“We’re all in this alone,” the comedian Lily Tomlin once quipped, in a startling home truth. Loneliness is the dirty little secret of our outgoing “people-person” culture. It’s not someone else’s problem, but one that afflicts almost 20% of Americans — roughly 60 million people.

Americans were once regarded as the most gregarious people on earth. No longer. They now report having fewer friends and confidants than ever before. Nearly 30 million of them live alone. And a recent study, drawing on a large cross section of the population, found that one-quarter of respondents had no one at all with whom to talk openly and intimately.

Two recent books tackle this problem, arguing that it is much larger than we suspected, but neither is entirely satisfying in its analysis or recommendations. Thomas Dumm’s “Loneliness as a Way of Life” (Harvard University Press, 208 pages, $23.95) is a quiet, meditative book that concerns — and sometimes reproduces — the excessive self-preoccupation that can afflict the lonely. Mr. Dumm’s portrait of loneliness is somber and sometimes quite hard to take. The lonely are, he says, refugees from others and strangers to themselves. Their isolation causes them to doubt everyone and everything. With so many people wretched in his or her way, Mr. Dumm worries about the collective drain on our culture. “Does it even matter for our politics whether we are lonely?” Yes, his book rightly insists, “it matters profoundly.”

“Loneliness” begins in a roundabout way, with a lengthy reading of Shakespeare’s “King Lear.” Mr. Dumm sees Cordelia’s loneliness as an allegory for our own. With her, “loneliness becomes a way of life. She is thus our first modern person.” Cordelia is of course a character spurned by a father too concerned with what her love should mean to notice its real value. But are we all Cordelias now? The more Mr. Dumm pushes this analogy, the less our plight seems to resemble hers. We are too preoccupied by status and materialism, he says, to notice how both factors can impoverish relationships: “Our unending desires remain unsatisfied … We move about the world obscurely ashamed of our pretense …” As the generalities pile up, the argument grows as hazy as Lear’s assessment of his daughter. Mr. Dumm also makes a decidedly wrong turn when he swerves into a detailed account of what went wrong in his own marriage. We’re obliged to relive arguments that he tells us have festered in his family for years. At such moments I would readily have preferred Lear and Cordelia’s harrowing drama.

John Cacioppo and William Patrick’s “Loneliness: Human Nature and the Need for Social Connection” (W. W. Norton, 288 pages, $25.95) left me wanting more Shakespeare — a lot more. Not just the beauty of his prose, but his insight into scenarios as complex and treacherous as those that unfold in “Lear.” Instead, even on a subject as profound and worrisome as loneliness, the new book offers a surprisingly perky account of what other people add to our lives. With his friendly acronyms, positive “feedback loops,” and reassuring platitudes (“birds of a feather flock together”), Mr. Cacioppo, the principal author, comes perilously close to cheerleading what he calls bumper-sticker wisdom. He tells us, “Happy people become less lonely people, and people who are less lonely tend to make more money.”

If only it were that simple. The endless clichés that Messrs. Cacioppo and Patrick trot out about positive thinking, mirroring neurons, and selfish genes muddy their snapshots of isolated individuals who are completely at sea in the communities that envelop them. If “loneliness is not a life sentence but simply a call to repair social connections,” as the authors declare, then stressing “the secret to success” not only understates the magnitude of the problem, it also trivializes how large numbers of Americans grew so isolated in the first place.

If Messrs. Cacioppo and Patrick are to be believed, genes are partly the reason. “Loneliness is about half heritable,” they assert, likening the problem to an evolutionary shock that prompts us to rejoin groups when we’re feeling cut off. But if that were the case, why are so many more Americans experiencing that shock now than ever before, and more sharply than in other nations?

After lengthy analyses of brain imaging, stress hormones, blood pressure, and immune responses, Messrs. Cacioppo and Patrick show that loneliness takes a physiological toll. They can identify pain on an fMRI scan, at least, even if the scan itself cannot distinguish between physical and social pain. But they are far less convincing at naming the underlying cause of that suffering and explaining why it has intensified in some countries, not others. At such moments, their arguments from evolutionary psychology teeter on the absurd. An anxious teenager at a party is, apparently, “just like a hunter-gatherer hearing an ominous sound in the bush.” The authors declare, “When we feel unsafe, we do the same thing a hunter-gatherer on the plains of Africa would do — we scan the horizon for threats.”

Doubtless it is easier to rehash such nonsense than to consider that loneliness may have become ingrained in our culture because of the way we live and work, often in circumstances we’ve not exactly chosen. Over generations, our habits have hardened into trends we cannot uproot merely by positive thinking. One in five Americans now moves at least once a year, often to communities that do not encourage spontaneous interaction with others. We spend more time commuting than ever before. And the hours we used to devote to group activities have shrunk as the cost of living has exploded. In short, stronger, more impersonal forces are in play. Fortunately, several excellent books assess the scale and gravity of that problem from different angles. They range from the socially scientific (Robert Putnam’s “Bowling Alone”) to the psychological (Deborah Luepnitz’s “Schopenhauer’s Porcupines: Intimacy and Its Dilemmas”) to the literary (Claudia Rankine’s “Don’t Let Me Be Lonely”).

Where Messrs. Cacioppo and Patrick are genial and banal, Mr. Dumm is hazy and diffuse. Yet the gray areas of his book are, in the end, its greatest appeal. “Loneliness as a Way of Life” doesn’t try to be cut-and-dried. It is quizzical, often deeply skeptical, about our intentions as a species. It recognizes that society — and sociability — are messy affairs that bring mixed blessings.

Especially in his last, rousing chapter on Emerson, freedom, and responsibility, Mr. Dumm offers a way out of the morass he has described. He makes no false claims that we can vanquish loneliness or fix our thinking in a fresh groove. Instead, he proposes the tempered idea that friendship and group accountability can — indeed, must — live together with our existential solitude.

Mr. Lane, the Pearce Miller Research Professor at Northwestern University, is the author most recently of “Shyness: How Normal Behavior Became a Sickness.”