Among Catholics in the United States surveyed in the study, 48% said they were in favor of legal abortion, while 47% said they were opposed to it and 5% said they didn’t know.

Unitarian Universalists are the most likely religious group to support legal abortion, at 90%, while Jehovah’s Witnesses were the least likely to support it, at 18%, according to the study.

Among both atheists and agnostics, 87% support legal abortion; as do 83% of Jews; 82% of Buddhists; 68% of Hindus; 55% of Muslims; and 27% of Mormons. Among Orthodox Christians, 53% support legal abortion.

The numbers may be surprising, as the Catholic Church is one of the most outspoken opponents of legalized abortion in the U.S. and teaches that abortion under any circumstance is a grave sin.

However, a closer look at other available data for Catholics helps to explain some of this discrepancy.

Overall, “the more frequently you go to Mass, the more likely you are to oppose abortion,” Mark Gray, a senior research associate with the Center for Applied Research in the Apostolate (CARA) out of Georgetown University, told CNA.

However, responses vary significantly depending on the frequency of Mass attendance of the respondent as well as on the phrasing of poll questions about abortion, according to data from the “General Social Survey” analyzed by CARA.

When asked if they would support abortion if a woman wants it for any reason, 85% of frequent Mass attendees (those who go weekly) said they would not support abortion, while 56% of Catholics who attend Mass less than monthly said they would oppose abortion if a woman wants it for any reason.

Responses changed the most among Catholics when were asked whether they would support abortion in situations in which the “woman’s health is seriously endangered.”

When posed this question, 26% of weekly Mass attendees said they would oppose abortion in this circumstance, compared with 5% of infrequent Mass attendees saying the same.

The discrepancy between these two different sets of responses may be attributable to a misunderstanding of the principle of double effect, an aspect of moral theology which can be used in evaluating acts which will have multiple effects.

The principle of double effect states that an act which is not inherently evil may be chosen for a good end, even if it is foreseen that this act will have an additional, evil effect, which is not disproportionate to the good end. The actor chooses the positive end and tolerates the evil effect as a consequence of achieving that good end. The act may never be chosen for the sake of the evil effect.

Therefore, in the case of an ectopic pregnancy, a physician may licitly choose the act of removing the affected area of the mother’s fallopian tubes to achieve the end of saving her life. The consequent death of the embryo or fetus owing to its removal is a foreseen, but unchosen, side effect of that act.

This principle of double effect is sometimes also invoked (incorrectly) to justify an abortion performed to save the life of the mother. However, the principle of double effect does not apply in this case, because the act of abortion is the direct killing of an innocent — an inherently evil act which is proscribed in all cases. Even if the act of abortion is chosen as an end to the means of saving the mother’s life, the act is itself nevertheless evil.