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.

    眼前的景象似乎總在最不應該的時候--或者在面試現場，或者在和老板開會，或者是在惴惴不安的第一次約會時，或者正出席一場重要的晚宴--被我們腦袋里涌出的歪念頭攪得一團糟。用這些開胃小菜來個食物大戰怎么樣？嘲笑主人磕巴的口齒？還是口無遮攔的發表種族評論？

    "單單一個想法就夠了，"埃德加·艾倫·坡在一篇關于這些邪惡念頭的小說《反常之魔》中寫道。"沖動增長為愿望，愿望變為欲望，欲望導致無法抑制的渴求。"

    他還補充道，"事實上，沒有什么激情能像它那樣惡魔般的蠢蠢欲動--當你在懸崖邊瑟瑟發抖，便以求一跳了之。"

    或者也可以好好考慮一下這個問題：我是不是病了？

    在不少情況下，答案是肯定的。但是大多數人絕少如此沖動的行事，事實上，他們對那些怪念頭的敏感正反映了一個正常靈敏的社會化大腦的工作機制，一篇上周發表在《科學》雜志上的論文如是說。

    "社交生活中我們目光所至的每個地方都充斥著各種誘惑，不只是小過失，甚至是可能發生的最糟的過失都能輕易的溜進腦中，"論文的作者，Daniel M. Wegner,一位哈佛大學的心理學家說。"并且在某些情況下，越是想它，它就越有可能發生。"

    歷史上不乏對于這類反常沖動的探索：從坡的小說和薩德侯爵到弗洛伊德被抑制的欲望和達爾文對許多表現"與主觀愿望大相徑庭"的行為的觀察。在過去的十年里，社會心理學家們證明了這些沖動的普遍性以及它們在何時最容易左右人的行為。

    基本上，社會化的活動意味著控制一個人的沖動。有研究表明，成年人的大腦用于抑制沖動的能量與用于行動本身的能量至少相當，而心理健康有賴于人們能忽略或壓制內心深處那些讓人不安的想法。例如，對于每個人都不可避免的死亡，這種忽略或壓制是一種普遍的、無意識或半意識的心理機制，它們通常自動發揮作用。

    當人們非常緊張的試圖避免犯某種錯誤或禁忌時，反常沖動似乎就出現了。這一理論一目了然：為了避免不經意間批評一個同事是徹頭徹尾的偽君子，大腦必須首先想象由此造成的可怕后果，而反過來，這種想象又增加了事情發生的幾率。

    "我們知道，腦中的既得信息會對我們的判斷和行為施加影響，這就是因為它在那兒，它漂浮在我們的意識表層。"密蘇里大學的心理學家Jamie Arndt說。

    有關這種影響的經驗證據近年來已被大量發現，正如Weger博士在他的新論文中證明的一樣。心理學家在實驗室中要求人們試著從思想里摒除某個念頭 --例如，一頭白熊--而后發現這個念頭會大約每隔一分鐘返回一次。與之類似的，雖然試著不去想起某個特定的單詞，但在快速的單詞聯想測試中，人們卻不斷的將其脫口而出。

    同樣的"諷刺性錯誤"--Wegner博士這樣稱呼它們--在現實世界中也非常容易發生。研究發現，被要求避免某個特定失誤（例如過度擊球）的高爾夫球手，在壓力下往往更容易犯錯。罰點球時被告知不要射向某點（比如右下角）的足球運動員，會比別的點更多注意那一點。

    對于試圖做到沒有偏見的努力，其效果尤為南轅北轍。在一項研究中，西北大學和里海大學的研究者們讓73名學生閱讀關于一個虛構的黑人男性Donald的小短文。學生們看了他的照片并讀到有關他和一位朋友去購物中心的一段記述。

    在擁擠的停車場，Donald從不在殘障車位停車，即使開著他祖母有殘障車牌的車也一樣，但他會一頭插到另一個司機的前面去搶非殘障車位。他斥退了一個為心臟基金募捐的人，而他的朋友則捐了一些零錢等等。這個故事有意識的用這樣一種模棱兩可的方式對主人公進行描繪。

    研究者讓一半學生在閱讀時試著壓制對黑人男性糟糕的刻板印象，然后通過例如誠實、敵對以及懶惰等量度判斷Donald的性格。與另一半沒有壓制刻板印象的學生相比，這些學生顯著的認為Donald更富有敵意--但同時也更誠實。

    簡而言之，試圖摒棄偏見的努力在某種程度上起作用了。但研究也"強有力的證明了對刻板印象的壓制將導致刻板印象的高影響度",作者總結道。

    煙民、重度酗酒者和其他藥物依賴者對這樣的困惑再了解不過了：在試圖抑制對煙或酒的渴望，給自己各種戒煙戒酒的理由的同時，那被試圖壓制的欲望似乎倒變的更強烈了。

    Wegner博士認為，人們把持不住自己的風險在某種程度上取決于他們所經受的壓力水平。集中注意使自己不要盯著一個新認識的人臉上突出的痣看，以及避免在交談中談及此事，都將增加說出這樣的話的風險："我們去痣（We went to the mole）--我的意思是說購物中心，購物中心（mall）!"

    "就讓最糟糕的事發生吧，克服這一點會使焦慮得以減輕，所以你沒必要再提心吊膽的時時監督自己。"Wegner博士說。

    所有這一切都難以解釋，當然，如果你剛剛在晚宴上神游太虛的話。