Ian Lancashire, an English professor, and Graeme Hirst, a computer expert, who both teach at the University of Toronto, compared the size of the vocabulary in Christie's final books with that of her earlier ones, The Guardian reported. They also looked for phrases being repeated and for the use of indefinite words like "something" or "anything."
Christie published her first book at 28 and her last at 82, with a lifetime total of more than 80.
"We found statistically significant drops in vocabulary, and increases in repeated phrases and indefinite nouns in 15 detective novels from 'The Mysterious Affair at Styles' to 'Postern of Fate,'" the researchers said.
"Elephants Can Remember," one of Christie's last books had a vocabulary 30 percent smaller than "Destination Unknown," written when Christie was 63, they said. The later book has 18 percent more repeated phrases and three times as many indefinite words.
Christie may have deliberately provided clues to her mental state, Lancashire told MacLean's Magazine. The title is a play on the saying that elephants do not forget and Christie also has a detective who is forced to summon Hercule Poirot because she is stumped.
Notable deaths of 2014 [PHOTOS]