The revelation in The Herald today that two sex offenders have been released on condition that they agree to electronic monitoring orders of eight and 10 years respectively will probably polarise opinion. Some will say that if these two men are so likely to re-offend that they must be confined to their homes for at least 12 hours a day, they should not have been released in the first place. Indeed, some believe certain categories of sex offender, such as paedophiles whose condition is not deemed to be treatable, should be incarcerated for life. "Lock them up and throw away the key," is a well-worn phrase.

On the other side will be those who argue these men have paid their debt to society and that imposing on them what amounts to house arrest for a further protracted period defeats the ends of natural justice. This dilemma could be compared with that of the terrorist suspects who, following more than three years of imprisonment without trial at Belmarsh Prison in London, were released on control orders that restricted severely their movements. Last August the Court of Appeal ruled the control orders were so draconian as to amount to a deprivation of liberty under Article 5 of the European Convention on Human Rights.

Part of the pressure to release sex offenders arises from prison overcrowding. Britain already imprisons more people per head of population than any country in Europe. On a practical level, this obstructs severely programmes aimed at rehabilitation. It is also extremely costly: keeping someone in prison is three times more expensive than electronically tagging and supervising them in the community. There is also an unacceptably high human cost in terms of the number of prison suicides. Tabloid newspapers prone to headlines along the lines of "Tagged lags on the loose" are the first to castigate the prison authorities and the parole board when people die miserably in prison by their own hand when humane alternatives are available. They cannot have it both ways.

Life is never devoid of risk, and constant surveillance of sex offenders is simply impractical. The electronic tagging of offenders is relatively new. Though it certainly represents a severe curtailment of a person's freedom, that may be an acceptable alternative to imprisonment for both the individual and society, under certain conditions. These include the proviso that it is used in the context of a programme of supervision and never as a substitute for it. Also, given the level of technical malfunction and human error associated with such technology, it requires rigorous monitoring. Over-reliance on tagging can provide a false sense of security, and public confidence is undermined when things go disastrously wrong, as in the case of Callum Evans, the 18-year-old who committed murder while wearing a tag that was set incorrectly.

The particular issue in this case is the length of time for which the men are likely to be restricted. Experience elsewhere suggests that the longer someone is subject to tagging, the more likely they are to breach their conditions, because they find them intolerable. If long-term, even lifelong, tagging is to become a regular part of our criminal justice system, it needs to be both flexible enough to be workable but rigorous enough to provide the public with the protection they rightly demand.