their sibling, but they don’t feel the same emotional closeness that two, more-equal adult siblings might.

“So little is known about the impact that a person with low IQ or mental illness has on the psychological and social development of his or her siblings, especially beyond childhood,” Taylor said.

This study, she and her colleagues write, is an “important first step” in understanding that dynamic.

SOURCE: Journal of Family Psychology, December 2008.