Phonology:

In a course at the LSA summer institute in 1991, Alan Prince and Paul Smolensky developed optimality theory—an overall architecture for phonology according to which languages choose a pronunciation of a word that best satisfies a list of constraints ordered by importance; a lower-ranked constraint can be violated when the violation is necessary in order to obey a higher-ranked constraint. The approach was soon extended to morphology by John McCarthy and Alan Prince, and has become a dominant trend in phonology. The appeal to phonetic grounding of constraints and representational elements (e.g. features) in various approaches has been criticized by proponents of 'substance-free phonology', especially Mark Hale and Charles Reiss.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". When did Prince and Smolensky develop their theory?
1991