Comparable methods have also been used to detect linguistic changes that occur to particular writers over their lifetime. Ian Lancashire and Graeme Hirst have used three measures of Agatha Christie's detective novels, written between the age of 28 (The mysterious affair at Styles) and the age of 82 (The postern of fate). The measures they used were the size of vocabulary, the number of repetitions of fixed phrases, and the use of indefinite words (such as "thing," "anything," "something").
Agatha Christie was born in 1890, and died in 1976. She was enormously prolific. According to Lancashire and Hirst, she wrote 85 novels and plays, and by 1990 two billion copies of her works had been sold. Each of her works was planned meticulously before she started to write it, and she received little or no input from editors. This last fact is important from the point of view of how far the writings that were analyzed were attributable solely to the author.
Lancashire and Hirst followed the method used by Peter Garrard and his colleagues (2005) in their analysis of the effects of early Alzheimer's Disease on Iris Murdoch. Garrard et al. found that changes of word-use by Murdoch indicated that when she was writing her last book she was already in the early stages of Alzheimer's, and that it was her word-use rather than her syntax that was affected.
For Agatha Christie, Lancashire and Hirst analyzed the first 50,000 words of each of 15 of her detective novels. They found that, in comparison with the detective novels she wrote between the ages of 28 and 32, those written between the ages of 81 and 82 showed a 15 to 30% loss in vocabulary and a 14% increase in the use of repeated phrases. Her last detective novel had 1.23% of indefinite words, as compared with 0.27% in her first. Lancashire and Hirst conclude that, like Iris Murdoch, Agatha Christie had started to suffer from dementia by the time of her last writings, perhaps Alzheimer's Disease.
Though we all decline in our abilities of recall with aging, Alzheimer's seems to have severe effects on cues for retrieval from memory. It is a sad decline, perhaps especially sad for writers.
Peter Garrard, Lisa Maloney, Jack Hodges, & Karalyn Patterson (2005). The effects of very early Alzheimer’s disease on the characteristics of writing by a renowned author. Brain, 128, 250–260.
Lancashire, I., & Hirst, G. (2009). Vocabulary changes in Agatha Christie's mysteries as an indication of dementia: A case study. Paper presented at the 19th Annual Rotman Research Institute Conference, Cognitive Aging: Research and Practice, 8–10 March, Toronto.