They jump off cliffs attached to powerless, rudderless gliders, hoping to fly on fickle winds. They drop, skis in tow, from helicopters onto slopes so steep and snowy that there's no telling where the runs might end.

They race in souped-up sports cars, driving fast enough to melt the tires. They climb with and without ropes up cliffs nearly perpendicular to the ground.

Some leap from bridges, trusting their lives to long rubber bungee cords. A few even attempt to scale Mount Everest

without oxygen or sail the Pacific in boats not much bigger than they are.

Risky sports -- feats of bravado to some, madness to most -- are attracting increasing numbers of seemingly normal individuals, from assembly-line workers and toll collectors to doctors and dentists. All are seeking a thrilling adventure. All are out to have fun, and no one expects to die in the process.

Yet die they do, as did nine "heli-skiers" in March who were swallowed by an avalanche while skiing in Canada's Bugaboo Mountains. The annual toll among participants in high-risk sports could make an undertaker rich.

And the far more numerous non-fatal injuries are forcing many orthopedists to become sports medicine specialists and prompting neurologists to try even harder to find a way to repair severed spinal cords.

Hang gliding, rock climbing, parachute jumping, scuba diving, sports car racing. . . . The list of daredevil sports keeps growing as modern life in mechanized society becomes ever duller and more unchallenging.

Who are these people whose idea of a good time is to risk their lives? And why do they do it? Is it just excitement and challenge?

Here is what half a dozen experts have to say about those who put their lives on the line in the name of fun.

"All risk-taking -- not just in sports -- involves magical thinking, the belief that while others may die, they will survive because fatal accidents result from mistakes that they wouldn't make," said Dr. George Serban, research psychiatrist at New York University.

In sports that challenge the environment, he said, "participants often realize that the activity is dangerous and has to be handled carefully. But they expect to overcome the obstacles and emerge as conquerors or celebrities," not corpses, he said.

From a cultural perspective, Dr. Willard Gaylin sees dangerous sports as "a substitute for the kind of derring-do that used to be necessary for survival."

"Sports were created to allow men to run mock risks to prove their manhood," said Gaylin, a New York psychiatrist. "Men no longer have to do daring feats to protect hearth and home. There are no longer any rites of passage, so we invent rituals in the form of risky sports to take their place."

Despite the feminist movement, the phenomenon is dominated by men. Fewer than 1 percent of hang gliders are female, for example.

"Most women don't feel the need to prove their womanhood through acts of courage, but men do," Gaylin said.

"We all live in a culture where the need for individual achievement is very strong and drummed into us by parents and teachers," said Dr. David Klein, a sociologist.

"But as work becomes more and more routinized, the opportunities to excel at work are very limited, so we turn to recreation for a sense of accomplishment," said Klein, an emeritus professor of sociology at Michigan State University who lives in Edmonds, Wash.

Klein has observed strong socioeconomic differences in the types of risky activities people pursue. Among less educated people who have not experienced the pleasure of seeing small improvements over the course of a long learning period, the choice is usually "a rapidly learned activity that is highly lethal, like snowmobiling or running a high-powered outboard motorboat."

More highly educated people, Klein maintains, tend to pursue different kinds of activities, such as learning to play chess or a harpsichord, where the risks involve the chances of successful completion, not physical danger.

Dr. David Campbell, a psychologist who directs the Center for Creative Leadership in Colorado Springs, Colo., also noted that "we are making the country so safe that people have to go out on a limb to find excitement."

Klein agreed, saying, "The safer and more routine we make work, the more we will push people into recreations where individual distinction and discretion, adventure and excitement play a part." That goes for doctors and dentists as well as blue-collar workers.

"Why do some people think it's fun to put their heads in the lion's mouth while others avoid it at all costs?" asked Dr. Richard Hernnstein, professor of psychology at Harvard University. People vary in "sensation seeking," he has found.

"You can test to see if people have an appetite for unusual or what some would consider frightening stimuli, such as becoming test pilots or astronauts," he said.

Criminals show "elevated scores in sensation-seeking tendencies," he said, a kind of Bonnie-and-Clyde syndrome.

In extreme sensation seekers, Hernnstein said, brain patterns show an unusually low level of arousal. When nothing highly stimulating is happening, the electrical patterns from their brain resemble those from normal sleep. These are the people, Hernnstein said, who might say "I only feel really alive when I'm doing something dangerous."

A "young-god syndrome" -- a self-image of invulnerability -- characterizes participants in high-risk sports such as hang gliding, said Dr. John R. Tongue, an orthopedic surgeon in Tualatin, Ore., who has studied hang-gliding injuries and fatalities and why they occur.

The overwhelming majority of participants are men 18 to 28, a group that tends not to be safety conscious in any endeavor, he said, noting that they are also most likely to take chances on the road and be injured or killed in motor vehicles.

The thrill of adventure often overcomes better judgment in high-risk sports. Tongue told of one hang glider who was so excited about jumping off the mountain that he forgot to attach his harness.

Participants are often further goaded on by the effort and cost involved in getting to the starting point, he said. After spending hours hauling his gear up the hill, a glider may try to take off even though wind conditions have become unfavorable.

The same is true for heli-skiers, who spend as much as $4,000 in advance and don't want to waste hours or days waiting for the weather to improve, or who take the chance that skiing on slopes of questionable stability won't trigger an avalanche.

Alcohol and other drugs can enter the picture, further clouding participants' judgment. Intoxicating levels of alcohol are found in 20 percent to 30 percent of those killed while hang gliding, Tongue reported. Hang gliders also tend to resist wearing helmets, saying the safety devices interfere with their sensation of flying like a bird.

"Hang gliders can get into very unstable air that can be quite dangerous, but by and large the people who are seriously injured deny being aware of the risks," Tongue said. He added that as people master the sport, the chance of death seems to rise because more experienced pilots take more risks.