"Tough on crime is catchy but it hurts — the pain is now too much."

Social policy that does not rise above emotive slogans nearly always ends badly.

The tough-on-crime mantra across the past four decades that has resulted in expertise being sidelined from sentencing law has predictably caused an incarceration crisis in the US and Australia.

Both countries spend about $200 per capita annually on detaining prisoners. This is money that could be spent on productive social services, such as education and health.

Worse still, governments are clueless about how to reduce prison numbers in a principled and methodical manner.

A key part of the solution involves identifying which prisoner cohort should be dealt with more leniently without undermining the community protection benefits of imprisonment. To this end, the starting point is to focus on the prisoner cohort whose numbers have been increasing most rapidly in recent years.

Incredibly, this unwanted designation belongs to people who normally give the community little to fear: the elderly. The number of old people in jail is rising so rapidly that prisons are morphing into old people’s homes — and woefully incompetent ones at that.

The number of prisoners aged 55 and older in the US has increased more than 15-fold across the past three decades, with the biggest increase relating to those aged 65 and older. A similar — although less pronounced — trend is occurring in Australia, with the portion of prisoners aged over 65 more than doubling during the past decade.

There are two main reasons for the rapid growth in aged incarceration levels.

First, average prison sentences are increasing in length and hence prisoners age more while in prison. For example, the average length of prison sentences in the US has increased by more than one-third since 1990 and the number of prisoners serving a life sentence without the possibility of parole increased 22 per cent between 2008 and 2013.

Second, there has been a stark increase in the number of elderly people entering prison. The increase far exceeds the general increase in the ageing population.

This represents a fundamental public policy failure. All people should be punished commensurate with the seriousness of their crime. This applies to aged offenders no less than to the rest of the community.

However, aged offenders are a discrete cohort in society so far as the proper objectives of the criminal justice and sentencing system are concerned. The relevant differences are not properly reflected in the sentencing principles and practices.

A key reason for distinguishing between aged and younger offenders is that aged offenders are far less likely to reoffend. Their recidivism rate is half that of other offenders.

The correlation between the passing of age and lower offending rates is stark. There appear to be two reasons for this phenomenon.

The first is that people become less aggressive and more risk averse as they get older. The second is simply a physical reality: infirmity reduces the capacity to harm others.

The futility of imprisoning aged offenders for long periods is exacerbated by the fact ageing inmates cost at least twice as much to incarcerate as other prisoners. This discrepancy in the cost of incarceration is primarily driven by the greater medical and healthcare needs of ageing inmates.

Aged offenders also suffer more while in prison. Older people are likelier to develop age-related illnesses and infirmities. This decline in health is accelerated by the effects of incarceration.

Elderly prisoners have a heightened risk of chronic illnesses, cognitive limitations and physical disabilities. Aged prisoners are also more susceptible to victimisation by other prisoners.

A principled and empirically grounded approach to aged offenders would result in offenders who are sentenced after turning 50 years of age receiving a penalty discount of at least 50 per cent, unless they have committed serious violent or sexual offences.

Further, all inmates who turn 65 and who have served at least half of their sentence and are not a significant danger to the community should be removed from prison and placed on electronic monitoring.

This would greatly reduce the cost of their confinement while not compromising community safety.

These reforms would limit the gratuitous suffering that is inflicted on many offenders and recognise the relevantly different situation of aged offenders. At the same time it would save billions of dollars without compromising community safety.

Better still, the empirically driven approach would result in a template for a principled wide-ranging reassessment of sentencing law.

Until this happens, our approach to sentencing will remain a black spot on our collective psyche and a needless fiscal burden.

“Tough on crime” is catchy but it hurts — the pain is now too much.