was not mandated nationwide until 1979. [ 17 ] [ 18 ] [ 19 ] The system of the work unit ( Danwei ) was crucial to implementing the one-child policy, as workers' reproductive behaviors could be monitored. As part of China's planned-birth policy, unit supervisors monitor the fertility of married women and may decide whose turn it is to have a baby. In 2003, it became possible to marry or divorce someone without needing authorization from the work unit. [ 20 ] The limit has been strongly enforced in urban towns, but the actual implementation varies from location to location. [ 21 ] In most rural areas, families are allowed to apply to have a second child if their first-born is a daughter [ 22 ] or suffers from physical disability , mental illness , or intellectual disability . [ 23 ] Second children are subject to birth spacing (usually 3 or 4 years). Additional children will result in large fines. Families violating the policy are required to pay monetary penalties and may possibly be denied bonuses at their workplace. Children born in overseas countries are not counted under the policy if they do not obtain Chinese citizenship . Chinese citizens returning from abroad are allowed to have a second child. [ 24 ] As of 2007, 35.9% of the population were subject to a strict one-child limit. 52.9% were permitted to have a second child if their first was a daughter; 9.6% of Chinese couples were permitted two