A WHILE AGO, a writer I know received an effusive e-mail from a young stranger who claimed to be an admirer. The admirer professed that she had always wanted to meet the writer. Not only had she read his books, she had once sat next to him in a café. She punctuated these confessions with exclamation points, OMGs, smiley faces, and exhortations of friendship. The writer did not reply. Then, the two happened to meet, whereupon the admirer told the writer that she had been sincerely hurt by his silence, convincing him all the more of her insincerity. He felt guilty and Facebook-friended her, which she declined.

Though this exchange may contain less gravitas than the idea of sincerity itself, and the various problems that sincerity—and its absence—unleash every day in the public and political arenas, I start here because R. Jay Magill, Jr. believes that the best place to mine the nuances of the moral ideal lies in the encounter between two individuals. But while these interactions are fruitful, his argument is larger. He posits that our always-conflicted relationship with sincerity has—at least since the 1980s, but especially since September 11, the explosion of the Internet, and the economic crash—become far more fraught. We acknowledge “[sincerity’s] decline with a kind of resentful remorse.”

Fortunately, Sincerity is not one of those philosophy books that bursts into a self-help manual. Magill has written a dense and intriguing cultural history, teasing out his subject’s variations and contradictions, and flirting with the influence of sincerity’s sort-of opposite, irony. Without irony, he notes, there would be no art, literature, Jon Stewart, or effective politicians. Ultimately, however, it is sincerity’s scarcity that concerns him. “The bolder, more democratic move,” he says, “would be to simply offend.”

Before he arrives at sincerity’s present-day rarity, Magill agilely traces his subject through the ages. The word first appeared as a description of objects. A sword was “sincerely” bronze, meaning pure. When, around 1533, sincerity first began to be used to describe people, it referred to faith. The definition broadened when, in the Elizabethan era, the birth of theater, along with the relaxing of sumptuary laws, meant that it was more difficult to tell sincerity by appearance.

For several centuries, sincerity was alternately celebrated and reviled. Machiavelli advised princes that they need only look sincere, since it was unlikely that any peasants would get close enough to tell the difference. (This has a familiar ring.) Montaigne was the first to prize sincerity enough to write about himself as he really was. The most appealing seventeenth-century commentator on the subject is La Rochefoucauld, whose maxims zero in on how easy it is to fake sincerity and how hard it is to be sincere. For instance: “What usually passes for sincerity is only an artful pretense designed to win the confidence of others.”