By Joseph Ax

NEW YORK, Sept 28 (Reuters) - Justices on the U.S. Supreme Court should offer a written explanation when they either recuse themselves from a case or deny a request for disqualification, the New York City Bar Association concluded in a report released Friday.

All too often, justices fail to disclose their reasoning when stepping down from a case or refusing to do so when asked, inviting speculation about hidden bias, according to the 39-page report, written by the bar's ethics committee.

"Opening the Court's 'black box' in this limited respect will help the parties and the public to have confidence that the judicial oath to hear cases 'faithfully and impartially' is honored in practice by the highest court in the land," the committee wrote in making its recommendation.

The recusal of Supreme Court justices is a longstanding matter of controversy, most recently in connection with the court's consideration of President Obama's healthcare overhaul. Justice Elena Kagan, who served as solicitor general when the Justice Department first began formulating its defense of the law, and Justice Clarence Thomas, whose wife has ties to conservative organizations opposing the law, both declined to step aside from the case.

"Justice Kagan herself, as well as Justice Thomas, made no public statement on the issue and simply proceeded to hear the case and cast their votes," the report said. "As this and other controversies swirled around the Court in connection with its consideration of the health care law, polls indicated that public confidence in the Court was reaching historic lows, while the belief that justices are influenced by personal or political considerations was reaching historic highs."

A bill introduced in Congress in 2011, the Supreme Court Transparency and Disclosure Act, has called for public disclosure of a justice's reasoning for either granting or denying a request for recusal. The ethics committee's report lauded that provision but did not offer support for the law's other sections, including a process for reviewing recusal decisions.

The bill has stalled in the House of Representatives.

According to the report, former justice William Rehnquist became the first justice to offer a memorandum explaining a recusal decision in 1972. He declined to stand down in Laird v. Tatum, a case involving the Army's surveillance of political activity, despite the fact that he had publicly commented on the merits of the case while working for the Justice Department's Office of Legal Counsel.

Since then, the report noted, such written explanations have been few and far between. Justice Antonin Scalia was one of the few, when he refused to step down from an energy policy task force case in which then vice president Dick Cheney was involved, even though he went on a hunting trip with Cheney weeks after the court agreed to hear the case.

"There is rarely any transparency with respect to how Justices make recusal decisions, even though questions of unfaithfulness and bias, which have arisen since the Court's inception, will only continue to be raised," the committee wrote.

The report noted that the American Bar Association's Model Code of Judicial Conduct calls for judges to "disclose on the record information that the judge believes the parties or their lawyers might consider relevant to the question of disqualification, even if the judge believes there is no real basis for disqualification."

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