It is well known that unlike English relative pronoun ‘who’, Spanish relative pronoun ‘quien’ is not allowed to appear in restrictive relative clauses when it functions as subject of the predicate. Many Korean learners of Spanish wonder why this contrast takes place. For this reason, this paper aims to give a new account of this phenomenon. Extending the fact that Spanish tends to avoid cacophony, I suggest that, in proportion to cacophony, this language also tends to avoid “semantic” cacophony, which is triggered when two words of the same meaning appear in adjacent positions. Based on the fact that the relative pronoun ‘quien’ and its antecedent refer to the same referent, I claim that they are actually two words of the same meaning and thus the sequence of these two words causes a kind of “semantic” cacophony that leads to ungrammaticality. On the other hand, I claim that languages like English and French have lower tendency to avoid cacophony than Spanish and, in proportion to this, have also lower tendency to avoid “semantic” cacophony. This is the reason why ‘who’ and ‘qui’, unlike ‘quien’, can be used in restrictive relative clauses when they function as subject of the predicate. In order to support my argument I provide some pieces of empirical evidence that show that Spanish is more sensitive to cacophony and has higher tendency to avoid it than other languages.