When did china set the 1 child policy and why?

In the late 1970s, the Chinese government introduced a number of measures to reduce the country's birth rate and slow the population growth rate. The most important of the new measures was a one-child policy, which decreed that couples in China could only have one child.

The one-child policy, a part of the family planning policy, was a population planning policy of China. It was introduced in 1979 and began to be formally phased out near the end of 2015 and the beginning of 2016. The policy was only enforced on Han Chinese and allowed exceptions for many groups, including ethnic minorities. In 2007, 36% of China's population was subject to a strict one-child restriction, with an additional 53% being allowed to have a second child if the first child was a girl. Provincial governments imposed fines for violations, and the local and national governments created commissions to raise awareness and carry out registration and inspection work.

The answer is cultural

Nation of conflicting ethnic groups