constraint vs incentive

constraint

noun
  • An irresistible force or compulsion. 

  • A condition that a solution to an optimization problem must satisfy. 

  • A linkage or other restriction that maintains database integrity. 

  • The repression of one's feelings. 

  • Something that constrains; a restriction. 

incentive

noun
  • Something that motivates, rouses, or encourages. 

  • A bonus or reward, often monetary, to work harder. 

adj
  • Inciting; encouraging or moving; rousing to action; stimulating. 

  • Serving to kindle or set on fire. 

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