"We can't be breeding right now," says Les Knight. "It's obvious that the intentional creation of another [human being] by anyone anywhere can't be justified today." Knight is the founder of the Voluntary Human Extinction Movement, an informal network of people dedicated to phasing out the human race in the interest of the health of the Earth. Knight, whose convictions led him to get a vasectomy in the 1970s, when he was 25, believes that the human race is inherently dangerous to the planet and inevitably creates an unsustainable situation. "As long as there's one breeding couple," he says cheerfully,...