This study explores how syntactic and discourse-based parsing principles direct English relative clause attachment preferences. Forty-nine highly advanced L1-Persian L2-English and thirty-six English native speakers completed a self-paced reading task involving temporarily ambiguous relative clauses that were semantically associated with either the first or the second noun phrase (NP) in a complex NP (NP1–of–NP2) (The resident called the nurse NP1 of the patient NP2 who was injecting penicillin/coughing severely). We manipulated the definiteness of the antecedent (a/the nurse & a/the patient) (...) to examine the extent to which a discourse-based definiteness principle — which motivates attachment to a definite NP — impacts attachment preferences. The results showed no L1/L2 differences, and both groups preferred an NP2 interpretation in relative clauses with a definite antecedent but no strong preference in relative clauses with an indefinite antecedent. The findings highlight the significance of definiteness and cast doubt on the hypothesis that L1 and L2 processing are fundamentally different. (shrink)