But it is not as bad as the US, you might say. The US has more than three times the imprisonment of Australia. However, the imprisonment rate of Indigenous Australians is now higher than that of African Americans. It is 13 times the rate of non-indigenous Australians.

Why has the imprisonment rate gone up so much? There are no doubt many factors, but one of those factors is not more crime or more serious crime. To the contrary, crime rates went down in that period in almost all categories.

For example, the homicide rate has halved in the two decades to 2018, yet the imprisonment rate has doubled.

It does not make sense.

However, my guess is that, if polled, most people would say, contrary to reality, that crime is rising and jail sentences are too lenient.

In Canada, in a similar environment of falling crime, the incarceration rate has also fallen. The sky has not fallen in in Canada and they have probably saved themselves a lot money.

In Australia we are putting people in jail more often and leaving them there longer not for any rational reason associated with sensible administration of justice.

Similarly, the astonishing rise the incarceration rate in the US between 1970 (220 per 100,000 adults) to more than 1000 in 100,000 in 2010 had nothing to do with dealing with a surge of crime or some new research suggesting this would improve society.

In Australia, as Leigh points out, a dominant factor seems to be longer sentences, or at least longer time spent in jail. The average age of prisoners over the three decades rose by seven years, from 29 to 36.

Other factors include a greater propensity to report crime, particularly assault; police taking people to court rather than cautioning; mandatory sentencing; more women and Indigenous people incarcerated; and new laws such as one-punch laws, knife possession, bushfire arson, and cyber crimes.

In all, though, Australia is following some of the unfortunate US trends of between 1970 and 2010 – populism and hysteria stirred on by both traditional media and social media combined with policy-makers promoting privately run prisons.

Our imprisonment rates have more than doubled, which is bad enough. We must not fall into the US trap of sending our rate up five-fold. We have to resist and reverse the trend.

I suspect that social media is having a perverse effect. As soon as a prisoner who committed an horrendous crime two decades or more ago seeks release, all the gory details of the crime are republished as if they had happened yesterday.

Then people get on social media attacking any release or parole. Politicians then respond by passing more laws to make parole more difficult or to impose mandatory sentences, such as “life without possibility of parole” which is so often imposed in the US.

This groundswell then creates a subtle pressure on parole boards and judges who should act independently. But we know they are influenced, on two grounds. First, the incarceration rate has gone up. Secondly, “community standards” are an element in sentencing.

The real problem here, though, is that prisoners are being made to serve a greater portion of their head sentence than they did a few decades ago. But the evidence suggests that the great bulk of crime is done by people in their teens and 20s. Once people, get into their 40s and 50s, they are much less likely to commit crimes and are more suitable for release.

Yes there are risks, but they have to be balanced against cost. Otherwise our jails will become old people’s homes. We need to concentrate more on rehabilitation and education in jail and more on policing because the certainty of being caught is a far greater deterrent than length of sentence.

Even the Americans are beginning to see the absurdity of “life without possibility of parole” – someone in their 90s shuffling about in a jail. The US incarceration rate has been falling in the past decade. We should not go there in the first place.

The full report can be seen here: http://www.andrewleigh.org/PDF/SecondConvictAge.pdf