The ‘Transgenic Spectrum’
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.25
McMahan surmises that ‘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’. And this certainly seems to be true.26 He offers the ‘Transgenic Spectrum’ as a straightforward argument against treating human species membership as a ‘source of moral status’. He asks whether it can be plausible that the moral status of any individual creature on the spectrum can depend solely on having a ‘sufficiently high proportion of human genes to count as a member of the human species’. He answers that it is not. In particular, he thinks it difficult to believe that an individual on the spectrum whose genes have given it the brain of a dull chimpanzee should come within strong moral protection simply because a sufficient proportion of its other genes are human, or that such an individual should be treated any differently from a neighbour 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. This is intended to demonstrate that moral status is not a matter of human genetics per se.
McMahan worries it may be mistakenly inferred that he is claiming there must be some sharp borderline between the human and the non-human in the spectrum experiment. That he is not. Rather, he is claiming only that, whether the property of ‘being genetically human’ is a matter of degree or not, it seems that membership in the human species itself is not the morally salient feature of creatures who have claims against being harmed for certain reasons. 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 real 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.
My own use of the ‘Transgenic Spectrum’ is meant to demonstrate something different. I want to assume for the moment that we have good reason to believe moral status supervenes on human species membership rather than on something else, like cognitive capacities. Granting this, I believe our reactions to the ‘Transgenic Spectrum’ suggest that the punctualist thesis is still difficult to believe. We must ask what the punctualist would have to say about the spectrum. It seems he will have to assert that one of the minute variations along the spectrum marks the difference between being a person and not being a person. But this is precisely what it is hard to accept. With which additional human gene does the transformation take place? There is no reason to believe of any single gene that it rather than another makes such a huge difference. Is it the substitution of the last chimp gene for a human one that brings a person into existence? It is hard to find reasons for this as well. Before the substitution of that final gene, the creature is only barely less human than it was before. It is far more human than it is chimp. So it will be tough to argue that the sharp borderline between persons and non-persons is the addition of that last gene. Needless to say, the same challenge will arise whichever additional gene is proposed as the borderline.27
Defenders of the species membership criterion of personhood may retort that the ‘Transgenic Spectrum’ has little bearing on their appraisal of abortion, since, if they are right, all embryos and fetuses will nevertheless warrant strong protection, being genetically complete human beings. But this would be to miss the lesson from the spectrum experiment. The point is not that hypothetical vague cases of human beings preclude clear cases. Obviously they do not. What the spectrum shows is that, whether the species membership criterion is correct or not, punctual- ism is nonetheless difficult to defend, since it seems clearly to be the case both that there are examples of persons and non-persons along the spectrum, and that no individual variation marks that difference. And if punctualism is not believable, as its implications for the spectrum suggests, then it cannot be relied upon by defenders of the conception threshold to disqualify later thresholds of personhood on the ground of sorites-susceptibility.
Does punctualism seem more credible if the criterion for personhood is switched to some particularly high level of sentience, characteristic only of developed humans? Let us amend the spectrum accordingly: