captivity vs helotism

captivity

noun
  • The state of being captive. 

  • The state or period of being imprisoned, confined, or enslaved. 

helotism

noun
  • The permanent oppression of a nominally free group of people 

  • A form of mutualism in which one species is forced to perform tasks for another, for their mutual benefit 

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