Nearly half of all Americans today say they are lonely. Why is that so, and what are the consequences? Here's everything you need to know:

How is loneliness defined?

Loneliness isn't determined by the actual number of friends or social contacts a person has. Social science researchers define loneliness as the emotional state created when people have fewer social contacts and meaningful relationships than they would like — relationships that make them feel known and understood. Essentially, if you feel lonely, you are lonely. One out of two Americans now falls into this category. In a recent study of 20,000 people by the health insurance company Cigna, about 47 percent of respondents reported often feeling alone or left out. Thirteen percent said there were zero people who knew them well. The U.S. is not unique in this respect: Loneliness is reaching epidemic levels throughout the developed world. Forty-one percent of Britons say the TV or a pet is their main source of company, and the U.K. has created a ­cabinet-­level minister to deal with the problem of rampant loneliness. A government study in Japan found that more than half a million people spent at least six months at home with no outside contact. "During my years caring for patients, the most common pathology I saw was not heart disease or diabetes," said former U.S. Surgeon General Vivek Murthy. "It was loneliness."

What impact does loneliness have?

It makes people sick. A 2010 study by Brigham Young University found that loneliness shortens a person's life by 15 years, about the same impact as being obese or smoking 15 cigarettes a day. Other studies have found connections between loneliness and a wide range of health problems, including increased risk for heart attacks, stroke, and cancer. Lonely people are more likely to suffer from insomnia, depression, and drug abuse. They are also more likely to suffer from more rapid cognitive decline in old age.

Why is physical health affected?

Stress. The feeling of loneliness, scientists say, is an evolutionary phenomenon. Just as hunger encourages animals to find food, loneliness forces humans to seek out the protection of the group, increasing the chances of survival. To produce this behavior, loneliness triggers the release of stress hormones, particularly cortisol. In small doses, these hormones help make solitary humans more alert to danger. But they damage health if the body is exposed to them over long periods of time. Stress leads to high blood pressure, increased inflammation, and a weakened immune system. Without an emotional support network, lonely people are also more likely to slip into unhealthy habits, such as substance abuse, overeating, and not exercising. For seniors, isolation can be especially deadly in the event of an emergency like a bad fall or a heart attack. "Denying you feel lonely makes no more sense than denying you feel hunger," said John T. Cacioppo, a neuroscientist who studied loneliness at the University of Chicago.

Is isolation more common?

It appears to be. Between 1985 and 2009, the average American's social network shrank by more than one-third, defined by the number of close confidants. One reason for this is the aging of the Baby Boomers, who had fewer children and more divorces than their parents, leaving many without companions in their old age. About 1 in 11 Americans age 50 or older doesn't have a spouse, romantic partner, or living child. That's roughly 8 million people. One in six Boomers lives alone. The increasingly transient nature of work is also making people lonely, as Americans leave family and hometowns behind in search of paychecks. Surprisingly, young people are actually most at risk of being lonely in modern society. In the Cigna study, Generation Z members ages 18 to 22 and Millennials ages 23 to 37 scored the highest for loneliness.

Why are so many young people lonely?

Americans are getting married and having children later in life; there are now more single people in the U.S. than at any time in the past 140 years. Not being part of a regular workplace also plays a role, with freelancers and "gig economy" workers reporting higher levels of loneliness. And despite seemingly infinite opportunities to connect online, social media may actually be making the problem worse. Scrolling through an endless stream of curated photos of parties, vacations, family gatherings, and weddings may increase feelings of being left out or dissatisfaction with one's own life. In one study of Americans ages 19 to 32, the top 25 percent of social media users were twice as likely to report feeling lonely as the people using it least. Some researchers say loneliness began becoming widespread long before the internet, when the Industrial Revolution broke up tightly knit agricultural communities. "I do think it speaks to one of the dilemmas of modern, mobile society," said Stephanie Coontz, a historian at Evergreen State College. "As we gain the freedom to become whatever we want to be, we've lost the sense of belonging."

Alone, angry — and intensely partisan

Some researchers believe that America's increasingly polarized politics — and the partisan viciousness on social media — may be at least partly the product of increasing loneliness. Psychiatrists Richard S. Schwartz and Dr. Jacqueline Olds describe loneliness as the "elephant in the room" of American politics. Social isolation, they say, makes people less empathetic and more likely to view the world in terms of "us" and "them." "I think comparing notes in a civil way is the antidote to a polarized society in which we don't understand a point of view other than our own," Olds says. "If we are so lonely that we have no one to compare notes with, we tend to become more polarized." Republican Sen. Ben Sasse of Nebraska believes that Americans are turning to political tribalism for the sense of community they used to get from simple connection to those around them. "The local, human relationships that anchored political talk have shriveled up," Sasse writes in his new book, Them: Why We Hate Each Other — and How to Heal. "Alienated from each other, and uprooted from places we can call home, we're reduced to shrieking."