A Saint Louis University study of twins concluded that those who are genetically predisposed to conduct disorders also have an increased risk of developing major depression and nicotine addiction, Science Daily reported July 17.

"Some people with a history of depression may become smokers as a way of self-medicating," said lead researcher Qiang John Fu, M.D., Ph.D., of the Saint Louis University School of Public Health. "Some people who are smokers might become depressed when they try to give up cigarettes and can't. When I tried to find something to explain this correlation, I discovered the answer lay partly in a person's genes that are associated with conduct disorder, which is extreme rebellious behavior of teens and children. My findings are an alternate explanation about why nicotine dependence and major depression go together."

Fu and colleagues studied 3,360 pairs of middle-aged twins. "Our data showed that both major depression and nicotine dependence were highly genetically correlated with conduct disorder," he said. "When [researchers or clinicians] see people with a history of conduct disorder, they may be able to predict those people who could develop major depression or nicotine dependence."

The study appears in the June 2007 issue of Twin Research and Human Genetics.