Daniel Engber has discovered the ZPG and voluntary extinction people:

Instead of burning down our numbers with oil and gas, we might follow the advice of the founder of the Voluntary Human Extinction Movement, who tells Weisman that everyone in the world should stop having kids all at once. Weisman isn't up for quite so drastic a measure, but he makes his own pitch, moderate in comparison: Let's cut the birth rate to one child per couple, for a few generations at least. The population would dwindle by about 5 billion people over the next century, he says, ensuring the habitability of the Earth for the 1.6 billion who remained. At that point, they could all reap the rewards of a more spacious planet, sharing in "the growing joy of watching the world daily become more wonderful." It seems like a notion from the fringe, but Weisman's book has become a mainstream best seller. Could population control be the next big thing in green culture?

He leaves out the strong probability that a planet that has only one child per couple will see its elderly abandoned quite early in their senescence. The elderly are extremely heavy consumers of labor intensive services. And unlike caring for children, caring for them is rarely rewarding, which is why they so often end up crammed alone, into nursing homes. This might be a good strategy for the planet, but it's a terrible strategy for the planet's future old people. Which I hope includes all of us.

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