involuntary vs obligatory

involuntary

adj
  • Without intention; unintentional. 

  • Not voluntary or willing; contrary or opposed to explicit will or desire; unwilling. 

obligatory

adj
  • Imposing obligation, legally, morally, or otherwise; binding; mandatory. 

  • Requiring a matter or obligation. 

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