Lower birth rates initially boost economies because it increases the percentage of the working age population which can drive a virtuous cycle in terms of savings and investment. In economics this is called the demographic dividend which if well managed can lift countries out of economic poverty.

Eventually though the benefit disappears as the demographic pyramid becomes a pipe and the share of working population shrinks again. The US demographic dividend was the era of the baby boomers who were a large cohort but had relatively few children.