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.

眼前的景象似乎总在最不应该的时候--或者在面试现场，或者在和老板开会，或者是在惴惴不安的第一次约会时，或者正出席一场重要的晚宴--被我们脑袋里涌出的歪念头搅得一团糟。用这些开胃小菜来个食物大战怎么样？嘲笑主人磕巴的口齿？还是口无遮拦的发表种族评论？

"单单一个想法就够了，"埃德加·艾伦·坡在一篇关于这些邪恶念头的小说《反常之魔》中写道。"冲动增长为愿望，愿望变为欲望，欲望导致无法抑制的渴求。"

他还补充道，"事实上，没有什么激情能像它那样恶魔般的蠢蠢欲动--当你在悬崖边瑟瑟发抖，便以求一跳了之。"

或者也可以好好考虑一下这个问题：我是不是病了？

在不少情况下，答案是肯定的。但是大多数人绝少如此冲动的行事，事实上，他们对那些怪念头的敏感正反映了一个正常灵敏的社会化大脑的工作机制，一篇上周发表在《科学》杂志上的论文如是说。

"社交生活中我们目光所至的每个地方都充斥着各种诱惑，不只是小过失，甚至是可能发生的最糟的过失都能轻易的溜进脑中，"论文的作者，Daniel M. Wegner,一位哈佛大学的心理学家说。"并且在某些情况下，越是想它，它就越有可能发生。"

历史上不乏对于这类反常冲动的探索：从坡的小说和萨德侯爵到弗洛伊德被抑制的欲望和达尔文对许多表现"与主观愿望大相径庭"的行为的观察。在过去的十年里，社会心理学家们证明了这些冲动的普遍性以及它们在何时最容易左右人的行为。

基本上，社会化的活动意味着控制一个人的冲动。有研究表明，成年人的大脑用于抑制冲动的能量与用于行动本身的能量至少相当，而心理健康有赖于人们能忽略或压制内心深处那些让人不安的想法。例如，对于每个人都不可避免的死亡，这种忽略或压制是一种普遍的、无意识或半意识的心理机制，它们通常自动发挥作用。

当人们非常紧张的试图避免犯某种错误或禁忌时，反常冲动似乎就出现了。这一理论一目了然：为了避免不经意间批评一个同事是彻头彻尾的伪君子，大脑必须首先想象由此造成的可怕后果，而反过来，这种想象又增加了事情发生的几率。

"我们知道，脑中的既得信息会对我们的判断和行为施加影响，这就是因为它在那儿，它漂浮在我们的意识表层。"密苏里大学的心理学家Jamie Arndt说。

有关这种影响的经验证据近年来已被大量发现，正如Weger博士在他的新论文中证明的一样。心理学家在实验室中要求人们试着从思想里摒除某个念头 --例如，一头白熊--而后发现这个念头会大约每隔一分钟返回一次。与之类似的，虽然试着不去想起某个特定的单词，但在快速的单词联想测试中，人们却不断的将其脱口而出。

同样的"讽刺性错误"--Wegner博士这样称呼它们--在现实世界中也非常容易发生。研究发现，被要求避免某个特定失误（例如过度击球）的高尔夫球手，在压力下往往更容易犯错。罚点球时被告知不要射向某点（比如右下角）的足球运动员，会比别的点更多注意那一点。

对于试图做到没有偏见的努力，其效果尤为南辕北辙。在一项研究中，西北大学和里海大学的研究者们让73名学生阅读关于一个虚构的黑人男性Donald的小短文。学生们看了他的照片并读到有关他和一位朋友去购物中心的一段记述。

在拥挤的停车场，Donald从不在残障车位停车，即使开着他祖母有残障车牌的车也一样，但他会一头插到另一个司机的前面去抢非残障车位。他斥退了一个为心脏基金募捐的人，而他的朋友则捐了一些零钱等等。这个故事有意识的用这样一种模棱两可的方式对主人公进行描绘。

研究者让一半学生在阅读时试着压制对黑人男性糟糕的刻板印象，然后通过例如诚实、敌对以及懒惰等量度判断Donald的性格。与另一半没有压制刻板印象的学生相比，这些学生显着的认为Donald更富有敌意--但同时也更诚实。

简而言之，试图摒弃偏见的努力在某种程度上起作用了。但研究也"强有力的证明了对刻板印象的压制将导致刻板印象的高影响度",作者总结道。

烟民、重度酗酒者和其他药物依赖者对这样的困惑再了解不过了：在试图抑制对烟或酒的渴望，给自己各种戒烟戒酒的理由的同时，那被试图压制的欲望似乎倒变的更强烈了。

Wegner博士认为，人们把持不住自己的风险在某种程度上取决于他们所经受的压力水平。集中注意使自己不要盯着一个新认识的人脸上突出的痣看，以及避免在交谈中谈及此事，都将增加说出这样的话的风险："我们去痣（We went to the mole）--我的意思是说购物中心，购物中心（mall）!"

"就让最糟糕的事发生吧，克服这一点会使焦虑得以减轻，所以你没必要再提心吊胆的时时监督自己。"Wegner博士说。

所有这一切都难以解释，当然，如果你刚刚在晚宴上神游太虚的话。