A 24-yearlong study by the Harvard Medical School suggests that one of the key ingredients to a healthy and happy life is marriage. Or, to break it down into one of marriage’s key domestic elements, living with somebody. The University of Chicago recently did a national survey on sex, and their findings were similar: They concluded that married couples, partners and roomies live a more satisfactory life than those of us who live alone.

According to New York University sociology professor Eric Klinenberg, more and more of us are choosing not to marry and prefer to live by ourselves. In his new book, Going Solo: The Extraordinary Rise and Surprising Appeal of Living Alone, Klinenberg says that in most major American cities, 40 percent of all households are single occupancy. In Manhattan and Washington, D.C., that number goes up to 50 percent.

Internationally, living solo is also the new norm. Single-dwellers in Stockholm account for 60 percent of the population. In Paris, it’s 50 percent. The number of one-person households in France, Germany, Britain and Japan outnumber those in the United States.

The big picture explanation seems to be that prosperity affords people the options of privacy, personal space and personal control of one’s time and assets. Surveys indicate that in the past, many adult children lived at home with their parents, got an apartment with one or more roommates and even got married because it was both the sociological norm and, simply cheaper.

After Klinenberg interviewed more than 300 “singletons” (his term for people who live alone) he realized that you have to make a critical distinction between living alone and being lonely. Living alone, by choice, is a way to center oneself, to gain control and organize of one’s life; it’s a way to recover from the now normal over-stimulated nature of modern society.

In some sense, living alone mirrors the general characterization and psychological profiles of introverts versus extroverts. Classic extroverts find large crowds, social situations and constant business energizing and stimulating. They become bored, anxious and uncomfortable when they are alone. Many extroverts actually fear being alone and become undone when left to their own devices.

Introverts, on the other hand, are comfortable being alone. They need to be alone, and when they are with others they prefer small groups rather than larger ones. Although they enjoy social situations, they need to find a balance between private time and public time.

Klinenberg’s research suggests that “singletons” are not antisocial monks. They enjoy and seek out the company of others. But they are selective with their social engagements and how they spend their time. After studying Klinenberg’s findings, I’m convinced that “singletons” are in some sense “gregarious introverts.” They need and crave time alone. But they also need and crave quality time with others.

The Old Testament is right. “It is not good that (a person) should be (always) alone,” it says. But I am convinced, and I think Klinenberg’s research backs me up on this, that the worst thing in the world is not being alone, but rather “being lonely when you are not alone.”

Al Gini is a professor of business ethics and chair of the department of management at Loyola University Chicago. He is also the co-founder and associate editor of Business Ethics Quarterly,and the author of several books, including My Job, My Self and Seeking the Truth of Things: Confessions of a (catholic) Philosopher.