High Court justices are more likely to find in favour of the federal government while the prime minister who appointed them remains in office, a new study finds.
An analysis of more than 400 High Court cases over more than two decades shows justices were roughly 70 per cent more likely to vote for the government of the prime minister who appointed them.
But the study found no evidence of political party loyalty once the appointing prime minister left office.
"Based on almost 2000 opinions given over a quarter of a century, we find no systematic evidence that Justices prefer their appointing party over the other party of government," the paper said.
"This is distinct from positive findings from the US Supreme Court, where lasting partisan effects are shown to exist."
Justices appointed by Liberal-National Coalition governments were 102 per cent more likely to vote in favour of their appointing prime minister than other prime ministers, with the data showing no significant personal loyalty from Labor government appointees.
"Justices appointed by [Kevin] Rudd, for example, were no more likely to vote in favour of an ALP government led by Julia Gillard than they were to a Coalition government lead by Tony Abbott," the paper said.
"Tentatively, this supports our hypothesis that personal - rather than political - loyalty is the more compelling form of judicial-executive tie in the Australian context."
The federal government has the power to appoint judges in secret and with no oversight, unlike the US where Supreme Court nominees must be confirmed by the country's Senate.
The power to appoint justices to the High Court rests with the Governor-General, but in practice the Governor-General makes appointments on the advice of the Prime Minister and the Attorney-General.
The paper - written by Dr Patrick Leslie and Professors Zoë Robinson and Russell Smyth - argues the Australian system gives greater power to prime ministers and the Attorney-General to appoint people to the High Court with personal connections.
"While there are probably a few exceptions throughout the Court's history, most appointments could be justified on the merits model," the paper said.
"However, the real issue is that the question of who is the best qualified candidate is subjective and one to which reasonable minds may give different answers."
Dr Leslie said it was important to understand judges were people too and subject to the same kinds of influences on decision making as anyone else.
"The way we appoint Justices to the High Court is a highly politicised process, even if we lack the drama of US style confirmation hearings, so in some ways it would be more surprising if we found no evidence of a loyalty effect to the appointing PM," Dr Leslie said.
Last year a study found justices of the High Court made decisions on ideological grounds, part of an increasing body of research challenging the long-held view Australian judges apply the law objectively.
That paper found a justice's ideological position before their appointment was a reliable predictor of their voting pattern in both rights and non-rights based High Court cases.
Prime Minister Scott Morrison announced last year Federal Court justices Simon Steward and Jacqueline Gleeson would be appointed to the High Court.
Then-Attorney-General Christian Porter said at the time the six-month selection process was exhaustive and extensive.
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