Reproduction rates are falling in developed countries. Why? It's not because sensible people look at a dark future and say No; the kind of people who do that bred themselves out of the genome long ago. Nature throws babies at the hostile future in the hopes that some will survive: most of those baby turtles swimming out to sea will be eaten or otherwise lost to hostile conditions; the Bachs had 20 children and only half survived their parents; the number of Bach descendants alive today is estimated as between zero and eight. But when conditions become so good that the babies become their own competition, Nature says, nah. (Cf the famous study that built the ideal environment for rats.)