The theory of acquired rights holds that everyone starts life without rights and only qualifies for rights once he or she demonstrates some characteristic or ability. Different qualifying criteria for rights have been proposed, all of which have problems.
In previous posts, I addressed the issues with using potential for rationality and demonstrated rationality as qualifications for acquiring rights. This post discusses the problems with using physical or physiological characteristics as the qualification for rights.
Various physical or physiological characteristics have been proposed by adherents of the theory of acquired rights as criteria for rights. For example, Jason Sorens advocated having a brain that can sense pain and demonstrate sleeping and waking states. Wendy McElroy argued that being no longer attached to the mother (i.e. birth) is the qualifier. Whatever physiological criteria are advocated, the following problems apply.
Physiological Criteria Are Arbitrary and Irrelevant
Physiological criteria have no relationship to the problem that rights are created to solve. Therefore, they are arbitrary and irrelevant to the question at hand. The purpose of ethical norms is to prevent conflict over scarce resources. Any criteria for rights has to have some relationship to the problem rights are solving: the need for objective rules to avoid fighting in a world of scarcity.
This is why rationality must play some role in the criteria for rights. Since rationality is the tool used to create moral rules and the tool used to agree such rules, it makes sense to argue that rationality is in some way involved in determining rights (the exact way is discussed here). Rationality creates its own issues for rights theory, but at least it is not arbitrary. In contrast, what have physiological criteria got to do with rights?
Physiological Criteria Entail Both Animal Rights and Human Infanticide
If rights are based on physiological capability, the logical implication is that drawn by Peter Singer: higher mammals have a right to life and human newborns do not.
Any merely physiological capability (as opposed to rationality) would include higher mammals. Some adult higher mammals demonstrate more physiological and mental capability than a human baby does.
Any rule granting rights to a human baby on the basis of its physiological capabilities would have to outlaw killing of animals, since the baby is less capable than a higher mammal.
Therefore, if rights are based on something physiological, you end up with Singer's conclusion that higher mammals are persons with rights and newborn humans are not. Animal rights are a logical consequence of demanding specific physical capability for rights.
If a theory ends up granting rights to animals, it is incoherent as a theory of ethics. We are no longer addressing the problem of how to create rules to avoid conflict over scarce resources, since animals cannot agree or uphold moral rules.