"And there's that perfidious mechanism in human biology that means that once prosperity hits a place and child mortality drops dramatically," Father Harman says, "people continue to have as many children as they would have when they were still expecting most of them not to make it; then, population drops off again. We have six billion people on Earth by my time; optimistic predictions claim that the whole earth can support ten. So there is a great lot of people in poverty still, as the fact people are rising out of misery automatically means there are more people, for a while."