instability vs levelheadedness

instability

noun
  • The quality of being unstable. 

  • A state that is not in equilibrium, or in which a small change has a large irreversible effect. 

levelheadedness

noun
  • The property of being levelheaded, stable, not overly swayed by passion. 

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