There’s a mountain of evidence suggesting that the quality of our relationships has been in steady decline for decades. In the 1980s, 20 percent of Americans said they were often lonely. Now it’s 40 percent. Suicide rates are now at a 30-year high. Depression rates have increased tenfold since 1960, which is not only a result of greater reporting. Most children born to mothers under 30 are born outside marriage. There’s been a steady 30-year decline in Americans’ satisfaction with the peer-to-peer relationships at work.

Former Surgeon General Vivek Murthy summarized his experience as a doctor in an article in September in The Harvard Business Review: “During my years caring for patients, the most common pathology I saw was not heart disease or diabetes; it was loneliness.”

Patients came to see him partly because they were lonely, partly because loneliness made them sick. Weak social connections have health effects similar to smoking 15 cigarettes a day, and a greater negative effect than obesity, he said.

Over the past five years, such trends have abruptly gotten worse. In 2012, 5.9 percent of young people suffered from severe mental health issues. By 2015 it was 8.2 percent.

Last year, Jean Twenge wrote a much-discussed article for The Atlantic, “Have Smartphones Destroyed a Generation,” charting the accelerating social collapse. Teenagers are suddenly less likely to date, less likely to leave the home without their parents, more likely to put off the activities of adulthood. Eighth-graders who are heavy users of social media are 27 percent more likely to be depressed.

Loneliness and social isolation are the problem that undergird many of our other problems. More Americans are socially poor. Yet it is very hard for the socially wealthy to even see this fact. It is the very nature of loneliness and social isolation to be invisible. We talk as if the lonely don’t exist.

I was really struck by this last week, when Mark Zuckerberg came through Washington. Most of the questions he faced at the congressional hearings and most of the analysis in the press were about Facebook’s failure to protect privacy. That’s the sort of thing that may be uppermost on your mind if you are socially wealthy, if, like most successful politicians and analysts, you live within a thick web of connection and feel as if your social schedule is too full.

But the big issue surrounding Facebook is not privacy. It’s that Facebook and other social media companies are feeding this epidemic of loneliness and social isolation. It’s not only that heavy social media users are sadder. It’s not only that online life seems to heighten painful comparisons and both inflate and threaten the ego. It’s that heavy internet users are much less likely to have contact with their proximate neighbors to exchange favors and extend care.

British anthropologist Robin Dunbar observes that human societies exist on three levels: the clan (your family and close friends), the village (your local community) and the tribe (your larger group). In America today you would say that the clans have polarized, the villages have been decimated and the tribes have become weaponized.

People try to compensate for the lack of intimate connection by placing their moral and emotional longings on their political, ethnic and other tribes, turning them viciously on each other.

Such big subjects didn’t come up in the Zuckerberg hearings because socially wealthy and socially poor people experience Facebook differently and perceive reality and social problems differently. It’s hard to quantify and communicate the decline in quality of relationships. But it is nonetheless true that many of us who are socially wealthy don’t really know how the other half lives.

— David Brooks is a columnist for The New York Times.