Pessimism is often wrong because people assume a world where there is no change or innovation. They simply extrapolate from what is going on today, failing to recognize the new developments and insights that might alter current trends. For too long, for instance, population forecasts have ignored the possibility that population growth would ease as the world became better off, because people who are wealthier and healthier do not feel the need to have so many children. A lot of the rhetoric about sustainability implicitly assumes that we will exhaust our natural resources, as though there will never be any substitution of one commodity for another in the future.