要究其原因，可以考虑各国的人均收入（以购买力平价衡量）在老龄赡养比达到或将达到峰值时的表现（以美国的经验为基准 ） 。
To see why, consider each country’s per capita income (in terms of purchasing power parity) when its old-age dependency ratio, benchmarked against the US experience, peaked or will peak.