The former head of New York's prison system is warning British policymakers that they do not need to jail more offenders in order to cut crime.
Michael Jacobson was in charge of New York's prison service in the 1990s when inmate numbers fell at the same time as crime fell substantially.
He described the "mass imprisonment" in the US as "a public policy gone mad".
He urged community-based prevention and non-jail alternatives for some breaches of release conditions.
Mr Jacobson, in London for a series of lectures, is currently director of the Vera Institute, a non-profit organisation which advises governments on the administration of justice.
He previously headed New York's probation service before becoming Corrections Commissioner.
The city's declining crime rates in the period were widely attributed to former Mayor Rudolph Giuliani's "zero tolerance" policy, in which police cracked down on minor offences to prevent more serious crimes.
But Mr Jacobson says the success of the policy was not due to tougher prison sentences as the use of jail terms fell.
"The New York experience challenges the conventional wisdom that the more people are imprisoned, the more crime declines," he said.
"The number of people in US jails and prisons reflects a public policy gone mad.
"Spending more and more money on incarceration and police does not mean crime will decrease."
He said public safety was influenced by many factors, not just the criminal justice system.
"Accessible health care, community-based mental health and child care, reasonable school class sizes and well-trained teachers, well-funded environmental and transportation agencies all protect public safety.
"Money spent on spiralling corrections cost has come at the expense of other crucial governmental services."
Mr Jacobson believes the UK, where jail numbers are at record levels and are forecast to rise further, can learn an "important lesson" from the US.
Benefits of imprisonment
Home Office figures last month showed the prison population in England and Wales has reached more than 76,000 for the first time on record.
However, one crime expert said prisons did not just prevent crime.
Professor Malcolm Davies, director of criminal justice at the social think tank Civitas, said: "Once people are locked up in prison - and the average prisoner commits about 140 crimes a year, according to the Prison Survey - you're stopping them doing that.
"So there's a positive benefit of imprisonment.
"And more crucially, in many ways, it's reassuring members of the public and victims that something's being done about crime."
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