The criminologist and well-known ex-convict Greg Newbold made private prisons sound almost too attractive when he described the overseas ones he'd visited as having "an atmosphere of vibrancy and enthusiasm".

Quite clearly, attractive is not what we want in a prison - and by "we" I mean for example Act's David Garrett, whose pursuit of the "three strikes and you're out" law seems to rely on keeping the prison system as unappealing as possible - which is to say in its present sorry, overcrowded and underfunded state. Otherwise, how could a life sentence with a 25-year non-parole period possibly be a deterrent?

No, better to keep Paremoremo "putrid" and Mt Eden "archaic", as an Australian union official described Auckland's prisons. We feel better knowing that prisoners are suffering, that they'll be crammed two to a cell if necessary while they serve out their preferably lengthy and uncomfortable sentences, and that they'll be denied the benefits of flat screen TVs or underfloor heating, even if these are in fact the most financially sensible choices in the
long run.

As Peter Williams, of the Howard League for Penal Reform, pointed out, spending money on fixing prisons isn't a vote catcher. But promising to swell prison numbers certainly is.

Somehow, National thinks that all these inherent contradictions will be solved by handing over the responsibility for running our prisons to private contractors. Through the magic of "competition" we'll get a first rate penal system, and private providers will make a profit without our having to spend any more money.

Given the lack of strong evidence for this proposition, this seems to be based squarely on faith and ideology.

It isn't hard to find examples of private prison failures.
For example, the American company Wackenhut (whose subsidiary ran the Auckland Central Remand prison until 2000) was stripped of contracts to run prisons in Texas and Louisiana in 1999, after accusations it had mistreated prisoners and tried to maximise profits at the expense of drug rehabilitation, counselling and literacy programmes.

In Australia, where about 17 per cent of the prison population is held in private facilities, the Victorian Government took back public control of the Metro Women's Correctional Centre in 2000 after an inquiry found widespread drug use, deaths in custody, poor training and cover-ups.

But most studies paint a more mixed picture of private prisons.

For example, a 2001 report by the Bureau of Justice Assistance of the US National Council of Crime and Delinquency found that "there are no data to support the contention that privately operated facilities offer cost savings over publicly managed facilities".

There was no evidence, it said, that services to prisons and conditions of imprisonment were "significantly improved in privately operated facilities".

In fact, staffing in private prisons was 15 per cent lower than in public prisons, management information systems were less well organised and the number of major incidents higher.

Private prisons also had a higher rate of assaults both on prisoners by other prisoners and prisoners against prison staff.

In Canada, the Government returned the country's one private prison to the public sector in 2006, saying that after five years "there had been no appreciable benefit". An assessment of overall performance found that the publicly run correction centre "performed better in key areas such as security, health care and reducing reoffending rates".

The UK experience is even more instructive. A 2003 report by the National Audit Office in the UK found that the performance of private prisons had been "mixed". The best were better than most public prisons, but the worst were at the bottom, among the least well performing public prisons.

Most British prisons were privately owned till the late 1870s, when they were nationalised amid concerns that commercial gain and the just treatment of prisoners were incompatible. Penal reformers argued that running a prison wasn't quite like running a mill - but that thinking had lost ground by 1992, when the prison system was again part-privatised.

As Tony Blair argued in 1993: "I have to say that I am fundamentally opposed both in principle to the privatisation of the prison service and indeed in practice ...

"I believe people who are sentenced by the state to imprisonment should be deprived of their liberty, kept under lock and key by those who are accountable primarily and solely to the state.

"There is a danger that if you build up an industrial vested interest into the penal system, and as part of that interest they are designed obviously to keep the prison population such that it satisfies those commercial interests ... there is a risk that that distorts the penal policy that otherwise you would introduce."

A leading campaigner against prison privatisation, Stephen Nathan, adds that "privatising prisons requires more people in the criminal justice system for longer and is squarely at odds with the public good".

The point is not how strong a business case we can mount for private prisons; this is about the morality of running prisons for profit, and the proper division between public and private sectors.

British prison expert, Professor Andrew Coyle: "The real issue is not about whether private prisons are managed more effectively than public prisons, or vice versa. The fundamental change which has come about with the introduction of privatisation is the concept of prison as a 'marketplace' and a business which will inevitably expand."