Having friends, a marriage or children is as important to living a long and healthy life as not smoking or becoming overweight.
Researchers at Brigham Young University in Utah found that people who have good social relationships are half less likely to die early than are more isolated people.
The psychologist who led the study, Julianne Holt-Lunstad, told Reuters: "A lack of social relationships was equivalent to smoking up to 15 cigarettes a day."
Having low levels of social interaction was equivalent to being an alcoholic, was more harmful than not exercising and was twice as harmful as obesity.
The researchers also found that having relationships makes a bigger impact on avoiding premature death than taking high blood pressure medication or being exposed to air pollution.
Holt-Lunstad said: "I certainly don't want to downplay these other risk factors because of course they are very important. We need to start taking social relationships just as seriously."
The study found that Americans are becoming increasingly isolated. Among the findings were less "intergenerational living', putting marriage off for longer periods than previous generations, and more single-residence households than before.
"More specifically, over the last two decades there has been a three-fold increase in the number of Americans who report having no confidant," the research team wrote.
"Such findings suggest that despite increases in technology and globalization that would presumably foster social connections, people are becoming increasingly more socially isolated."
Holt-Lundstad would like to see the government encourage social interaction by making policies that enable friends or relatives to more easily take part in the medical care of each other, and for city planning that encourages more social contact.