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In any criminal investigation where the perpetrator writes an original document, law enforcement can turn to forensic linguists to analyze the writing. Forensic linguists can compare documents written by suspects to that of the perpetrator to determine whether they were written by the same author.
This analysis is possible because even among people who speak and write the same language, different people use it differently. One person might prefer a certain word or phrase over another word or phrase that says the same thing, or have a different writing style or interpretation of grammar from another person. The result is that each person has his or her own personal version of the language, called an idiolect. In some cases this personal language may be so unique that a linguist can say two documents were written by the same person.
This analysis is difficult in most criminal cases, where the relevant document is very short, anything from a text message to a ransom note. These documents tend to be ten words or fewer, not nearly enough to analyze the author's idiolect. Some cases, however, involve long, elaborate documents that exhibit unique linguistic patterns such as word choice or writing style.
The most well-known case where law enforcement used forensic linguistic experts was the Unabomber. After sending or placing several bombs in universities and airlines, the serial bomber sent a very long manifesto called Industrial Society and its Future to several publications demanding it be published. When they obeyed, a man named David Kaczynski read the manifesto and found it disturbingly familiar; the word choices and philosophy resembled those of his brother Theodore. There were particular phrases David recognized as Ted's, including a reversal of the common saying "have your cake and eat it too;" Ted preferred to say "eat your cake and have it too." These were unique enough to be instantly recognizable, but were not the only indicators.
Forensic linguists analyzed the document, comparing the phrasing of the manifesto's philosophical statements to that of documents provided by the brother, and later, further documents found in Kaczynski's cabin. They concluded that the documents had all been written by the same author.
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