If U.S. Sen. Ben Sasse is right, the nation's most-discussed political problem is entangled with the least-understood public health problem. The political problem is furious partisanship. The public health problem is loneliness. Sasse's new book argues that Americans are richer, more informed and "connected" than ever — and unhappier, more isolated and less fulfilled.

In "Them: Why We Hate Each Other — and How to Heal," Sasse's subject is "the evaporation of social capital" — the satisfactions of work and community. This reflects a perverse phenomenon: What has come to count as connectedness is displacing the real thing. And matters might quickly become worse.

Loneliness in "epidemic proportions" is producing a "loneliness literature" of sociological and medical findings about the effect of loneliness on individuals' brains and bodies, and on communities. Sasse says "there is a growing consensus" that loneliness — not obesity, cancer or heart disease — is the nation's "number one health crisis." "Persistent loneliness" reduces average longevity more than twice as much as does heavy drinking and more than three times as much as obesity, which often is a consequence of loneliness. Research demonstrates that loneliness is as physically dangerous as smoking 15 cigarettes a day and contributes to cognitive decline, including more rapid advance of Alzheimer's disease. Sasse says, "We're literally dying of despair," of the failure "to fill the hole millions of Americans feel in their lives."