Yet for scientists who study mental illness and addiction, impulsive behavior " the tendency to act or react with little thought " has emerged as an all-purpose plague.

In recent years, studies have linked impulsiveness to higher risks of smoking, drinking and drug abuse. People who attempt suicide score highly on measures of impulsivity, as do adolescents with eating problems. Aggression, compulsive gambling, severe personality disorders and attention deficit problems are all associated with high impulsiveness, a problem that affects an estimated 9 percent of Americans, according to a nationwide mental health survey completed last year.

Now researchers have begun to resolve the contrary nature of impulsivity, identifying the elements that distinguish benign experimentation from self-destructive acts. The latest work, in brain research and psychological studies, helps explain how impulsive tendencies develop and when they can lead people astray. A potent combination of genes and emotionally disorienting early experiences puts people at high risk, as do some very familiar personal instincts.

"What we're seeing now," said Charles S. Carver, a psychologist at the University of Miami in Coral Gables, Fla., "is a rapid convergence of evidence indicating that when the prefrontal cortical areas of the brain, the brain's supervisory management system, are not functioning well, this interferes with deliberative behavior, and the consequences are often unpleasant."

Few experts dispute that impulsiveness pays off in some situations and, perhaps, had evolutionary benefits. When life is short and dangerous, and resources are scarce, there is a premium on quick response. In studies of baboons and monkeys, researchers have found that animals that are impulsive as adolescents often become dominant as adults, when they moderate their confrontational urges.

In humans, impulsive behavior typically peaks in adolescence, when the prefrontal areas of the brain continue to develop, or soon after, in the young adult years, when it is culturally expected that people will test their limits, psychologists have found.

Yet new research suggests that a taste for danger or conflict is not enough to produce persistent, ruinous impulsivity.

In a study published online last month in The Journal of Psychiatric Research, Janine D. Flory, a psychologist at the Mount Sinai School of Medicine in Manhattan, led a team of investigators who studied 351 healthy adults and 70 others with impulse-related disorders like antisocial and borderline personality disorders. The participants took a battery of tests to measure inhibition, appetite for risk and the inclination to plan.

Analyzing the responses to questions intended to gauge thrill seeking like, "I like to explore a strange city or section of town by myself, even if it means getting lost," and, "I like to try foods I've never tried before," the researchers found that an appetite for risk was associated with smoking in both groups.

But in the healthy volunteers, the appetite was also associated with higher education. In previous studies, healthy risk seekers scored highly for curiosity and openness to new experiences. On measurements of instinctive planning " "I am better at saving money than most people" and "I hate to make decisions based on first impressions"" the researchers found that less deliberative habits were related to heavy drinking in the healthy group and the troubled group.

In cases with personality disorders, deficits in planning were also associated with a history of suicide attempts. The combination of sensation seeking and lack of deliberation characterizes millions of healthy people but appears to be extreme in those whose impulsivity leads to chronic trouble or mental illness, Dr. Flory said.

"The way I think of it is that one factor has to do with the urges people have, and the other has to do with the brakes they apply," she said.

How and when people apply the brakes is crucial to distinguishing those who can flirt with regular heroin or cocaine use while finishing an Ivy League degree and those who die trying.