Those who do consider population to be a key to the problem typically say little about which policies would spare the planet many more billions of people. Should we restructure tax rates to favor small families? Propagandize the benefits of small families for the planet? Reward family-planning workers for clients they have sterilized? Each of those steps alone or in combination might help bend birthrates downward for a time, but none has proved to affect demographic trends over the long term or, critically, to gain and keep public support. When the government of India rewarded health workers for meeting sterilization quotas in 1976, the zeal of some of them for wielding scalpels regardless of their patients’ wishes contributed to the downfall of Indira Gandhi’s government in 1977.

And how can we reduce consumption? Ideas such as cap-and-trade plans for limiting greenhouse gas emissions and allowing companies to trade emission rights are based on the same principle: raise the price of what harms the environment to reduce consumption of it. Beyond the consumption cuts, however, such schemes don’t have much to recommend them. Governments can also eliminate subsidies of polluting behavior, an approach that is more palatable—except to the often powerful interests that benefit from the subsidies. Or governments can subsidize low consumption through tax deductions and credits, but the funds to do so on the needed scale will likely be increasingly scarce.

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