The Jeju Island Treason Case of Ulhae-oksa(乙亥獄事) ‘Guilt by Association’ Criminals and the Strengthening of Control Measures in 18th Century

After the Ulhae-oksa(乙亥獄事) incident in 1755, a large number of guilt-by-association individuals were concentrated on Jeju Island. These exiles, mostly descendants of high-ranking officials, experienced significant socio-economic differentiation in their new environment. They divided into two distinct groups: the Manggeon-pae(網巾牌), who became craftsmen and labourers, and the Yangban-pae(兩班牌), who maintained their aristocratic lifestyle through family wealth.<br/> The Yangban-pae cultivated relationships with local officials and spread anti-government sentiment by criticising King Yeongjo’s Cheonuisogam(闡義昭鑑). They raised hopes among local people that they would eventually return to central politics. When these activities were reported to the government by Sim Naebok, both groups faced severe consequences, with many members being executed for treason.<br/> This incident prompted the government to introduce stricter control measures over exiles, including increased oversight by local magistrates, strict regulations on movement and marriage, and a shift from concentrated housing to nationwide dispersal. These reforms transformed the exile system from a means of isolation into a comprehensive surveillance mechanism, and had a significant impact on exile management throughout the 18th and 19th centuries.