Two Thought Experiments Against the Pro-life View


Almost all pro-life advocates hold that human species membership is sufficient for personhood. Zygotes, embryos, fetuses, 5 year olds, 20 year olds, and 80 year olds, are all equal in dignity and moral value by virtue of belonging to the human species.

However, thought experiments can tease out the extremely counter-intuitive nature of this view. In this post, I will briefly outline two such thought experiments: the transgenic spectrum and the embryo rescue case.

The Transgenic Spectrum

The transgenic spectrum is a thought experiment devised by Jeff McMahan in his book The Ethics of Killing. It forces the reader to reflect on the real phenomena of transgenic animals in order to show that human species membership is not inherently valuable.

Consider the following: The Spectrum begins with a chimpanzee zygote that has an unaltered genome. In the next case, a single human gene is inserted into a chimpanzee zygote and a chimp one swapped out. In the third case, two human genes are inserted. In each case further along in the spectrum, one more human gene is inserted while the corresponding chimpanzee gene is deleted.

Thus, at the far end of the spectrum is a case in which all of the chimpanzee genes are replaced by corresponding genes from a human source, and the creature is fully human. In all cases the genetically altered zygote is implanted in a natural or artificial uterus and thereafter allowed to grow to adulthood. Individuals at one end of the spectrum with only a tiny proportion of human genes are unambiguously chimpanzees and that ‘those at the other end with only a tiny proportion of chimpanzee genes are unambiguously human beings.

However, moral status can’t depend on having a sufficiently high proportion of human genes to count as a member of the human species. First, consider the case of an individual on the spectrum whose genes have given it the brain of a dull chimpanzee, yet a sufficient proportion of its other genes are human. It is truly hard to fathom why such an individual should gain strong moral protection simply because it has enough genes to count as a member of the human species.

On other hand, consider a case of an individual on the spectrum whose proportion of human genes just fell short of making it overall human, even though its brain functions more like that of a human than that of a chimpanzee. The pro-life theory of personhood implies that such an individual is morally equal to the prior individual who got by with just enough genes to qualify as human. This result is absurd on its face.

The humanity of the transgenic creatures will clearly vary if it attaches to their brute proportion of human genes, yet this does not appear to be the basis on which we should morally distinguish between them. The creatures that are more valuable than others along the spectrum will be so in virtue of their enhanced cognitive capacities, not their pure number of human genes relative to chimp ones. In other words, human species membership is wholly irrelevant to how we should treat individuals along the transgenic spectrum. If that’s true, then the pro-life theory of personhood is false.

The Embryo Rescure Case

Perhaps the most well-known thought experiment relating to abortion ethics is the embryo rescue case. Suppose you are faced with the choice of saving either an infant or 100 human embryos from a burning down fertility clinic. Should you save the infant or should you save the 100 human embryos?

At least prima facie, the pro-life theory of personhood entails the later. The reason for this is simple: saving the 100 human embryos amounts to saving the greatest number of innocent people. Remember, the pro-life theory of personhood implies that an embryo and an infant are equal in moral value and dignity. Given this, saving the 100 embryos prevents the greater moral tragedy from occurring. But, intuitively, saving the 100 human embryos is not the most reasonable course of action. In fact, it strikes almost all of us as morally monstrous, allowing an infant to die in order to save embryos that have never even experiences consciousness. The most straightforward reason why almost everyone agrees that we ought save the infant over the 100 embryos is that the moral status of the infant vastly outweighs that of any embryo. Hence, the pro-life theory of personhood is false.

A pro-life advocate might be tempted to resist this implication by appealing to extrinsic factors that can justify choosing to save the infant over the 100 embryos. After all, the infant will presumably feel pain when being burned to death—something that clearly isn’t true of the embryos. Further, the infant almost certainly has a family that will mourn their death. Again, this isn’t true of the 100 embryos in the fertility clinic. Unfortunately, these appeals to extrinsic factors are of no help to the pro-life advocate—for we can just stipulate that the infant is orphaned and is reversibly comatose. In such a case, the infant will not experience pain by dying, and no family members will mourn their death. Even still, the intuition that we ought save the infant is as strong as ever.

What about the distinction between killing and letting die? Can’t a pro-life advocate appeal to this distinction to justify saving the infantover the 100 embryos? Can this give pro-life advocates a much need reprieve? It cannot. In her book Arguing About Abortion: Personhood, Morality, and Law, legal scholar and philosopher Kate Greasley gives a thought experiment that she calls the embryo killing case.

Suppose that someone, under duress, is instructed to kill either 5 human embryos or one little 5 year old girl, in circumstances where failing to make any choice would result in everyone involved being killed. Let us stipulate further, the infant will actually be saved if the 5 embryos are killed. There are no underhanded tricks present in this thought experiment. In such a scenario, it’s clear that killing the embryos is not only one morally permissible option, but the only morally permissible course of action.

Crucially, it misses the point of this objection to point out that intentionally killing innocent people is always wrong, even in extreme circumstances such as those in the embryo rescue case, since such a response presupposes that embryos do count as persons—the very judgement our instinctual reactions to the thought experiment are meant to challenge. As Greasley points out elsewhere, “Given a different choice between, say, killing one little girl or killing 5 different ones(and no way out that would not result in the death of everyone), I agree that there would be grounds for moral hesitation about whether it is permissible to kill anyone, and that this hesitation would be reflected in most people’s attitudes when presented with the dilemma. But this only strengthens the observation that, when confronted with the [embryo killing case], our attitudes reflect a strongly rooted belief in the lesser moral status of human embryos”(In Defense of Abortion Rights, 32)


It seems, then, that things are very grim for the pro-life advocate. The transgenic spectrum case, the embryo rescue case, and related thought experiments expose gaping holes in the pro-life theory of personhood. Specifically, they reveal strong difficulties with the view that human species membership is sufficient for significant moral status. Until pro-life advocates can overcome these difficulties, these thought experiments represent a major strike against their theory of personhood.


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