In January of this year, James Watson, an Australian scientist who works for the Wildlife Conservation Society, noticed an image that had been tweeted by a friend of his, a physician in Sydney. With a chain of progressively larger circles, it illustrated the relative frequency of causes of death among Australians, from the vanishingly rare (war, pregnancy and birth, murder) to the extremely common (respiratory disorders, cancer, heart disease). It was a simple but striking depiction of comparative risk. “I thought, ‘Why hasn’t anyone done something like this for the rest of nature?’ ”Watson recalled.