IT'S ONE THING, A BEAUTIFUL THING, to watch a falcon swoop off a 2,500-foot cliff. But to watch a man run to the edge and hurl his earthbound body into the void is to feel ill. Primates just aren't supposed to do this. Don't try telling that to Ted Davenport, though. When the 28-year-old Aspenite started BASE-jumping a few years ago, he found his destiny.

"I always wanted to fly. I love anything involving big air," says Davenport, who's also a champion extreme skier.

On this winter morning in western Colorado, the wind is blowing steady at about 25 miles an hour, whipping my hair around my face. We're at the top of an outcrop that Ted calls the "W Hotel," because of the letter shape its gray gullies make against the cliff face. Davenport and six of his buddies are standing near the precipice, hands in pockets, shuffling their feet. The guys are nervous as calves at a branding—some of them more so than others.

These seven friends, most of whom live around Denver, do a lot of crazy stuff together. They go to Moab to jump into canyons and build big bonfires and shoot skeet. They dive off hotels in Denver and try to shake the police. But it's not so much their jumps that I'm here to watch; it's their brains. What makes one person fling himself off a cliff with abandon and another stay home to watch the Food Network? Even among rad adventurers, why do some hesitate when things get extreme? During the past half-decade—and especially the past year—scientists have been using high-tech imaging, advanced neurochemistry, and even computer games to tease out an answer to that question. They are opening a window on one of humanity's most mysterious traits: the way we hear the call of adventure.

Atop the cliff, Collin Scott, 34, tries to light a wind indicator, a rolled-up piece of paper he'll toss into the abyss. But Ted isn't waiting for any indicators. "It's windy as fuck," he declares. Calling out to 24-year-old Matt Hecker, he says, "Get your gear on, pussy." Ted wouldn't say that to the others, but he knows Matt has both a similar level of experience and a similar worldview—one that often includes the surface of the earth approaching at near-terminal velocity.

Ted dons a helmet and parachute, then stuffs a walkie-talkie in one pants pocket and a bottle of water in the other. He grins for a picture, then asks me to send it to his family if it's the last shot of him ever taken. He roars with laughter. Then he gets serious. He pauses for a second, walks briskly to the lip, and launches belly first into the air.

A few seconds later, his chute opens and he lands lightly on the rocky side of a ravine. The men up top cluster around their receiver. Ted's voice comes in scratchy, as if he's miles away. He's all business. "I'm not going to lie," he crackles. "There's definitely turbulence pushing you around. Keep away from the wall. There's no wind at all down here. Wooooyeah!"

Collin shakes his head. "I've made the decision," he says. "I'm not going." Collin is a software salesman, the only man here with a wedding ring. Peter Konrad, a 34-year-old pilot who until recently worked for a Denver telecom company, nods in agreement. He's just three weeks past a car accident, the stitches on his forehead still red. Denver real estate investor Kevin Cochran is not jumping either. So there's at least a little bit of sense in this crowd.

But then off the rest go. Next into the air is Jacob Fuerst, a 25-year-old former marine who after seven months in Iraq has found a measure of peace jumping off tall objects and taking photographs, often at the same time. Next Matt, without much ado, jumps into the void. Three years ago he spent 16 days in a coma after his paraglider seized up in north Boulder. It doesn't seem to have slowed him down. Jeremy Puhal, 30, wants to jump, but he's taking his time, waiting for a lull in the wind that never comes. Finally, he shakes out his cold arms, runs to the edge, and flies off.