Gallup routinely asks, “Do you think abortions should be legal under any circumstances, legal only under certain circumstances, or illegal in all circumstances?” For the last thirteen years, it has subdivided that middle option into “legal under most circumstances” and “legal only in a few circumstances.”

A pretty typical recent result: In May 2005, 23 percent of respondents said abortion should legal under “any” circumstances, 12 said “most,” 40 said “only in a few,” and 22 said “illegal in all circumstances.” Pro-lifers find this polling data congenial, since it yields a 62 percent majority choosing the two most pro-life of the four options (40 + 22). (Some polling by other organizations makes these findings plausible.)

Other polls, however, find results that appear to conflict with these. The ABC/Washington Post poll asks, “Do you think abortion should be legal in all cases, legal in most cases, illegal in most cases, or illegal in all cases?” This question has typically yielded a majority for the two most pro-choice options. In Dec. 2005, for example, 17 percent picked “legal in all cases,” 40 “legal in most cases,” 27 “illegal in most cases,” and 13 “illegal in all cases.” That’s a 57 percent pro-choice majority.

I’m looking for explanations for the divergent results–specific explanations, that is; something that goes beyond the point that many people are ambivalent about abortion policy. (Ideally with some evidence to back up the explanation.) Can switching from the word “legal” to the word “illegal”–that is, from “legal only in a few circumstances” to “illegal in most cases”–cause a 19-point shift in measured public opinion? Is there some difference between how people respond to “cases” and how they respond to “circumstances?” Note also that neither set of results appears to be a fluke: They are consistent over time in generating predictably divergent results.

Also: Is there any reason to think that one of these ways of phrasing the question is better than the other, in terms of shedding more light on public opinion?