Like the monarchy or the sex industry, the criminal justice system is best viewed at a distance, preferably by an idealist wearing rose-tinted lenses. The high principles on which it professes to base itself often bear little relation to the grubby goings-on behind the scenes.

Every man is innocent . . . until persuaded by his lawyer to plead guilty at the last possible minute in order to maximise the legal aid fees. There is no escaping the long arm of the law . . . especially if it contaminates the evidence and doctors the witness statement.

It was the high-minded 18th-century jurist Sir William Blackstone who articulated what is now known as the Blackstone ratio: “It is better that ten guilty persons escape than that one innocent suffer.” It was left to a pragmatic Chinese professor to ask: “Better for whom?”

I don’t have much truck with the idea that a once noble and fair Britain is under attack from policies and practices designed to erode our personal freedoms. It was never that great in the first place. Google can monitor my internet usage as long as it lets me find the train times at the click of a mouse. I’m happy for Tesco to know what brand of shampoo I prefer if it makes it easier for the company to deliver it to my doorstep. These are trade-offs which seem perfectly reasonable.

So when Stephen House, the chief constable of Strathclyde Police, calls for the DNA database to be extended to the entire population, I can see his point. At present the Scottish DNA database holds the genetic fingerprints of 193,000 souls, all convicted criminals. In 2005-6, according to the Home Office website, more than 3,100 Scottish matches were found — an impressive 68% success rate.

The Home Office is more reticent about the more pertinent statistic of how many matches led to convictions, but it makes up for this omission with plenty of grim anecdotes about grisly crimes solved because the brute happened to shoplift years before and was required to give a DNA swab.

It must be particularly galling for the Scottish police to see how much more freedom their English counterparts have. In Scotland, samples from those who are found not guilty are automatically destroyed, or so they tell us. In England the tentacles of the DNA database reach ever further.

The pesky requirement to destroy the DNA samples of the innocent was removed in 2001. Three years later, anyone arrested for a recordable offence, however trivial, and detained in a police station, however unjustly, would have their DNA extracted mandatorily and stored indefinitely. The same applied to anybody who helped police with their enquiries by giving a sample for elimination — the relatives of murder victims included. Nearly a quarter of the samples on the entire DNA database come from children aged ten to 18.

Self-interest dictates that I should back House’s call for a blanket database for Scotland. I support the police. I want to see the bad guys put away. I have no fear of new technology. My family and I, along with just about everybody I know, are far likelier to be victims of crime than perpetrators.

Yet House’s call strikes me as deeply distasteful. We would, in one wave of a saliva-coated cotton bud, go from being a nation of upright citizens to a population of latent criminals. It would be a subtle, yet fundamental shift in our relationship with government.

That is before any practical considerations are taken into account. Your Honour I give you: the cavalier attitude of civil servants to the security of our personal information; the money-grubbing antics of authorities keen to make a fast buck by flogging any piece of data they can; the huge potential for sloppy policing and the ease with which it would be possible to frame the innocent. DNA is considered infallible, but so once was fingerprinting. The Shirley McKie case blew that fallacy wide open. It was one of the most shameful episodes in the history of Scottish criminal justice.

Prosecutors tell jurors that the probability of DNA matching someone other than the defendant is infinitesimal, which is true if crime-scene evidence is compared to the DNA of one random person. But when DNA samples of thousands of people in a database are compared to each other, the number of comparisons and the probability of a match soar.

House’s call at least has a moral logic to it which is lacking in the opportunistic English system. Either the DNA database should contain samples from only convicted criminals — as the Scottish system does now — or it should contain everybody.

The Home Office website boasts that the English database is now the largest in the world and contains profiles of 5.2% of the population. But the morality of building a database by stealth and adding anybody at random who comes into the orbit of the police is dubious to the point of being deeply unconstitutional. It is not a system that Scotland should match, whatever the temptation.

gillian.bowditch@sunday-times.co.uk