Bella DePaulo, a visiting psychology professor at the University of California, Santa Barbara, whose work focuses on single people and friendships, notes that in many studies, friendship has an even greater effect on health than a spouse or family member. In the study of nurses with breast cancer, having a spouse wasn’t associated with survival.

While many friendship studies focus on the intense relationships of women, some research shows that men can benefit, too. In a six-year study of 736 middle-age Swedish men, attachment to a single person didn’t appear to affect the risk of heart attack and fatal coronary heart disease , but having friendships did. Only smoking was as important a risk factor as lack of social support.

Exactly why friendship has such a big effect isn’t entirely clear. While friends can run errands and pick up medicine for a sick person, the benefits go well beyond physical assistance; indeed, proximity does not seem to be a factor.

It may be that people with strong social ties also have better access to health services and care. Beyond that, however, friendship clearly has a profound psychological effect. People with strong friendships are less likely than others to get colds , perhaps because they have lower stress levels.

Last year, researchers studied 34 students at the University of Virginia , taking them to the base of a steep hill and fitting them with a weighted backpack. They were then asked to estimate the steepness of the hill. Some participants stood next to friends during the exercise, while others were alone.

The students who stood with friends gave lower estimates of the steepness of the hill. And the longer the friends had known each other, the less steep the hill appeared.