One child
This rule was fairly well known around the whole world, and due to the massive population of Chinese people, families were only allowed to have one child each. This was seen as the only way the government could control the population and deal with such a massive amount of people at once.
But the long-term effects of this were awful and many female babies were killed or thrown away as a result because most families wanted a male. This huge imbalance in society has caused many social problems, recently though families are now allowed two children. Imagine finding a wife with so much male competition.