Why not just say something such as "from 1980 to 2000" or "between 1980 and 2000"? I would also mention that without any further context, saying "a criticism of the theories of the 1980s and 90s" suggests disapproval of all of the theories in that time frame. If you don't plan to discuss all of the theories, you could omit the word "the" and, depending on what you mean exactly, it might be better to say that your paper is a "critique" or a "critical discussion" of theories in that time frame.
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