Critics and fans have often complained that Agatha Christie’s later novels were not (to use an old British phrase) up to snuff.

We may now know why. Hidden language clues in the novels themselves suggest she was suffering from dementia. Furthermore, she may have sprinkled at least one of her later novels with evidence that she was aware of her mental decline — although she was never, apparently, officially diagnosed with dementia.

As the hosts (Jan Abumrad and Robert Krulwich) of WYNC’s always-entertaining “Radiolab” explained Wednesday, the dementia-related clues in Christie’s writings were uncovered by Ian Lancashire, a University of Toronto English professor, who “takes a text, puts it into a computer, and turns it into data.”

In other words (no pun intended), Lancashire creates and then analyzes concordances (alphabetical listings of every word in a book). It’s a process that’s been used for centuries to better understand a text (usually a religious one) and, more recently, to better understand authors themselves.

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Before computers, putting together a concordance was an extremely tedious and time-consuming task. The earliest concordance, according to the Radiolab show, was compiled by some 500 monks back in the early Middle Ages. They created an alphabetical listing of each word in the Bible, noting where it appeared and its contextual meaning. The process, which took the monks an entire lifetime, can now be done by computers in 15 seconds. (Good thing the monks aren’t alive to hear that.)

But back to Agatha Christie …

Lancashire began creating computer concordances for authors (such as Coleridge, Shakespeare, Chaucer and Milton) in the 1980s. He decided to do the same for Christie because, as he notes, although she died in 1976 (at age 85, from natural causes), she remains among the world’s all-time most popular authors, having sold more than 2 billion copies (so far) of her 85 or so novels and plays.

Christie’s biographers have suggested that her mental powers declined significantly in her later years, as Lancashire points out in his analysis of Christie’s writings (which he presented to fellow researchers [PDF] last year):

[S]he had angry fits (in one she cut off all her hair) and did not always make sense in conversation…. Although she was never assessed for dementia, her last novels reveal an inability to create a crime solvable by clue-detection according to the rules of the genre that she helped to create.

For his study, Lancashire analyzed the language in 16 of Christie’s novels, beginning with her first groundbreaking book, “The Mysterious Affair at Styles,” which she published in 1920, when she was 28 years old.

He found that her use of language was relatively consistent — until her 73rd novel. In that book, her use of indefinite words (such as thing, anything, nothing) suddenly increased sixfold, while the size of her vocabulary (her choice of different words) plummeted 20 percent over her earlier novels.

“That is astounding,” Lancashire told the “Radiolab” hosts. “That is one-fifth of the vocabulary lost.”

He believes the changes he found in her word choices may have been early signs of Alzheimer’s disease.

Of course, as the Radiolab show points out, this study isn’t the only one to find a correlation (but not a cause-and-effect) between the degree of writing complexity and dementia. One of the findings from the University of Minnesota’s well-known and ongoing “Nun Study” was that women who wrote grammatically complex and idea-dense autobiographical essays at age 18 were less likely to develop dementia in old age than their peers whose language skills in youth were less complex and dense.

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Lancashire suggests that Christie may have been aware of her decline in her mental abilities. On this point, the clues are not so subtle. Christie called her 73rd book “Elephants Can Remember,” and she made its main character an aging female novelist who is suffering from memory loss.

“I began to see that Christie was heroic, still writing despite this handicap,” says Lancashire.

(Similarly heroic was the British novelist Iris Murdoch, whose last book was found in a 2005 study to show a declining pattern of vocabulary and syntax similar to that now uncovered in Christie’s later novels.)

As “Radiolab” host Krulwich says, “The muse wouldn’t quit, but the tools all left the room.”

You can listen to the entire 16-minute “Radiolab” podcast here.