For quite some time it's been known in the medical community that a link exists between depression and heart disease. For example, large epidemiological surveys typically find that 1.5 to 3 percent of the population is depressed at any given time. Among patients with heart disease, however, the rate of depression is closer to 18 percent. Similarly, about 1 in 6 people in the general population has an episode of major depression during their lifetimes, compared to about 50 percent of people with heart disease. Finally, a Canadian study revealed that of 222 patients who had suffered heart attacks, those who were depressed were four times as likely to die within the next 6 months.

Amassing this evidence is one thing, but explaining it is quite ...