The following article comes from the U.S. News ebook, How to Live to 100, which is now available for purchase.

They are known as singles, singletons, the never-married, the divorced, and the widowed. What they share is that they are part of the country's fastest-growing living unit—more than 31 million one-person households in 2010, according to the U.S. Census.

Traditionally, relationship researchers have found that people living alone are on the bottom rung of the wellness ladder. They lack the emotional, financial, and daily help of a committed partner, which are major reasons why people in successful marriages and other strong two-person relationships fare better in measures of health, happiness, and longevity.

"When people succeed in having a good intimate relationship, it has so many benefits," says UCLA psychology professor Ben Karney. "Your body works better, your immune system functions better, your body produces more antibodies. Study after study shows that people in good relationships live longer." Even severely ill people who were in good relationships recovered faster and lived longer than comparably ill people who were not in good relationships.

Single men, in particular, take especially poor care of themselves. "Unmarried men are more likely to have bad health habits than married men," says Linda Waite, a sociologist at the University of Chicago. "They drink too much, don't eat well, don't wear seat belts, have more unprotected sex" and don't enjoy the kind of social supports they would in a committed relationship. Single women, by comparison, fare better, precisely because they have better social connections and are used to taking care of themselves.

However, many experts say the health and happiness disadvantages of living alone are disappearing. Social science research tends to look at a long-distance rearview mirror, analyzing large groups of people over many, many years. Current trends are easily documented.

"Over the past 30 years, the health gap between the married and never-married has narrowed to almost nothing," says Debra Umberson, a sociologist at the University of Texas. "Being not married has increasingly become an accepted option."

"Once they accept [being unmarried] and make their peace with it, they fare just as well as anyone else," says Deb Carr, a Rutgers University sociologist. "We see them expanding their definitions of what is a family. Not only do they have larger numbers of friends [than married people], but they have more frequent contact with them and closer relationships with them." Carr says society has become friendlier to "never marrieds" as well, and that people are more tolerant and supportive of a broad range of different ways people choose to live.

"I think that there is a really important distinction to be made between social isolation and choosing to live alone," Umberson says. "People who are socially isolated are the ones more likely to die" at earlier ages.

Eric Klinenberg is a sociologist at NYU and author of a recent book about living alone called Going Solo. It supports, if not celebrates, the emergence of the one-person household as an increasingly preferred living choice, not only in the United States but even more so in many Western European nations.

Klinenberg is careful to distinguish among different types of one-person households when assessing their occupants' health and well-being. He also thinks that much of the pro-marriage research is based on either misleading or flawed assumptions.

"Many, if not most, studies of the health consequence of marriage compared currently married people versus never married people," he says. The adverse health consequences of divorce and widowhood are well-documented but are usually viewed separately from the positive health effects of people who remain married. No one gets married thinking the marriage will fail or their spouse will die, Klinenberg notes. And while staying married produces benefits, he says it's impossible to conclude that simply getting married improves a person's well-being and longevity compared with staying single.

In looking at the effects of living alone, Klinenberg says, "I make a very careful distinction between living alone, being alone, being isolated, and feeling lonely. These are four different things. And most researchers, even the best of them, conflate them."

"It's really a specific minority of people who live alone who are vulnerable," he says. "And we could do much more to provide care and support for them than we do now. We could do more to connect them to other people and services. And that's what would make them safer."

The opportunities and challenges of living alone differ greatly, depending on a person's age and marital history. About 5.5 million young adults under age 35 live alone, Klinenberg says. Especially in larger urban areas, they have an unparalleled mix of social options. Coupled with the explosion of online media and networking tools, there is no societal reason for these people to be lonely, and many reasons why living alone can produce a fulfilling and happy experience.

Among people ages 35 to 65, he explains, most of the those who live alone were previously married. "What's new today is that they are not going to remarry the wrong person." Social pressure to be married has receded, and single people are getting a lot more affirmation about making the best decisions for themselves. "People who live alone do get lonely," Klinenberg says, "but so do people in marriages."

Among people over age 65, there are 11 million one-person households. It's here, many experts fear, that loneliness and isolation can take an enormous toll on health and happiness. Many of these people are widowed, and most of them are women who have outlived their husbands.

Building new friendships and social networks is an effective strategy to ward off the negative consequences for older people who live alone. Moving into a senior living complex may make sense for those who may need help finding companionship, social activities, and help with their daily lives. Throughout the country, hundreds of groups of seniors have formed virtual senior communities to provide organized support to one another as they continue to live alone in their homes.

The key to healthy aging, sociologist Laura Carstensen says in her book A Long Bright Future, is to build a plan that anticipates the needs of older age. Renew your social networks. Find younger friends and new activities and social organizations. Build daily routines and a lifestyle that matches what you've previously envisioned as the way you want to live.