A Vatican study on whether it could permit condoms to battle AIDS has a very narrow scope: married Roman Catholic couples in which one partner has the virus. But its theological underpinnings are centuries old, and could lay the groundwork for an end to the church's blanket ban on contraception.

The principle of "double effect" entered mainstream Catholic debate more than 300 years ago and draws on questions about the "lesser of two evils" raised by theologians such as St. Thomas Aquinas in the 13th century. The concepts broadly ask: Can a questionable act be morally justified when the good effect outweighs a bad consequence?

Answering the question with an empathic "yes" are scholars, health professionals and others who want a change in the Vatican's stance that abstinence is the only acceptable way to prevent the spread of AIDS. They have argued for years that condom use as a defense against HIV infection, under specific circumstances, does not contradict the Catholic ban on artificial birth control.

Some groups, including the Southern Africa Catholic Bishops Conference, have even given a tacit nod to condoms for married couples with one partner infected. The Vatican — however tentatively — now could be moving to formally recognize that position.

"It's a reality that's finally moving into a wider arena," said Sister Alison Munro, coordinator of the AIDS project for the Southern African Bishops Conference.

There's no chance the Vatican would fundamentally revise its opposition to contraception, which has been reaffirmed and reinforced since the famous 1968 encyclical "Humanae Vitae."

But even the targeted discussions under way are further evidence of Pope Benedict XVI shedding the tradition-bound reputation he earned during more than two decades as the chief doctrinal watchdog for his predecessor, John Paul II.

Benedict, a widely respected theologian, has shown a willingness to re-examine church attitudes toward advances in genetic engineering and in-vitro fertilization. But none approach the sensitivity of whether to open the door — even a crack — for condoms.

"The Vatican is like a submarine. On this one, it has put up its periscope, looked around and submerged again," said the Rev. James Keenan, a moral theologian at Boston College. "It's still not clear at all what — if anything — the Vatican will eventually say on the subject."

Helen Hull Hitchcock of Women for Faith & Family, a traditionalist group based in St. Louis, predicted it could be "deeply confusing" for Catholics if the church made any concessions.

"People would say, `Now wait a minute. If it's OK for this couple to use it, why can't another couple use it,"' Hitchcock said. "We think that it would be very worrisome."

Cardinal Javier Lozano Barragan, who heads the Vatican's office for health care, would only confirm a "dialogue" is under way as part of a larger examination of bioethical issues. The study on condoms only concerns married couples in which one partner has the virus, his office said.

Notably, there have been no official announcements of an upcoming document or details of the discussions. But a possible signal came last month from retired Milan Cardinal Carlo Maria Martini, who was quoted by the Italian newsweekly L'Espresso as saying condoms were the "lesser evil" in combatting AIDS.

Martini, once considered a top papal contender, was not the first Catholic leader to make this connection. The timing, however, was widely interpreted as a hint of the Vatican's leanings.

"Martini was not reprimanded or asked to correct himself," said the Rev. Michael Fahey, a professor of theology at Marquette University. "This seems to say that the Vatican is moving in this direction or at least wants to send a trial balloon."

If the Vatican allows condoms as an AIDS control measure within a marriage, it would open the way for Catholic groups to take a more direct role in anti-AIDS campaigns in ravaged places such as Africa, where the virus is often transmitted from husbands to wives. Catholic charities in Africa offer health care and many other services to AIDS sufferers, but come under sustained criticism for their refusal to distribute condoms.

Sub-Saharan Africa accounts for more than 60 percent of the 40 million people infected with HIV worldwide.

In 2001, more than 30 bishops from southern Africa denounced the use of condoms, but noted that married couples should "listen to their consciences" — widely viewed as recognition the "lesser of two evils" scenario.

The Vatican initially came down hard on such rationales. But it gradually retreated as influential theologians and clergymen made the case that condom use — in cases such as between an HIV-infected person and spouse — would fall under the "double effect" rubric, which says a good intent (not passing the virus) has a bad consequence (using the condom).

The principle is often used to rationalize causalities in a "just war" or a procedure to end a pregnancy to save the woman's life.

The related "lesser of two evils" views boil down to moral damage control. A priest should always advise against doing "evil," but encourage a "lesser evil" if they can't stop the act.

In 2000, Monsignor Jacques Suaudeau of the Pontifical Council for the Family wrote an article in the Vatican newspaper, L'Osservatore Romano, strongly supporting sexual abstinence to control AIDS, but noting specific cases where condom use could be considered a "lesser evil" — including prostitutes in legal brothels.

The idea since has been echoed by even more powerful figures. Belgian Cardinal Godfried Danneels has said an HIV-positive person would be committing a sin by having sex without a condom.

"Let's hope the Vatican brings some clarity to this issue," said theologian Keenan. "It would finally take the stigma off the condom. Then it's all over. The condom will be freed of this whole, heavy moral debate."