The United States should not rush into a change as large as repealing the ban on gays serving openly in the military without making sure the people it affects are on board, Defense Secretary Robert Gates said Wednesday.

Gates and Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff, said an 11-month study into the effects of lifting the ban will examine practical questions such as how the change would affect the numbers of people who decide to remain in the service when their terms expire.

"There is very little objective data on this. It is filled as you know with emotion and strongly held opinions and beliefs," Mullen said a day after announcing his own opposition to the ban as unfair to gay troops.

"That's the work we have to do over the course of this year. We need to understand that in terms of what the senior military leadership's principal concern is, which is the readiness and military effectiveness of the force," he said.

The study is seen by advocates of a quick repeal as an unnecessary delay, or a political convenience designed to stretch any real action to lift the ban until after congressional elections this fall.

Answering critics from both parties during testimony on the proposed defense budget for next year, Gates offered his own resume as a cautionary tale. He noted that he has unusually broad management experience, having run three large public institutions — the CIA, Texas A&M University and the Defense Department.

"In each of those I have led and managed change," Gates told the House Armed Services Committee. "I've done it smart and I've done it stupid. Happily, I think, the stupid was early."

Gates said he learned that imposing change from on high does not work, and he is determined not to repeat that mistake. Pentagon press secretary Geoff Morrell said later that Gates was referring to a period in the early 1980s when he headed the CIA's analytical division and made policy changes and sometimes harsh assessments that he came to regret.

"Stupid was trying to impose a policy from the top without any regard for the views of the people who were going to be affected or the people who would have to effect the policy change," Gates said.

Gates has commissioned two reviews, one by the outside consultant Rand Corp. and one to be led by a four-star Army general and the Pentagon's top lawyer. The reviews will look at attitudes about openly gay service among the armed forces, with particular emphasis on those in combat.

The reviews are supposed to look at the effect that lifting the ban could have on soldiers' trust and reliance on one another, as well as practical and legal issues, military officials said.

Mullen is the president's senior military adviser, but he said he does not speak for all the Pentagon top brass. Several of the top uniformed leaders in the military services have deep reservations about repealing the policy known as "don't ask, don't tell," and Mullen was unable to unite them behind his view before Tuesday, when he became the first sitting chairman of the Joint Chiefs to say the policy should be changed.

The attitudes of troops under fire, the ramifications for recruiting, family benefits and other considerations are likely to get a fuller examination later this month, when leaders of the Army, Marine Corps and other services testify separately on Capitol Hill.

"I have discussed this with them at considerable length," Mullen said of the Joint Chiefs leaders. "I would sum up their view to say that they need to understand that impact as well, should this policy change."

President Barack Obama pledged to work this year to repeal the 17-year-old policy that says gays may serve in the armed forces only if they keep their sexuality private. In the decades before that, the Pentagon expelled gay troops but the policy was not written into law.

Congress enacted the ban in 1993 as a compromise short of fully allowing openly gay military service, as then-President Bill Clinton had said he wanted.