This week, as usual, television viewers will be subjected to an undeclared marketing campaign on behalf of the forces of law and order which amounts to a none-too-subtle type of light brainwashing.

The police documentary is now such a staple of the terrestrial and cable TV schedules that variations on the same basic theme are to found every night of the week in The Force, Sky Cops, Vice Squad or Police Stop, in Street Law, Traffic Cops, Police Camera Action and many others.

The format is seductively simple. A production team is given inside access to the crime-busting business - the briefings, the investigation, the CCTV footage, the car chases, the dawn raids and, most excitingly of all, the arrests.

Seeing the look of panic, defiance, resignation or despair settling on a suspect's face as he is read his rights by a policeman offers a thrill which no fictional cop show can rival.

What is required of the director in return is simple: the story of these dramas is told from the perspective of the police. They are both the heroes and, essentially, the directors of the show. Everybody wins from this arrangement, or so it seems. The TV companies get cheap programmes. The police are able to explain to the public how they go about doing their difficult work. The viewer is both excited by the action and reassured by the underlining moral message: the good guys are out there, looking after us all and the bad guys are getting caught and put away.

If that were the arrangement for an occasional TV show, there would be no particular problem, but it is not.

When it comes to the police documentary as entertainment, the set-up is pretty much always the same.

There will be the good-hearted, sincere cop explaining to camera some terrible problem of law and order: drunkenness, people trafficking, bad driving, drug-dealing, kerb crawling. We follow an investigation to its exciting conclusion. Sometimes the faces of those arrested are concealed, but often, when he is later found guilty, it is not. The concerned copper will reappear, perhaps outside the court, to assure us all that the streets are that little bit safer tonight.

Reality shows are all about ritual, but the frequency of these programmes with their one-sided view of life, their ironing-out of any kind of complexity, has the drip-drip effect of propaganda.

For half-an-hour, the real world is presented in unquestioning black-and-white terms.

A strand on a recent Channel Five series called Vice Squad saw the police catching kerb-crawlers by getting a policewoman to stand on a kerb in an area where there was a problem.

If a car slowed down, she ambled over to the driver and chatted to him. When he propositioned her, she would say loudly: "Sex? I thought you were asking for directions!" The police would move in: another satisfying result for them, and the viewer. Perhaps it does not matter that no questions arise about what seems to be a policy of entrapment. Kerb-crawling is unpleasant and, if an errant husband has his face shown on national television, he has only himself to blame.

Yet everyone knows that life and crime can be complex. A marketing documentary covering this year's G20 demonstrations - Demo Cops, perhaps - would be unlikely to have told the whole story.

They feel sleazy, these shows - even slightly sinister. Ritualised parables, reassuring the public that the authorities are looking after them, are bad for the viewer and for the police.

Here is a form of TV's dumbing-down which requires some attention.