Researchers have determined that major depression increases the risk of heart disease more than any genetic or environmental factor. The connection between chronic depression and cardiovascular malfunction is well established, but this study expands on the nature of that relationship, noting that depression significantly increases the likelihood of heart disease regardless of genetic predisposition or environmental stressors (smoking, etc). 

An extended study performed by researchers at Washington University School of Medicine in conjunction with the VA surveyed 1,200 male twin Vietnam veterans in both 1992 and 2005 on a variety of health issues including depression. The twins were placed in groups to account for the severity of their genetic and environmental risk factors. 

Focusing on adverse cardiovascular "incidents" (stroke, heart stents, angina) among depressed subjects and controlling for variables, researchers then determined that subjects who reported a history of depression in 1992 were more than twice as likely to develop heart disease by 2005. Most significantly, researchers noted no rise in heart disease rates among those who carried the requisite genetic markers but had no history of depression. This finding means that depression's influence on cardiovascular health surpasses even that of genetics: if two twins carry the genes for heart disease but only one suffers from depression, the unaffected brother will face a considerably lesser risk of developing a related cardiovascular condition. 

This study also casts light on a preceding report which found that medicating the depressed reduces the risk of subsequent cardiovascular incidents. These two complimentary conclusions make very clear the necessity of immediate treatment for depression, especially among subjects with any form of heart disease in their immediate family histories (a description that applies to most everybody).