According to the theory of acquired rights, an individual starts life without rights and only qualifies for rights once he or she demonstrates some characteristic or ability. One of the many problems of this theory is defining the qualification criterion.
One version of the theory is that an individual has rights when he has the potential to be rational in future. As discussed in a previous post, this argument actually supports the theory of inherent rights, not acquired rights.
Another version of the theory holds that the qualification required to have rights is demonstrated rationality. According to this version, an individual acquires rights once he demonstrates rationality. Therefore, before an individual acquires rationality, he has no rights. This theory is logically indefensible for the reasons set out below.
It Permits Infanticide
If an individual's ability to reason is his qualification for rights, then children have no rights because children are not rational. A newborn baby cannot be said to demonstrate rationality any more than a newborn foal does. Children cannot be said to exhibit rationality until some significant period of time after birth, unless rationality is redefined so broadly that it would have to include all higher mammals (making the theory incoherent).
A norm of withholding rights until an individual demonstrates rationality would logically permit infanticide, since a newborn child cannot be said to demonstrate rationality. This implication is not one that most advocates of acquired rights theory are willing to accept, yet this must be accepted if one is to argue that children only qualify for rights once they demonstrate rationality. Most advocates of the theory choose to ignore this implication and thereby contradict themselves. Occasionally, one chooses to accept the reductio and make an absurd "libertarian" case for infanticide.
It Is a Conflict-Generating Norm
A norm of respecting rights only once an individual has demonstrated any capability would lead to conflict, and is therefore an invalid norm. It is invalid as a moral rule because of the presumption of rights: you can't demand proof that someone deserves rights, you must presume they do until proven otherwise.
It Is a Non-Universal Norm
Any rule for withholding rights until a specific capability is demonstrated breaks the universality requirement of norms as inter-subjectively ascertainable based on relation of person to physical thing. This is another reason that it is invalid as a moral rule.
The failure of a norm to comply with the universality requirement always has disastrous implications. In this case, the idea of rationality as the qualification for an individual's rights implies justification of class rule by a self-declared elite who may judge the rights-worthiness of others (i.e. the self-declared rational judging whether others meet their definition of rational).
Reason Is Intermittent, Rights Are Not
Reason is by nature an intermittent capacity, so basing rights on demonstrated rationality logically entails removing rights from everyone at numerous points in their lives, or else the theory requires an endless list of arbitrary exceptions. Does someone lose their rights whilst asleep? Does someone lose their rights whilst intoxicated? What about if they are in a coma? People in all these cases fail to demonstrate rationality, so the theory would deprive them of their rights or else fall into a mess of contradictions.
Conclusion
Although reason does play an important role in determining the scope of rights, demonstrated rationality cannot be a criterion for rights. The correct role for reason in rights theory is in the theory of inherent rights, described here.