Rule naturalness and the acquisition of phonology

Standard generative phonology: privileged status to " natural " phonological rules (those with a clear phonetic motivation). Making it easier for theories to express common processes (which tend strongly to belong to the natural category) is often considered a primary goal. " Unnatural " phenomena are typically relegated to a " peripheral " or secondary status. The usual (tacit) assumption: they require some other apparatus in the grammar. If nothing else, the child ought to first attempt to apply basic principles of the phonology — biased toward naturalness — and only when this fails will she resort to secondary principles. An implication of this sort of approach — implicit or explicit — is that a child learner equipped with the universal phonological faculty will more easily learn processes that are natural. (1) Morphophonological patterns that are in accord with natural phonological predispositions will enter early and will seldom lead to errors. (2) Phonological rules with a heavy natural component (e.g. voicing assimilation) appear to be acquired early. Problem: If children are biased toward natural processes, processes ought to become more natural over time, or at least not become less natural. Language transmission is imperfect, and bias toward natural processes should err in favor of them. But phonological rules tend to become less natural over time (cf. Hyman 1975: 181f): (3) Although sound changes are sometimes blocked by considerations within a paradigm [...] no corresponding force has been discovered which would strive to keep rules natural. Instead, the above examples show the great tendency for rules to become unnatural [...] that is, to lose their phonetic plausibility and become morphologically conditioned. Natural and unnatural rules in the same language can have similar status. Kashaya (Pomoan: N. California) has lowering to [a] after a uvular (natural); and /i/ becomes [u] after [d] (unnatural, and arose out of a morphological reanalysis: Buckley 2000).