Policeman's Register

Once upon a time, the only way to tell that a suicide note had been faked was by matching its faded e's and crooked g's to the keys on the murderer's typewriter. Not any more. You might think that these days you could just text 'goodbye cruel world' to everyone in your victim's phone book before chucking their mobile off the balcony after them – a perfect crime, so long as you didn't forget to wear your rubber gloves. Except that John Olsson, 'the world's only full-time forensic linguist', could well, even then, be able to bust you.

In his latest book, Wordcrime: Solving Crime through Forensic Linguistics, Olsson documents a couple of dozen of the hundreds of cases he's worked on over the years: analysing, for example, the text messages sent among a gang of teenagers who beat a middle-aged cyclist to death in an underpass in Luton; scrutinising witness statements accusing a former mayor of a town in Rwanda of complicity in the genocide; or more banally comparing The Da Vinci Code to the novels of Lew Perdue, one of several people to have accused Dan Brown of plagiarism.

Olsson acknowledges Jan Svartvik as 'the "father" of the discipline' for the part he played in the posthumous pardoning of Timothy Evans, John Christie's neighbour at 10 Rillington Place. Evans was hanged in 1950 for the murder of his wife and child, which Christie was almost certainly guilty of. More than a decade later Svartvik showed that Evans's 'confession' was largely written in 'what is known as "policeman's register"’.

But Olsson is otherwise fairly protective of his turf, and ruthlessly scathing of the part-timers. He doesn't so much as mention Don Foster, for example, and devotes one of his chapters to exposing the incompetence of 'Professor Willerby (not his real name)', an expert witness for the prosecution of a man falsely accused of anonymous libel. No prizes for guessing whose evidence led to the case being thrown out of court.