Why do some people risk their lives regularly by placing themselves in extreme and challenging situations? For some, such as astronauts, the extreme environments are a requirement of the job. For others, they involve the thrill and competition of extreme sports, or the achievement of what seem like unimaginable goals to some - such as being the first to reach the South Pole or climb Mount Everest. Whether for sport or a career, these people have made the personal choice to put themselves in places where there is a significant risk. What drives such people? What skills and personality traits...