Once upon a time, or so we’re told, the princes were charming, the villains were grotesque, and distinguishing between them was as simple as examining their manners and their minds. When Rapunzel meets her overeager home invader of a nobleman, she finds him “friendly”—so much so that, when he promptly asks her for her heart, she happily obliges. On the other hand, “It must not be denied but I am a plain-dealing villain. I am trusted with a muzzle,” Don John, the cruel and moody brother in “Much Ado About Nothing,” says. Deciding who gets muzzled and who receives ladders of golden locks is trickier these days, thanks largely to a modern figure who is neither purely good nor wholly evil, who’s both arresting and unsettlingly weird. In Dickens’s time, such a person might be described as eel-like or clammy. Today, he’s best known as a creep.

Are creeps, and their defining quality—creepiness—to our age what anxiety was to postwar life? The proposition is embarrassing, but, then, so is everything the pruney fingers of creepiness touch. Half a century ago, there were squares and libertines, stalwarts and histrionics, private lives and public personalities. Today, in our self-scrutinizing, liberated time, these categories have got scrambled, and distinguishing between a charmingly revealing Instagram post and a bomb of oversharing requires daunting feats of judgment. Looming behind many missteps is the threat of creepiness: a fear that, out of all the free paths open to the modern social actor, you have picked the one that is invasive, obviously needy, and perverse.

It is tempting to posit that, when it comes to creepiness, we know it when we see it. But we don’t, really, not for sure, and that’s the trouble. Creepiness is the nightmare version of seduction: it can crop up in an instant, in a gesture or an ill-starred turn of phrase, and yet it takes much longer to expunge from the imagination. Is John Travolta creepy as a disco predator in “Saturday Night Fever”? Hard to say. Is he creepy as a weathered, black-clad gentleman who leers at women while fondling their chins? Yes, very much, and now forever more. What changed?

Creepiness is often judged to be a male problem, like baldness or the wearing of loafers without socks. That’s probably because creepiness carries a vague, erotically tinged threat, and men are classically predatory. (It’s telling that many iconically creepy women—Mrs. Danvers, the austere and homoerotic housekeeper in “Rebecca,” or Catherine Tramell, the cool putative murderess of “Basic Instinct”—seem to pose a physical and sexual menace; the Miss Havishams of the canon, bizarre and aloof, are usually mere eccentrics.) Creepiness suggests unknown potential: the creepy person may appear so phony, or so mistuned to social constraints, that we don’t know what he could do. And so we seek the evidence of minor maladjustment—heavy breathing, strange mustaches, leering conduct onstage at the Oscars—to predict a deeper disconnection. Creeps don’t let their freak flags fly. They try to curb their wild tendencies and blend in, lacking the restraint or fine-grained social acumen to do so in a totally persuasive way. In this sense, possibly, they’re more like normal people than a lot of us would like to say.

The more fragile that boundary gets, the more fretful we grow. Print use of “creepiness,” after holding steady for a couple of postwar decades, began climbing precipitously during the George H. W. Bush and Bill Clinton Presidencies, a fact that will shock no one. (The same is true of “creepy.”) In Victorian England, where the world rested on niceties, the great terror was being thought ill-mannered: behavior of that kind marked an unpredictable person, teetering on the edge of the social compact—what Thackeray variously called a “cockney dandy” and a “padded booby.” Today, when manners are much looser and the boobies lie low, creepiness is our red flag. It helps to brand the people whom, for our own safety, we might rather keep at bay.

Analysis couldn’t be far behind. In a new book, titled, simply, “Creepiness,” Adam Kotsko, an assistant professor at Shimer College, tries to trace the quality as it has evolved through characters in media and pop culture. “My intent is not to ‘diagnose’ the characters as though they were real people, but rather to show how they conform to certain ideal types,” he writes. The creepy types are haunting, he believes, due to their “peculiar power of attraction.” Kotsko’s previous books are “Awkwardness” and “Why We Love Sociopaths: A Guide to Late Capitalist Television,” so it is possible that his feelings of attraction to the offbeat are a little overindulged. Still, his study is illuminating, maybe in more than the manner strictly intended.

Kotsko’s big idea is that creepiness played a considerable but unrecognized role in the oeuvre of Sigmund Freud. “Freud’s theory is about the inherent creepiness of human desire,” he writes. Specifically, Freud elaborated an ineffable quality that he termed the “unheimlich”—usually expressed in English as “uncanny.” On Freud’s list of suggested translations, Kotsko also finds “uncomfortable, uneasy, gloomy, dismal, uncanny, ghastly; (of a house) haunted; (of a man) a repulsive fellow.” What Freud was actually talking about, Kotsko thinks, was creepiness.

In some sense, such a revelation seems a century too late. Freudian analyses are démodé today, partly because Freud, a conspiracy theorist of the mind, had problems separating scientific reason from hokum. Kotsko nods to such objections but sweeps them aside. If Freud was our great theorist of creepiness, he suggests, then he remains the thinker for our times. Taking up Freud’s ideas, he starts his own book with an icon of creepy behavior: the mask-wearing Burger King mascot who became known, in 2004, as “The Creepy King.” This character was infamous for cropping up in the bedrooms of men, ogling them with his sourire figé. Kotsko writes: