Paedophiles and rapists in the West Midlands will be made to take lie detector tests as part of their release from prison.
Up to 450 convicted sex offenders are to take part in a study which monitors their heart rate, breathing and sweat glands while they are being quizzed about their fantasies.
It is hoped the polygraph tests will help to better monitor them once they are living in the community.
Two pilot studies will be carried out in the Midlands over the next three years. So far, 25 sex offenders are lined up to take the tests, with an estimated 450 to be tested over three years.
It could lead to mandatory testing of all sex offenders on their release from prison.
West Midlands Probation chief officer Mike Maiden said: “Sex offenders are known to manipulate situations in order to avoid detection and delude themselves about the risk they present to others.
“We are keen to examine whether lie detector tests can help us better to protect the public by giving us reliable forewarning of any escalation in the risk that a particular offender poses.”
The lie detector study follows an original pilot in the West Midlands between 2003 and 2005 which involved voluntary testing of sex offenders.
Probation chiefs said those low-key trials showed polygraph tests had helped control the behaviour of sex offenders. In three cases the tests prevented the subjects from committing further crimes.