This study investigated how Turkish-speaking children and adults interpret negative sentences with disjunction (English "or") and ones with conjunction (English "and"). The goal was to see whether Turkish-speaking children and adults assigned the same interpretation to both kinds of sentences and, if not, to determine the source of the differences. Turkish-speaking children and adults were found to assign different interpretations to negative sentences with disjunction just in case the nouns in the disjunction phrase were marked with accusative case. For children, negation took scope over disjunction regardless of case marking, whereas, for adults, disjunction took scope over negation if the disjunctive phrases were case marked. Both groups assigned the same interpretation to negative sentences with conjunction; both case-marked and non-case-marked conjunction phrases took scope over negation. The findings are taken as evidence for a "subset" principle of language learnability that dictates children's initial scope assignments.