That's a great question. Especially for those of us who write (or read) mysteries!
In situations where authorship is in doubt, law enforcement and legal officials turn to a forensic linguist. Such specialists rely, in turn, on one of two methodologies 1.) intuitive, where the linguist reads and assesses the message using non-rational methods and 2.) computational, where the available data is subjected to software that counts patterns. In 2000, the Daubert trilogy established that trial judges are the gatekeepers for assuring that expert testimony is based on scientific knowledge. With that ruling, the intuitive analysis approach has been limited to showing differences or commonality, but it can't state conclusions.
Fortunately, the computational method has a champion in Carole E. Chaski, PhD, a forensic linguist and a scientist who spoke last week to our local chapter of Mystery Writers of America. Dr. Chaski's philosophy is that the truth comes first, and that nothing should be allowed to obstruct truth in the service of justice. She's a former academic who recognized a new calling when the Raleigh NC police force asked her to help solve a perplexing case involving a suicide note written on a computer and printed onto a disk.
The suicide note was supposedly written by Michael Hunter, a North Carolina student, who died from an injection of lidocaine, Benadryl and Vistaril. Michael was rooming with two other men, and the police suspected his suicide was actually a homicide. So Dr. Chaski examined the pattern and placement of words in sentences in the message. She counted the nouns, verbs, adverbs and prepositions and studied the syntax, which she explains is as individual as a signature. According to Dr. Chaski, writing is an intuitive process, "but it is the minor, tiny things that are different because we understand each other."
These differences are unique. Once you become an adult, you are consistent in how you writing. As a result, Dr. Chaski was able to tell the police she was "99.9%" sure that Hunter had not written the suicide note. When confronted with her testimony, one of Michael Hunter's roommates confessed to the killing.
Dr. Chaski's method requires about 200 words, but more is better. Since that seminal court case, she has developed computer programs designed to break documents down by doing an analysis of sentence structure and examining parts of speech and their relationships. Using statistical analyis, she has an accuracy rating in the high 90 percentiles.
Her research has also proven that spelling is an unreliable indicator of authorship. "There are people who spell poorly in one document, but who spell (words) right in another," she explains. Furthermore, everyone who spells words correctly looks alike!
Dr. Chaski is often called upon to examine threatening letters, and to determine the likelihood of a threat being carried out. She mentioned four of the 13 characteristics she uses to make an informed assessment: conditionality ("if you don't do so-and-so, I will do such-and-such,"), profanity (true threats include a lot of profane language), modal verbs ("you should"), and specificity (both Dylan Klebold and Eric Harris, the Columbine killers, detailed their violent plans and turned them in as "creative writing" assignments).
So...if you are ever tempted to write a letter, and sign it "Anonymous," you better hope that no one shares your work with Dr. Chaski. You won't be anonymous for long!You can read more about Dr. Chaski's computer programs at her website.
I found her work to be fascinating. What do you think?