The visions seem to swirl up from the brain’s sewage system at the worst possible times — during a job interview, a meeting with the boss, an apprehensive first date, an important dinner party. What if I started a food fight with these hors d’oeuvres? Mocked the host’s stammer? Cut loose with a racial slur?

“That single thought is enough,” wrote Edgar Allan Poe in “The Imp of the Perverse,” an essay on unwanted impulses. “The impulse increases to a wish, the wish to a desire, the desire to an uncontrollable longing.”

He added, “There is no passion in nature so demoniacally impatient, as that of him who, shuddering upon the edge of a precipice, thus meditates a plunge.”

Or meditates on the question: Am I sick?

In a few cases, the answer may be yes. But a vast majority of people rarely, if ever, act on such urges, and their susceptibility to rude fantasies in fact reflects the workings of a normally sensitive, social brain, argues a paper published last week in the journal Science.

“There are all kinds of pitfalls in social life, everywhere we look; not just errors but worst possible errors come to mind, and they come to mind easily,” said the paper’s author, Daniel M. Wegner, a psychologist at Harvard. “And having the worst thing come to mind, in some circumstances, might increase the likelihood that it will happen.”

The exploration of perverse urges has a rich history (how could it not?), running through the stories of Poe and the Marquis de Sade to Freud’s repressed desires and Darwin’s observation that many actions are performed “in direct opposition to our conscious will.” In the past decade, social psychologists have documented how common such contrary urges are — and when they are most likely to alter people’s behavior.

At a fundamental level, functioning socially means mastering one’s impulses. The adult brain expends at least as much energy on inhibition as on action, some studies suggest, and mental health relies on abiding strategies to ignore or suppress deeply disturbing thoughts — of one’s own inevitable death, for example. These strategies are general, subconscious or semiconscious psychological programs that usually run on automatic pilot.

Perverse impulses seem to arise when people focus intensely on avoiding specific errors or taboos. The theory is straightforward: to avoid blurting out that a colleague is a raging hypocrite, the brain must first imagine just that; the very presence of that catastrophic insult, in turn, increases the odds that the brain will spit it out.

“We know that what’s accessible in our minds can exert an influence on judgment and behavior simply because it’s there, it’s floating on the surface of consciousness,” said Jamie Arndt, a psychologist at the University of Missouri.

The empirical evidence of this influence has been piling up in recent years, as Dr. Wegner documents in the new paper. In the lab, psychologists have people try to banish a thought from their minds — of a white bear, for example — and find that the thought keeps returning, about once a minute. Likewise, people trying not to think of a specific word continually blurt it out during rapid-fire word-association tests.

The same “ironic errors,” as Dr. Wegner calls them, are just easy to evoke in the real world. Golfers instructed to avoid a specific mistake, like overshooting, do it more often when under pressure, studies find. Soccer players told to shoot a penalty kick anywhere but at a certain spot of the net, like the lower right corner, look at that spot more often than any other.

Efforts to be politically correct can be particularly treacherous. In one study, researchers at Northwestern and Lehigh Universities had 73 students read a vignette about a fictional peer, Donald, a black male. The students saw a picture of him and read a narrative about his visit to a mall with a friend.

In the crowded parking lot, Donald would not park in a handicap space, even though he was driving his grandmother’s car, which had a pass, but he did butt in front of another driver to snag a nonhandicap space. He snubbed a person collecting money for a heart fund, while his friend contributed some change. And so on. The story purposely portrayed the protagonist in an ambiguous way.

The researchers had about half the students try to suppress bad stereotypes of black males as they read and, later, judged Donald’s character on measures like honesty, hostility and laziness. These students rated Donald as significantly more hostile — but also more honest — than did students who were not trying to suppress stereotypes.

In short, the attempt to banish biased thoughts worked, to some extent. But the study also provided “a strong demonstration that stereotype suppression leads stereotypes to become hyperaccessible,” the authors concluded.

Smokers, heavy drinkers and other habitual substance users know this confusion too well: the effort to squelch a longing for a smoke or a drink can bring to mind all the reasons to break the habit; at the same time, the desire seemingly gets stronger.

The risk that people will slip or “lose it” depends in part on the level of stress they are undergoing, Dr. Wegner argues. Concentrating intensely on not staring at a prominent mole on a new acquaintance’s face, while also texting and trying to follow a conversation, heightens the risk of saying: “We went to the mole — I mean, mall. Mall!”

“A certain relief can come from just getting it over with, having that worst thing happen, so you don’t have to worry about monitoring in anymore,” Dr. Wegner said.

All of which might be hard to explain, of course, if you’ve just mooned the dinner party.