What factors contributed to the development of population policies during the 20th century?

There was many factors that contributed to the development of population policies during the 20th century. For example, the demand for labor force increased. Also, people from 3rd world countries started to migrate to different countries in search for a better life for them and their families. And, the fertility rates started to decrease, mostly through the first half of the 20th century. Also, because of the world wars that happened, it started a huge migration movement that caused a lot of people to migrate into new countries.