Here we go yet again:

BBC: Paedophiles to undergo lie tests

Paedophiles are to undergo lie detector tests to see if they are likely to re-offend, the Home Office confirmed.

OK, for the sake of argument, let’s assume the following:

• lie-detectors have an accuracy of 90% (although I’m sure it must actually be far lower than this)
• there is a 20% chance of a convicted paedophile re-offending if released (This figure is very difficult to estimate. The latest official recidivism figure for people conviced of child sex offences is 14%, but this doesn’t take into account recidivists who are not re-convicted, nor people who would re-offend, but are never released.)
• 100 convicted paedophiles are tested to decide whether they should be released

20% (i.e. 20) of the 100 paedophiles will re-offend if released. The 90% accurate lie-detector will detect 18 of these. So, presumably, they will not be released, and the other two will.

80% (i.e. 80) of the 100 paedophiles will not re-offend if released. The 90% accurate lie-detector will say 72 of these people should be released, and 8 should not.

Overall Results:

• 74 people released from prison, of whom two (3%) will re-offend
• 26 people remain in prison, of whom 8 (31%) would not have re-offended

If, instead of 90%, I had assumed a lie-detector accuracy of, what seems to me, a far more realistic yet still generous 60%, the result would have been a 14% recidivism rate, with 44 people kept in prison, of whom 32 (73%) should have been released.

Does this inspire you with confidence? If you were being accused of a crime, would you prefer your liberty to be determined by one of these machines, or by a group of your peers?

But we’re talking about convicted paedophiles, so I guess it’s all right, then, isn’t it?