"Sometimes when I'm driving on the road at night I see two headlights coming toward me, fast, and I have this sudden impulse to turn the wheel quickly, headlong into the oncoming car. I can anticipate the explosion, the sound of shattering glass, flames rising out of the gasoline."

-- from Woody Allen's "Annie Hall"

At one time or another, you may have carried on a similar (though probably less dramatic) deliberation, wondering what it would be like to pull into oncoming traffic, drive off a bridge or ram another car. Some people may ponder the consequences of jumping from a great height, while others wonder what it would feel like to cut themselves.

Some people, in the quiet of a church or concert hall, inexplicably think about standing and shouting obscenities. Then there is a range of violent sexual and murderous fantasies, which may be quite lurid. And we all think these thoughts far more often than you'd imagine.

"These are the 'whispers' of obsessive thoughts," explains Donald Weatherley, who teaches abnormal psychology and psychopathology at the University of Colorado. "An obsession is a thought with a peculiar quality. It's one you wish you didn't have, because it often has a content that's destructive or unpleasant."

Weatherley notes that the major difference between such an obsession and obsessive-compulsive behavior is that the individual doesn't feel driven to perform the act in question (and usually feels guilty for thinking about it at all).

"With such obsessions, what's lacking is the urge to do the thing," he says. "It's just a thought without motivation, and you're not really having to restrain yourself. The people who actually do godawful impulsive acts don't often do this kind of thinking, while the kind of people who get obsessions tend to act less and think more. It's extremely rare that people with these obsessions actually do the thing they fear, so what you do with people is reassure them that they're not going to act when they have this obsession."

The exceptions, where someone follows through on an obsessive thought, may happen as a result of chemical changes in a person. "When people lose control because of alcohol, drugs or organic changes from senility, then all bets are off," Weatherley says. "That may change a person's self-control capacity, and tends to make doing something that is not useful or adaptive more likely."

"Obsessive rumination is a different process from mentally rehearsing something, where someone is rehearsing an act he's getting closer and closer to carrying out," concurs Erwin Dreiblatt, a psychologist at the University of Washington in Seattle. "I get calls from people who say, 'I'm really afraid I'm going to molest a child,' or 'I'm afraid I'm going to rape someone.' Most of these people have been troubled by these obsessions for years, but the thoughts tend to be irrational and have no motivation behind them. The people rehearsing, on the other hand, tend never to call before they do it."

Another peculiar quality about obsessions is their usually impersonal nature. Obsessive thoughts seem to come randomly. When one thinks about, say, running someone down with a car, it's usually with a sense of cold objectivity. Emotional feelings are detached from obsessive thoughts by a defense mechanism called isolation, which Weatherley describes as an exaggeration of the normal defense a doctor has when he thinks of a patient in an impersonal way.

"When people go to look at an accident, it's not necessarily that they're curious about it in a morbid way," explains Dr. William Crary, a clinical psychiatrist with the University of Southern California Medical Center. "They actually want to know if they could stand to do it themselves."

So if obsessions are seldom acted upon, why do they show up? "The answer is we don't know," says Wayne Goodman, director of Yale University's Obsessive-Compulsive Clinic. "In the past, emphasis was put on symbolic meanings, where we would trace the roots of obsession to childhood conflicts. More recent work de-emphasizes that."

According to Weatherley, they can serve as symptoms of a person's emotional condition. "The most interesting question to ask people is 'Why is this happening to you now?,' " he says. "In this symptom, the person is expressing some doubt about the smoothness and certainty with which he can control his emotions and impulses. Since a little slip while driving, for example, would be disastrous, the person comes up with this preoccupation of having that slip. It may be self-destructuve, it may be a code that says, 'Something in my life is screwed up.' "

Often, a repressed emotion or desire lies at the root of obsessive thoughts. For example, you may dislike living in Boulder, Colo., "and you have this awful obsession come up when you're driving in the mountains," Weatherley continues. "Then you decide you have to get out of here, you leave and the thing goes away. So it turns out to have been a way of getting yourself out of Boulder instead of confronting whatever problem you had here directly."

Although obsessions are common, people often feel guilty for having them. "Most people don't actually think they're going to do these things, but thinking about it makes them feel dirty," Weatherley says.

Still, obsessive thoughts shouldn't be considered twisted or warped. "These usually aren't the sorts of things people have their whole lives," says Weatherley. "They tend to come in a flurry, then recede into the background. I get a lot of heads nodding in agreement in class when I talk about this, because almost everyone has had them. What's called abnormal actually occurs in everyday life; it just becomes abnormal when it's carried to such an extreme that it interferes with everyday life. Most people have some aspects of the symptoms that I study. Having them doesn't mean you're screwed up."

David Menconi writes for The Boulder Daily Camera, from which this article was excerpted.