This year David Zald, a psychology professor at Vanderbilt University, performed some of the first tests on so-called thrill-seeking personalities using PET scans. He discovered that his subjects’ brains actually had fewer dopamine shutoff valves—or autoreceptors—than the average human’s. So when they have a new experience, they feel more euphoric. Zald refers to these individuals not as risk takers, but “novelty seekers.” The distinction, he says, is important: “The world would be a boring place if we didn’t have people drawn to the new and exciting.” But once the mountain has been climbed, the experience becomes predictable. It doesn’t evoke the same feel-good response. “So for them to get that rush again,” Zald says, “they have to try something new and climb an even higher mountain.”

That’s where risk comes in. Zald explains that the pull of euphoria can sometimes overpower a novelty seeker’s ability to properly evaluate risk. “The reward is just too great, so they’ll do things that most other people wouldn’t do.” To compensate, novelty seekers—at least those who are successful—tend to be meticulous planners. A researcher at the Queensland University of Technology in Australia published an article last July showing that extreme athletes like alpinists and BASE jumpers overwhelmingly strive to minimize risk. They also tend to delude themselves, somewhat, about the real dangers.