Donald Trump’s health secretary, who defended the administration’s child separation policy as “charity”, once argued in a private paper that repeat juvenile offenders cannot be rehabilitated and ought to be jailed until they reach middle age, according to a memo obtained by the Guardian.

A 1991 legal memo written by Alex Azar, a former drug industry executive who is now secretary of health and human services (HHS), rejected the notion that a juvenile who committed a crime was a “salvageable human” who could be treated rather than punished.

Instead, the memo, written for a senior attorney at a private law practice, outlined the legal rationale for a criminal justice policy that favoured reducing and deterring crime via mass incarceration.

Azar is closely linked personally and ideologically to Brett Kavanaugh, the Trump administration’s nominee to the supreme court. The two men were among the first young prosecutors hired by Kenneth Starr when he became independent counsel and investigated the Clinton administration. In a tweet following Kavanaugh’s nomination to the high court, Azar said the judge had been a “close friend for 30 years”.

Azar has recently come under scrutiny because HHS has been charged with overseeing the supervision of more than 2,000 children separated from their parents as part of the Trump administration’s “zero tolerance” immigration policy. The separation policy was designed to deter illegal crossings into the US.

Azar has defended the policy. In a July interview on CNN, he said the care of immigrant children represented “one of the great acts of American generosity and charity”. He also said the children were being cared for in federal facilities by “well-meaning, altruistic individuals” and were happy and loved in a “compassionate environment”.

The policy of separating parents from their children has ended but it is unclear how hundreds of children who remain in US custody will be reunited with their families.

The 1991 memo on criminal justice policy, which explores the legal rationale for mass incarceration, was written when Azar was a fresh graduate of Yale Law School, working as a summer associate for Richard Willard, a Reagan administration lawyer and attorney at Steptoe & Johnson.

Facebook Twitter Pinterest Alex Azar with Donald Trump during a White House announcement on drug policy in May. Photograph: Ron Sachs / POOL/EPA

Willard is listed as a contributor to the rightwing Federalist Society, which helped choose Kavanaugh as a nominee, and is a leading conservative voice on legal matters. According to one person who has followed Azar’s career, the young graduate’s work for Willard was seen as a way to develop his conservative credentials.

In the memo, Azar wrote that he had collected evidence to “undergird” Willard’s thesis, which the two had discussed.

Willard favoured incarceration as a deterrent to criminal behaviour, “locking up” repeat juvenile offenders until they were in their 30s – an age when the memo said individuals were no longer prone to commit acts of violent crime. The memo supported the growth of privately run prisons to handle overcrowding. Azar also made the assertion that young black men committed the vast majority of violent crimes, citing as evidence the frequency of their arrest.

The analysis did not critically analyse whether there was racial bias in policing and did not address the moral implications of decades-long jail sentences for juveniles.

“Special juvenile courts were created in late 19th century with [the] idea that [a] juvenile was a salvageable human who needed treatment rather than punishment in criminal court,” Azar wrote, citing a report. “We see today that that is not the case.”

Caitlin Oakely, a spokeswoman for HHS, said the memo did not necessarily reflect Azar’s current view on criminal justice matters. But she declined to comment on what that view was.

“Doing research at the instruction of a senior partner during a brief summer associateship 27 years ago says nothing about Secretary Azar’s personal opinions or views,” she said. “This is a ridiculous insinuation even by the standards of the Guardian. He was a summer associate; he was tasked with research for someone else’s project for someone else’s purposes.”

Legal and criminal justice experts who analysed the memo for the Guardian said it in some ways reflected conservative views about criminal justice in the early 1990s, when crime in America was at its peak.

“It’s fairly clear that Azar was given a thesis in quest for a basis and then tasked with backfilling that thesis with any factual support he could muster,” said Ramzi Kassem, a professor at CUNY School of Law. The thesis favoured over-enforcement, hyper-policing of communities of colour, and mass incarceration to bolster privatisation.

“With the benefit of hindsight since 1991, we all know this approach has proven to be an unmitigated disaster,” Kassem said. “The profit incentive that has come with privatisation has driven mass incarceration in black and other communities of colour to epidemic proportions, with all sorts of social, economic, and political consequences, including disenfranchisement.”

Ames Grawert, senior counsel at the Brennan Center for Justice, said the thesis presented by Azar reflected a time when policymakers had lost faith in rehabilitation and believed incarceration was the only solution to crime.

“Unfortunately those views are still held by a few people,” he said. “And the attorney general, and some of the president’s policy advisers, happen to be among them.”