It is impossible to take a stance on such important problems as climate policy or healthcare prioritisation without making controversial assumptions about population ethics.
In many decision situations, at least in expectation, an agent’s decision has no effect on the numbers and identities of persons born. For those situations, fixed-population ethics is adequate.
But in many other decision situations, this condition does not hold. Should one have an additional child? How should life-saving resources be prioritised between the young (who might go on to have children) and the old (who are past reproductive age)? How much should one do to prevent climate change from reducing the number of persons the Earth is able to sustain in the future? Should one fund condom distribution in the developing world?
In all these cases, one’s actions can affect both who is born and how many people are (ever) born. To deal with cases of this nature, we need variable-population ethics: ‘population ethics’ for short.
Extant theories in population axiology include totalism, averagism, variable value theories, critical level theories, and “person-affecting” theories. Each of these theories is open to objections that are at least prima facie serious. A series of impossibility theorems shows that this is no coincidence: it can be proved, for various sets of prima facie intuitively compelling desiderata, that no axiology can simultaneously satisfy all the desiderata on the list. One’s choice of population axiology appears to be a choice of which intuition one is least unwilling to give up.
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