https://www.un.org/en/development/desa/population/publications/ageing/replacement-migration.asp https://www.un.org/en/development/desa/population/publicatio...

https://www.un.org/en/development/desa/population/publications/ageing/replacement-migration.asp https://www.un.org/en/development/desa/population/publicatio... It's not a theory. It's established UN doctrine publicly available on the UN website. The "theory" part is whether or not it's a good or bad thing. Nobody wants to be replaced but something has to be done about birth rates falling one way or another.