An unhappy marriage can break your heart—literally. A new study from sociologists at Michigan State University indicates that people in contentious marriages are much more likely to develop heart disease or suffer a heart attack or stroke than their more happily wed contemporaries. Previous research has shown that married people tend to be healthier overall, but this study found that the stress of an unhappy union outweighs the usual benefits of wedlock. "It's not that every marriage is better than none," says study author Hui Liu. "The quality of marriage is really important."