Protecting potential targets of my patients' aggression is one of the responsibilities as a senior resident in psychiatry. Although dangerousness is difficult to predict, there is a part of the routine psychiatric assessment that focuses on whether a person is thinking of harming -- or even killing -- someone.

It is a difficult question to ask, and I used to squint and say something like, "I hope you're not offended; I need to ask all my patients this: Have you had thoughts of harming anyone?"

The squint was a mix of embarrassment and fear. Tapping into someone's rage feels a lot like exploring his or her sexuality. Both are intensely private emotions and even after nearly three years of residency, some remnant of the child in me still wonders what business it is of mine. Another part is afraid of the answer: that exploring such feelings, in fact, has become my stock in trade. I am responsible and, even more formidable, trusted to explore them.

In the face of possible risk to another person, I have a legal duty to either hospitalize a dangerous patient, alert the police or warn identifiable potential victims. This obligation is the result of a landmark 1974 malpractice decision by the California Supreme Court known as the Tarasoff case. Tatiana Tarasoff, a student at the University of California at Berkeley, was murdered by a graduate student who, disappointed that she was not responding to his romantic advances, told his psychiatrist that he fantasized about killing her. Although the psychiatrist notified university police, he did not contact city police, involuntarily hospitalize the patient or warn Tarasoff, whom the patient subsequently stabbed to death.

I have to check myself as the patient answers my questions about violence, because I always hope for a clear disavowal of violent intent. The promise of safety clarifies my allegiances, allowing me to focus my attention less on the outside world and more on the person seeking help. There is great comfort in the traditional confidentiality of the doctor-patient relationship, so much so that part of me would like to accept as sufficient an ambiguous response like, "I don't think I have it in me to ever really hurt him." Yet another more skeptical part of myself forces me to say: "I need to know if there is even a chance that you might."

Dangerousness is as much a feeling as a calculation. It takes its dimensions from a patient's past history, as well as current thoughts, appearance and behavior. One patient might talk openly of homicidal feelings toward a lover. Another may deplore violence but have his or her judgment clouded by psychoses. Still another, known to be seriously assaultive in the past, might come to the emergency room voluntarily -- perhaps carrying a pocket knife, only to then refuse to answer questions.

There is a gravity in the moment when it becomes clear a patient cannot convince me that he or she is not a risk to others. It is as if an interpersonal hall of mirrors suddenly reveals a gaping new distance between us. I try closing it by reminding myself how frightening it must feel to be alone with so much rage. I can sometimes hear the patient's revelation as a plea to be held -- literally and figuratively. Other times, on call without sleep, seeing the patient in the emergency room at 2 a.m., I think of the hours it can take to arrange for a bed on a locked ward. But I check myself, again.

Sometimes, I feel a duty to warn a potential victim even when the patient does not require further observation or hospitalization. A patient's promise that he or she won't hurt another person relies heavily on the patient's own appraisal of his or her self-control. Often, a patient is able to maintain self-control with the support and structure of a psychiatric ward. Some patients make threats when they are inebriated and then vehemently retract them following a night spent in the emergency room, after the alcohol has left their systems. "I said what? That's crazy," patients frequently tell me, "I was just drunk." But what about the next binge, I wonder?

Assessing dangerousness requires an appraisal of the patient's reliability. One person may confide violent fantasies that cross the line of confidentiality, while another may be utterly convincing in assuring me that thought will never turn to action.

I side with caution. I do not need to be convinced beyond a reasonable doubt of a patient's dangerousness. The doubt itself is reason enough for me to protect the community. I am not quick to risk my own safety, but I am slower still to risk the safety of others. The imprecision of this science engenders a healthy respect for the unpredictable.

I recall sitting in the center of a Boston emergency room last year, in a glass booth, doing paperwork. My patient had finally slept off his binge and assured me that his threat to kill his mother was just so much vodka. He loved her, he said, and would never hurt her.

I looked through the observation window at him, turned around and picked up the phone. I dialed, and that same embarrassment returned, the feeling that I was taking myself too seriously, playing emergency room psychiatrist. I wondered if I might be setting in motion an irrational chain of fear. Was murder really a possibility?

"I've been working with your son in the emergency room," I told the man's mother.

"He's been drinking again," she sighed. "Is he all right?"

"He's better now." I closed my eyes. "Actually, one reason I'm calling is to get your thoughts. When your son was intoxicated, he spoke of wanting to harm you," I said, squinting. "He even mentioned killing you. Has he ever hurt you physically?"

"Never. He'd never lay a hand on me. He talks nonsense sometimes. Send him home."

I continued: "He does seem calm now. He denies wanting to hurt you. But I wonder if you know how to get a restraining order, should you need one." I could feel that my face was flushed.

Since that night, I have had other, more personal, encounters with dangerousness. I have been assaulted by a state hospital patient who ended his flurry of punches with a simple, "No hard feelings" -- and meant it. I have been threatened by an outpatient brandishing a knife. I have sat with people convicted of rape and murder who were perfectly pleasant to me. I have been shocked at hearing that patients whose names and faces I recognize from our wards or emergency rooms have gone on, in fact, to kill themselves or others.

And I have lost the embarrassment I used to feel when I found myself confronting what Freud called "the half-tamed demons that inhabit the human breast."

Now, I anticipate them. I don't think myself overly dramatic when I try to convey what seem even distant risks to the uninitiated. I know that the fear I sometimes inspire in potential victims is real and justified. I find myself repeating the warning, documenting it, checking to make sure that my new familiarity with danger does not make me jaded, immune to recognizing its subtleties or to communicating them.

Keith Russell Ablow is a senior resident in psychiatry at New England Medical Center in Boston.