ST. LOUIS (AP) - Ever wonder why smoking and depression seem to go together? A Saint Louis University School of Public Health study finds the connection is genetic. Researchers found that the genes that increased a person's risk of developing major depression and nicotine addiction are found in those who have conduct disorder, such as stealing, vandalizing, running away from home and fighting. These people are likely to become addicted to other drugs and behave impulsively. Researchers analyzed responses from more than 33-hundred pairs of twins. They used a mathematical model to estimate the genetic and environmental influences on nicotine addiction and major depression.