An opinion this consistent and nearly universal must be based on something. The late political scientist James Q. Wilson gave the most persuasive explanation. In his 1994 essay, “On Abortion,” he argued bluntly that “people treat as human that which appears to be human; people treat as quasi-human that which appears quasi-human.” Sympathy, in his view, grows with resemblance. This explains why the miscarriage of an embryo is (generally) treated differently than the death of a newborn. It is also the reason, according to Wilson, that we recoil from “the thought of killing an infant that does not differ from the newborn in any respect other than that it receives oxygen and food via an umbilical cord instead of through its nose and mouth.”

“Life emerges,” Wilson said, “or more accurately, the claims that developing life exert upon us emerge, gradually but powerfully.” As a fetus becomes more recognizably human, it invokes “attachment that is as natural as any sentiment that ever enters the human breast.” Wilson placed the decisive stage of development, as many Americans seem to place it, at 10 to 12 weeks of gestation.

Wilson was broadly criticized, by both pro-life and pro-choice advocates, for attempting to turn sentiments into principles. As a moral matter, I share that criticism. His gradations strike me as ethically arbitrary, and even universal opinions do not add up to moral rules. But Wilson’s theory of natural “moral sentiments” on abortion does seem to describe the way most Americans think about this issue. Which makes it politically predictive.