From the NYTimes, by Tara Parker-Pope,  a new piece that nuances the old theory that married people are happier; she’s arguing that evidence suggests now that it is the quality of the relationship.

In 1858, a British epidemiologist named William Farr set out to study what he called the “conjugal condition” of the people of France. He divided the adult population into three distinct categories: the “married,” consisting of husbands and wives; the “celibate,” defined as the bachelors and spinsters who had never married; and finally the “widowed,” those who had experienced the death of a spouse. Using birth, death and marriage records, Farr analyzed the relative mortality rates of the three groups at various ages. The work, a groundbreaking study that helped establish the field of medical statistics, showed that the unmarried died from disease “in undue proportion” to their married counterparts. And the widowed, Farr found, fared worst of all.

But while it’s clear that marriage is profoundly connected to health and well-being, new research is increasingly presenting a more nuanced view of the so-called marriage advantage. Several new studies, for instance, show that the marriage advantage doesn’t extend to those in troubled relationships, which can leave a person far less healthy than if he or she had never married at all. One recent study suggests that a stressful marriage can be as bad for the heart as a regular smoking habit. And despite years of research suggesting that single people have poorer health than those who marry, a major study released last year concluded that single people who have never married have better health than those who married and then divorced.

All of which suggests that while Farr’s exploration into the conjugal condition pointed us in the right direction, it exaggerated the importance of the institution of marriage and underestimated the quality and character of the marriage itself. The mere fact of being married, it seems, isn’t enough to protect your health. Even the Healthy Marriage Initiative makes the distinction between “healthy” and “unhealthy” relationships when discussing the benefits of marriage. “When we divide good marriages from bad ones,” says the marriage historian Stephanie Coontz, who is also the director of research and public education for the Council on Contemporary Families, “we learn that it is the relationship, not the institution, that is key.”