Ecologically-unschooled leaders in both countries believe it will be America.
Immigration is minuscule, so births and deaths alone determine the population size.
Assuming they stay constant, Japan's population will peak in 2005 at 130 million, and diminish by 2100 to 55 million. Pension specialists in Japan and the US alike shudder when baby boom generations have to be supported by smaller numbers of baby bust workers. But a high ratio of children to adults is an anomaly in industrial societies. Each child poses huge demands for schools, job creation, housing and infrastructure. The 1995 world average was five acres.