Why does China need a one-child policy?
They wanted to substantially curtail population growth. In spite of the one-child policy they are still growing. The theory in 1979 was that radical action was needed to avoid unsustainable population levels in cities and prevent wide-spread famine. (China in the 1950s averaged 5 children per family, in the 1960s 4 per family.) But not all Chinese are subject to this policy, it’s actually less than half of the population. It’s always been limited to Ethnic Han in urban areas. It was a temporary measure in 1979, but renewed with every 5-year plan.