pejoration vs retrogression

pejoration

noun
  • The act or process of becoming worse; worsening or degeneration. 

  • The process by which a word acquires a more negative meaning over time. 

retrogression

noun
  • A deterioration or decline to a previous state. 

  • A return to a less complex condition. 

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