But the view that viability is the standard for fetal personhood is common -- maybe because it's the law of the land (sort of). The U.S. Supreme Court decided in Roe v. Wade, and reaffirmed in Casey v. Planned Parenthood, that viability is the point at which the state first has a serious interest in protecting the life of a developing human being. (That doesn't mean post-viability abortions can be prohibited under the Court's rulings. The post-viability "health" exception, detailed in Doe v. Bolton, is so broad that virtually any reason is sufficient to justify an abortion late in pregnancy, and thus we in effect have a policy of abortion on demand throughout the entirety of pregnancy.)
Why is viability so relevant? Here is the Court's entire argument, in its 1992 Casey v. Planned Parenthood ruling (decided by a 5-4 vote):
Viability, as we noted in Roe, is the time at which there is a realistic possibility of maintaining and nourishing a life outside the womb, so that the independent existence of the second life can in reason and all fairness be the object of state protection that now overrides the rights of the woman.That's it. As Francis Beckwith writes, this is fallacious:
For the Court to make its argument valid, it would have to add to its factual premise [the fact of fetal nonviability through roughly the first six months of pregnancy] the normative premise: whenever a human being cannot live on its own because it uniquely depends on another human being for its physical existence, it is permissible for the second human being to kill the first to rid the second of the burden.That controversial view was assumed (not argued for) by the Court in both Roe and Casey. So American abortion policy -- and all the unborn human lives sacrificed because of it -- hinged on an unmentioned, undefended moral assumption by five philosophically-untutored (clearly) lawyers.
Problems with the viability criterion are not difficult to find. "Viability" is determined not just by the physical maturity of the fetus, but by the current state of medical technology, what facilities and resources are available, and the skill of doctors. That's why the typical point of viability has moved earlier in pregnancy as medical technology has advanced. But if viability determines personhood, then one's basic rights depend on the technology and doctors available, and those rights can be gained or lost depending on the circumstances or time period one finds oneself in (e.g., a developing-world country). That is absurd.
A conjoined twin depends entirely on the body of another for survival, but no one suggests that she is not a person who merits full respect. Further, we are all "nonviable" relative to our environment. Writes Beckwith: "If any one of us were to be placed naked on the moon or the earth's North Pole, we would quickly become aware of our nonviability. Therefore, the unborn prior to the time she can live outside her mother's womb is as nonviable in relation to her environment as we are nonviable in relation to ours."
Ultimately, viability is a measurement of dependency on other people and things. But we are all dependent (on other people and things) to varying, and often-changing, degrees. So dependency seems a shaky basis for determining whether someone has the right not to be killed. It seems more accurate to say that the dependency and vulnerability of some members of the human family impart on us a special obligation toward them -- not a justification for killing them! The viability criterion has it backwards.