In a dramatic turnaround that could strain the long-standing ties between the psychology profession and the military, the American Psychological Association has reversed its policy of encouraging members to assist in the interrogation of suspected terrorists at Guantanamo Bay, Cuba, and other overseas prison sites.

The professional association’s new policy, which was reached by a referendum, goes beyond telling members, even those who are military personnel, that it is off-limits to participate in interrogations at detention centers abroad. Members would be prohibited from working at such sites in any capacity that directly assists the government. The prohibition would apply to psychologists who work as psychological profilers or even as clinicians who treat detainees as mental health patients.

“This goes beyond interrogations,” a Boston psychologist who has sought to change the APA’s position, Stephen Soldz, said. “The thought is that if you are there and a part of the military chain of command, then you are part of the system.”

The new policy represents “a significant change” in the association’s policy on the involvement of psychologists in interrogations, the association said in a statement. A spokeswoman, Rhea Faberman, declined to make any officers at the APA available for comment. According to the bylaws of the APA, the policy does not go into effect for another year.

Previously the APA has generally encouraged a policy of “engagement” — or involvement in national security interrogations — for the purpose of stopping “interrogations that cross the bounds of ethical propriety,” as the director of the APA’s ethics office, Stephen Behnke, wrote in a letter earlier this year. APA officials also had encouraged engagement in the interrogation process by psychologists, on the grounds that psychologists have expertise to lend and ought to assist in the country’s anti-terrorism efforts.

The APA had already banned its members from participating in any of 19 interrogation techniques, including the use of hoods, forced nakedness, and waterboarding.

Since June 2006, the Defense Department has relied increasingly on psychologists to staff the behavioral science consultation teams, which advise interrogators on how to attempt to elicit information from detainees. Before then, psychiatrists had participated on such teams, but the Defense Department announced it would increase its reliance on psychologists after the American Psychiatric Association began a policy of instructing its members not to participate.

The role that psychologists played in advising interrogators is not well-documented but is increasingly coming under scrutiny. During a court proceeding at Guantanamo last month, lawyers informed the court that a military psychologist would invoke her right under the military’s equivalent of the Fifth Amendment, were she called as a witness. At issue was the psychologist’s role in devising the conditions of detention and the tactics of the interrogation of a detainee facing war crimes charges, Mohammad Jawad. The detainee’s attorney, Major David Frakt, claims in court papers that the psychologist advised that Mr. Jawad be put under extremely isolating conditions and that interrogators exploit his concerns about his family.

While not all licensed psychologists are members of the APA, a majority are, according to information provided by the association. The APA’s military psychology group has 442 members, although it was not clear whether all of those were uniformed military personnel. Because the APA can conduct investigations against its members for violating APA ethics codes and forwards any adverse findings on to state psychologist licensing boards, the new policy goes far beyond a statement of principles.

It is unclear how the military will respond to the APA’s new policy and whether it will remove psychologists from teams that advise interrogators. The new policy also would apply to any detention sites run by the Central Intelligence Agency, but would allow psychologists to be present at such sites if they were employed by an “independent third party working to protect human rights,” such as the Red Cross.

The measure could put pressure on military psychologists involved in detainee programs to seek other work.

“These people are going to want to go back into the civilian work force some day,” Dr. Soldz said. “This will make it harder for the military to recruit psychologists, if the military asks them to do things that are unprofessional.”

The new policy was decided by a vote put to the 90,000 members of the APA’s voting membership. Of about 15,000 members who returned ballots, 59% voted for the resolution and 41% against.

The chief executive officer of the group Physicians for Human Rights, Frank Donaghue, said the vote was a “blow against medical complicity in torture.”

The text of the resolution states, in part, that “psychologists may not work in settings where persons are held outside of, or in violation of, either International Law or the US Constitution.” Because the conditions at prisons in America are occasionally found, during the course of a civil rights lawsuit, to violate the Constitution, a strict reading of the new policy would suggest that APA members could not work in such facilities.