proclivity vs tendency

proclivity

noun
  • A predisposition or natural inclination, propensity, or a predilection; especially, a strong disposition or bent. 

tendency

noun
  • A likelihood of behaving in a particular way or going in a particular direction; a tending toward. 

  • An organised unit or faction within a larger political organisation. 

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