There are many branches of forensic linguistics, but the basic premise is that the evidence of linguists can be used in both civil and criminal court cases. For example, an expert on spoken language may be asked to identify a voice on a recording, or perhaps decide if a suspect was really able to comprehend what was asked of him or her during an interrogation. In the case of written language, an expert may be asked to give evidence relating to whether a document was written by a particular person.
There are a number of famous cases which have involved corpora and forensic linguistics. The Derek Bentley Case in the UK is a good example of an instance where the use of corpus linguistic techniques allowed a conviction for murder to be overturned and a pardon granted, albeit posthumously. The linguist called upon to give evidence was Professor Malcolm Coulthard from Birmingham University, UK.
One area of analysis focused on the use of the word 'then' in Bentley's statements. By comparing the number of occurrences in a mini corpus of statements with the number of occurrences in a corpus of everyday English, Coulthard showed how Bentley used this particular word with significant frequency (one in every 58 words, compared with once in every 500).
Further analysis of the texts from the Bentley corpus showed that Bentley's use of the word 'then' had more in common with the use found in a corpus of police statements than it did with use of 'then' in everyday speech. In particular, Bentley's use of 'then' immediately after the subject of a clause, was shown to be more typical of the discourse of police statements than of everyday speech.