The abrupt decline in the Total Fertility Rate (TFR) of Puerto Rico since 2000 makes the prospect of a sustained population decline a real possibility. From 2000 to 2021 the TFR declined from 2.1 to 0.9 children per woman, one of the lowest in the world. Population projections produced by the United States Census Bureau and the United Nations Population Division show that the island population may decline from 3.8 millions in 2000 to slightly above 2 million by 2050, a dramatic 47% population decline in 50 years. As dire as this prospect may be, this may be an optimistic scenario. Both projections have the TFR increasing to 1.5 by 2050, but a fertility projection conducted by us show that fertility can remain much closer to 1.0 until 2050. Bayesian Hierarchical Probabilistic Theory has been used by the United Nations to incorporate a way to measure the uncertainty and to estimate the projection parameters. However, the assumption that the fertility level in countries with low fertility will eventually increase to 2.1 has been widely criticized as unrealistic and not supported by evidence. We modified the assumptions used by the United Nations considering countries with TFR similar to Puerto Rico and find that by 2050 Puerto Rico may have a TFR of 1.1 bounded by a 95% credibility interval (0.56,1.77). This indicates that there may be a larger population decline than what current projections show.

翻译：自2000年以来，波多黎各总和生育率（TFR）的急剧下降使得人口持续减少的前景成为现实可能。从2000年至2021年，其总和生育率从每名妇女2.1个孩子降至0.9个孩子，成为全球最低水平之一。美国人口普查局和联合国人口司发布的人口预测显示，该岛人口可能从2000年的380万减少至2050年的略高于200万，50年内人口降幅高达47%。尽管这一前景令人忧虑，但或许仍属乐观情景。两项预测均假设总和生育率到2050年将回升至1.5，而我们进行的生育率预测表明，生育率可能更接近1.0的水平直至2050年。联合国采用贝叶斯分层概率理论来量化不确定性并估计预测参数。然而，该理论假设低生育率国家的生育水平最终将回升至2.1的做法，因脱离现实且缺乏证据支持而受到广泛批评。我们基于与波多黎各总和生育率相近的国家对联合国所用假设进行修正，发现到2050年波多黎各的总和生育率可能为1.1，其95%置信区间为（0.56,1.77）。这表明实际人口降幅可能大于当前预测所显示的结果。