In this article I examine the proposition that severe cognitive disability is an impediment to moral personhood. Moral personhood, as I understand it here, is articulated in the work of Jeff McMahan as that which confers a special moral status on a person. I rehearse the metaphysical arguments about the nature of personhood that ground McMahan’s claims regarding the moral status of the “congenitally severely mentally retarded” (CSMR for short). These claims, I argue, rest on the view that only intrinsic (...) psychological capacities are relevant to moral personhood: that is, that relational properties are generally not relevant. In addition, McMahan depends on an argument that species membership is irrelevant for moral consideration and a contention that privileging species membership is equivalent to a virulent nationalism (these will be discussed below). In consequence, the CSMR are excluded from moral personhood and their deaths are less significant as their killing is less wrong than that of persons. To throw doubt on McMahan’s conclusions about the moral status and wrongness of killing the CSMR I question the exclusive use of intrinsic properties in the metaphysics of personhood, the dismissal of the moral importance of species membership, and the example of virulent nationalism as an apt analogy. I also have a lot to say about McMahan’s empirical assumptions about the CSMR. (shrink)