Governments, organisations, and individuals employ a range of linguistic deception strategies. These include silence, denial, PR, propaganda, spin and outright lying. Verbal (including written) communication is an extremely dense medium for information and it makes sense for deceivers to invest in wordsmithing. The aim of deception is to induce a decision error on someone else's part in a way that advantages you and disadvantages them. What it involves in practice is interference with the opponent's information-set so that the decision you want them to take appears, to them, optimal.

Knowing how to do this in practice is hard, not easily reduced to general principles, and involves a lot of empirical knowledge about the systems in which you're operating. Successful deception in a game like poker depends on knowing how 'weak' looks when you're strong, or vice versa. Deceiving shareholders into thinking you're profitable instead of loss-making depends on knowing what profitable firms look like, and pushing out signals that resemble them and hiding ones that don't. Convincing enemy governments that your intentions are other than they are can involve extremely subtle and complex activity that is itself designed to resemble deception about something else.

The 1989 Hillsborough disaster, in the news again in April following the verdict at an inquest that the victims were unlawfully killed, gives us a practical example of organisational deception. In the aftermath of the disaster, police statements made on or close to the day were altered - 'sanitised' - before being made public, a fact only unearthed by the Hillsborough Independent Panel more than two decades later. Many of the alterations seem clearly designed to absolve the West Yorkshire Police of whatever responsibility they may have felt they bore.

Examples of this 'sanitisation' (among many) can be found in the changes made to the report of PC Martin McLoughlin. These include altering "...I was stood with PS 266 Jacques, the only officer in our serial who had a personal radio..." to "...I was stood with PS 266 Jacques, who had a personal radio...", and "I heard an officer asking desperately for a gate to be opened" to "I heard an officer asking for a gate to be opened".

This 'sanitisation' is different from redaction. Redaction works if there is such a wide range of possibilities for the redacted words (names and addresses make ideal redactees) that it doesn't much matter that people know where the gaps are. It's a very honest kind of deception. The sanitisation of the police statements widens the reader's uncertainty more subtly. The publicly-released statements were not untrue, but their information content has been reduced, widening the range of possibility to include (for example) that there might have been many officers with radios, or that the request to open the gate was a rather routine one.

A known unknown This is considerably more deceptive than redaction because it conceals the deception itself and can therefore rely on linguistic implicatures: "if PS 266 Jacques had been the only officer with a radio, surely PC McLoughlin would have told us, so we have no reason to think he was..." If we're looking for cover-ups, we shouldn't be looking for redacted documents. Real cover-ups - good ones - don't leave a trace.