Abstract: Syntactic islands are structures that typically block the extraction of their elements. Movement out of such islands is ungrammatical, resulting in reduced acceptability—a phenomenon termed the island effect. The source of this effect, whether syntactic constraints or cognitive processing limitations, remains debated. This study investigated the sensitivity of 89 English major students to English wh-island constraints and examined the potential influence of working memory on a hypothesized super-additive syntactic effect. Results indicate that: 1) Participants demonstrated sensitivity to wh-islands, rating embedded clauses containing island structures significantly lower in acceptability judgment tasks, and the predicted super-additive syntactic effect was observed; 2) A strong correlation was found between participants' working memory capacity and the magnitude of the super-additive effect. The findings suggest that while syntactic principles may primarily underlie the island effect, individual differences in working memory capacity can modulate its strength, with higher capacity facilitating recognition of the syntactic constraints. We conclude that both syntactic island constraints and working memory capacity contribute to the observed island effect.