Former Lib Dem minister Chris Huhne and his ex-wife Vicky Pryce were released from prison today.

Both were jailed for perverting the course of justice after Pryce took Huhne’s speeding points ten years ago.

Huhne said prison had been a “humbling and sobering experience” while his former wife, a Greek-born economist, thanked fellow inmates and prison officers for the support they gave her on the inside.

Fair enough. But many people were left wondering why the pair had been let out after serving just two months of an eight-month sentence.

‘Good behaviour’

It was very widely reported today that Huhne and Pryce had been given time off “for good behaviour”.

But this phrase is a legal myth. There is no such thing as a reduction in sentence for good conduct in British prisons.

The only way your behaviour inside can affect your sentence is if you are judged to have breached prison discipline and an adjudication panel awards you a maxiumum of 42 extra days as a punishment.

Tags for lags

Most prisoners are released after serving half their sentence behind bars.

They are then “on licence” and can be recalled to prison if they fail to stick to conditions like keeping appointments with probation officers, staying away from victims and so on.

But Huhne and Pryce were released under the terms of the Home Detention Curfew scheme, in which some prisoners are eligible for release after just a quarter of the jail term handed down by the judge.

They were eligible because they were given a sentence between three months and four years and they weren’t convicted for violence, sex crimes or any other of the long list of offences deemed unsuitable.

The former power couple would also have had to pass a risk assessment.

Once on the outside, they wear a tag around the ankle or wrist which sends a signal to a receiver stashed somewhere in the home. If the signal shows they have broken the curfew they are liable to get hauled back to prison.

Does tagging work?

It depends what you mean. There is some research which suggests that Home Detention Curfew is a safe and cost-effective alternative to prison.

Ministry of Justice data suggests that tagged prisoners are very slightly less likely to go on to commit more crimes in the two years after leaving jail.

In 2006, the National Audit Office said it cost £1,300 to monitor a prisoner on a tag for 90 days compared to £6,500 for the same period in custody.

Of course, you could argue that prison is less of a punishment or a deterrent if more people get out after serving a quarter of their time.

How prevalent is tagging now?

Home Detention Curfew was launched in 1999 and was quickly embraced by the criminal justice system, with more than 20,000 prisoners a year being let out early by 2002.

The popularity of tagging has fallen in recent years: between 12,000 and 13,000 prisoners a year have released on the scheme since 2010. There were 2,800 prisoners on Home Detention Curfew at the end of 2012.

By Patrick Worrall