Identity figures much more widely in phonology proper, though perhaps less obviously. According to Optimality Theory, "faithfulness" constraint demand that the output be as close as possible to the input, along all the dimensions upon which structures may vary. Derivation is determined to a large degree by the interaction between faithfulness constraints, demanding identity, and constraints on output structural configurations, which may favor modification of the input, contravening faithfulness. Input-output faithfulness and base reduplicant identity, we argue, are effectively the same thing, controlled by exactly the same set of formal considerations, played out over different pairs of compared structures.
- McCarthy & Prince (2004), a pag.370-371