In a world of uncertainties, some people avoid risk, while others thrive on it.

The United States is a Type T nation (for "thrill"), says University of Wisconsin psychologist Frank Farley, and as many as 30 percent of Americans are thrill-seekers who pursue risk for its own sake.

The personality identified by Farley as Type T prefers the unknown to the known, the new to the familiar. Type T's thrive on variety, complexity, intensity and conflict.

In a word: risk.

"Risk means engaging in an uncertain outcome," Farley said. "You name it, and the Type T's are engaged in it. They're consumers of novelty, of the unknown."

Type T personalities include not only hang gliders, mountain climbers and round-the-world balloonists, but also artists, scientists and entrepreneurs. Albert Einstein as well as Evel Knievel.

The Type T personality has both good and bad aspects (which Farley labels T-plus and T-minus). It fosters both creativity and crime, adventurousness and violence, courage and drunken driving.

Type T young adults are twice as likely as others to be involved in highway accidents, Farley said.

"They drink more, they experiment more with drugs, they experiment more with everything," he said. "They're the great experimenters of life."

Mountain climbers and other adventurers pursue their goal not "because it's there," Farley said, but because it offers stimulation and challenge. He speculates that the Type T personality may even be based on a "fundamental biological need" that sets risk-takers apart from the unadventurous.

The United States -- a country of immigrants, pioneers and cowboys, where the system of laws encourages considerable freedom of individual choice -- is in Farley's view largely a Type T nation.

The positive side of that national personality, he said, is American energy, resourcefulness and creativity. The negative side is a high rate of crime and violent behavior.

The challenge for society, he said, is to get the T-minus person "out of that destructive track and into creative risk."