Mandatory minimum sentences. They're the modern "Stone Age" policy.

So say criminal justice advocates who are slamming U.S. Attorney General Jeff Sessions's latest tough-on-crime policy as a return to the bygone "War on Drugs" era they had hoped was well on its way to extinction.

Under the new law-and-order administration of President Donald Trump, that era of harsh penalties, even for potentially small-time drug offenders, appears to be roaring back.

Sessions said Friday the power for prosecutors to recommend the strongest sentences possible for drug offenders is a "bedrock" responsibility. A two-page memo announcing the policy change instructs the Justice Department's prosecutors to pursue and charge the "most serious, readily provable offense" they can determine, a move that can trigger lengthy mandatory minimum sentences.

Critics warn it could strain already crowded federal prisons and raise recidivism rates.

"For someone who's not a hardened criminal, but who's been locked away for 25 years, re-entry (into civilian life) is very hard," said criminologist John Pfaff, author of Locked In: The True Causes of Mass Incarceration—and How to Achieve Real Reform.

"You don't come out with savings…You have fewer skills, less health, fewer social ties, all these things that encourage people to desist from committing crimes."

The new memo rescinds a 2013 policy from former Obama-era Attorney General Eric Holder that attempted to ease the burden on the federal corrections population by advising leniency. Prosecutors were given discretion to tailor sentences to individual defendants, depending on mitigating factors.

Obama oversaw drop in prison population

Holder had urged federal attorneys to overlook the quantity of drugs as a means for upping the ante for non-serious offenders. It was a way to keep defendants below mandatory minimum thresholds.

That policy was credited for making Barack Obama the first president in nearly four decades to oversee a drop in the federal prison population. Although numbers declined by fewer than 8,000 people, or 0.5 per cent of the total prison system population of 1.6 million, it was still viewed as progress.

Experts on criminal sentencing and prison policy say the Sessions directive is a retrograde callback to the tough-on-drugs approach which, while popular during the Richard Nixon administration, began falling out of favour in the 1990s.

"The Trump administration's approach represents a return to the Stone Age of criminal justice policy," says Daniel Medwed, a professor of law and criminal justice at Northeastern University. "It's like we haven't moved on. It's all reflecting comments made 30 years ago."

U.S. attorneys obligated to push for the highest sentences would do so by stacking charges, resulting in decades-long incarceration for even non-violent offenders with little or no criminal records.

DOJ has taken an unwise step backward to discredited criminal justice policies. The need for reform still exists. <a href="https://t.co/Q0GT99qYBz">pic.twitter.com/Q0GT99qYBz</a> —@EricHolder

For example, a low-level, small-quantity cocaine dealer might be arrested as part of a wider drug operation, tagged for the entire amount of drugs sold cumulatively in the network, then named as a co-conspirator in a massive criminal enterprise.

The offender may have never even been in contact with many of the other co-conspirators.

Previously, however, "the people who benefited from the Holder approach were often the minor players in drug conspiracies, like the girlfriend (of a trafficker) whose apartment the drug deal was conducted in," says Sonja Starr, a law professor at the University of Michigan who specializes in prison sentences.

"In some cases, it might not be necessary to use the public's resources to incarcerate someone for a long time, and it may just be gratuitous suffering imposed to a person and his or her family in a case that doesn't necessitate it."

'Unwise step backward'

A prosecutor pursuing the harshest charges might lump in whether a gun was in an offender's home at the time of a drug bust, or whether more than one weapon was present. Criminal law experts say prosecutors might consider whether an offender "brandished" or "possessed" a gun, resulting in different charges.

Holder criticized Sessions's rollback of his own policies as "an unwise step backward," noting that sentencing reform efforts enjoyed bipartisan support.

"The policy announced today is not tough on crime. It is dumb on crime," he wrote in a statement.

Republicans and Democrats have agreed on a need to reduce mass incarcerations, with fiscal hawks, libertarians and civil-rights advocates making the call to reduce the ballooning prison population. In the 1980s and '90s, Starr said, both parties were in favour of the expansion of the criminal justice system, before politicians across the aisles eventually started to rethink that hardline approach.

"Sessions was not one of those people," Starr said.

Some conservatives are reserving judgment on the Sessions memo.

John Malcolm, vice-president of the Institute for Constitutional Government at the conservative-leaning think tank the Heritage Foundation, believes pushing for mandatory minimums can be effective, provided the right criminals are targeted.

"This is designed to increase the general deterrence that mandatory minimums provide, and if it's used to target kingpins and cartel leaders…of large-scale drug conspiracies, it may very well have the desired effect of getting bad people off the streets for a very long time," Malcolm said.

(Starr and other justice-reform scholars dispute the idea that severe sentencing law significantly deters drug offenders.)

Shouldn't target low-level criminals

Malcolm's only concern is whether small-time, non-violent criminals will get caught and locked up for long periods of time if the net is cast more widely.

"If it ends up catching people at the very bottom of the totem pole of very large conspiracies, that will be an inefficient use of federal resources," he said.

"Mandatory minimum penalties are a very powerful tool that can be effective, if used in the right way. I just hope it will be used in the right way."