The study doesn’t explain exactly why marriage makes a difference. It’s possible that married patients just have better health insurance. But, the authors note, “even in nations with universal access to free care, such as Denmark, sociodemographic factors affect outcome in a multitude of health conditions.” (Insurance differences also wouldn’t do much to explain the disadvantage of widowhood, because widows tend to be covered by Medicare.)

“The most likely reason,” the authors conclude, “is that married patients have better adherence with prescribed treatments than unmarried patients.” They have someone to watch over them.

What about couples who aren’t formally married but in long-term relationships? What about widows with supportive kids? What about people with great friends? And what about unhappy marriages?

The study didn’t leave these people out. If it had — if it had measured only the difference between ideal marriages and lonely, miserable singles — the differences would presumably be even more distinct. In the big picture, marriage makes a difference.