Files are maintained on every North Korean by security officials and party cadres from age 17 and updated every two years. In general, songbun is difficult to improve, but it can be downgraded for a variety of reasons such as a lack of political enthusiasm, marrying someone of lower standing, or being convicted—or having a family member convicted—of a crime, political or otherwise. Before the late 1960s, it was possible to conceal that a relative had bad songbun; however, the ancestry of all citizens was thoroughly checked starting with a 1966 census. These investigations have been suggested to have been a response to the Chinese Cultural Revolution which began in 1966. Kim Il Sung, afraid that Beijing would also interfere in his country, whether by invading or sponsoring a coup d'état (Chinese soldiers had been sent previously on "provocative incursions" into Korea), aimed to increase internal security by classifying his citizens. These investigations were repeated several times in subsequent years, for reasons varying from suspected corruption in previous checks to weeding out possible opposition.
