mental instability vs obsession

mental instability

obsession

noun
  • An idea that engenders a compulsive or irrational preoccupation, or the preoccupation thereby engendered. 

  • The quality of being obsessed. 

  • Influence or control by evil spirits without possession. 

  • An activity or entity that inspires a compulsive and potentially unhealthy fixation, or the fixation thereby inspired. 

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