"So for instance, our relationships have been linked to lower blood pressure, better immune functioning, and even inflammatory processes that are implicated in a number of diseases," said Holt-Lunstad. "And so our relationships can influence our health in a variety of ways that are all very important."

Even though Holt-Lunstad and her colleagues looked at almost 150 different scientific studies, most of them didn't assess the quality of the relationships.

"And certainly relationship quality matters, and not all relationships are entirely positive," she conceded. "And so it's possible that the effects that we have reported may be a conservative estimate, and that the odds of survival associated with high-quality relationships may actually be larger."

Speaking via Skype, Julianne Holt-Lunstad said her research suggests that policy makers need to consider relationships as a health issue. So just as no-smoking zones have expanded over the years, she suggests that city planners, for example, should consider whether their decisions might promote or hinder the development of personal relationships.

Julianne Holt-Lunstad's study is published in the journal PloS Medicine.