A long-standing challenge for welfare economics is to develop welfare criteria that can be applied to allocations with different population levels. Such a criterion is essential to resolve the optimal population problem, i.e., the tradeoff between population size and the welfare of each person alive. A welfare criterion that speaks to this issue inherently requires evaluating the welfare of nonexistent people, because some people exist only in some allocations but not in others. To make progress, we consider the population problem in an environment where population is variable, but there is a fixed supply of souls, who may experience multiple incarnations over time. Rather than pondering the value of nonexistence, from the souls’ perspective comparing larger or smaller populations merely involves valuing shorter or longer waits until the next incarnation. We argue that such comparisons are possible on the basis of introspection and lead to intuitive welfare criteria with attractive properties. We emphasize that one does not have to believe in reincarnation to accept the resulting criteria—rather, reincarnation serves as a metaphor to facilitate the necessary utility comparisons.

福利经济学的一项长期挑战是制定可适用于不同人口水平的福利标准。这样的标准对于解决最佳人口问题至关重要，即人口规模与每个人的生存权衡之间的权衡。谈到这个问题的福利标准固有地需要评估不存在的人的福利，因为有些人仅存在于某些分配中，而没有存在于其他分配中。为了取得进步，我们考虑了人口可变的环境中的人口问题，但是有固定的灵魂供应，随着时间的流逝，他们可能会经历多种化身。与其思考不存在的价值，从灵魂的角度比较较大或较小的人口仅涉及评估下一个化身之前的等待时间更长或更短。我们认为，这种比较可能是基于内省的，并导致具有有吸引力的属性的直观福利标准。我们强调，人们不必相信转世就可以接受由此产生的标准，而是转世是一种隐喻，有助于进行必要的效用比较。