Scientists have known for more than 20 years that smoking and depression are linked. It's estimated that 10 percent of all people who see a doctor for any health problem are depressed. More recent research suggests this figure may be much higher, closer to 50 percent.

Some scientists think depression leads to smoking. Others say smoking leads to depression. So far, the only thing that's clear is that many of the risk factors for smoking and depression are the same. These include low income or unemployment, low educational level, divorce or separation, and stress. Smoking and depression are also linked to back pain.