feudalism vs forced labor

feudalism

noun
  • A social system based on personal ownership of resources and personal fealty between a suzerain (lord) and a vassal (subject). Defining characteristics are direct ownership of resources, personal loyalty, and a hierarchical social structure reinforced by religion. 

forced labor

noun
  • Work which one is compelled to perform against one's will, especially in a condition of involuntary servitude as a prisoner or slave. 

How often have the words feudalism and forced labor occurred in a corpus of books? (source: Google Ngram Viewer )