Jinlu Liu, Ming Wu, Haitao Liu
Abstract Understanding how different dimensions of syntactic complexity interact with processing demands is fundamental to language acquisition, pedagogy, and assessment. While structural measures like Clausal Density (CD) and Embedding Depth (ED) have been examined separately, their relationship with processing load, as proxied by Mean Dependency Distance (MDD), remains underexplored. This study investigates this interplay in 42,654 complex sentences from the Brown and LOB corpora, using statistical modeling to examine the nature of the CD-MDD relationship, ED’s independent contribution to MDD, and their interaction. Results reveal a non-linear CD-MDD relationship peaking at CD = 3, a distinct processing cost at the transition to double embedding (ED = 2), and a significant interaction where deeper embedding reverses the CD-MDD relationship. These findings demonstrate a dynamic trade-off in syntactic complexity, challenging additive models and supporting cognitive constraint theories. The results have direct implications for refining complexity measures in second language acquisition and for informing psycholinguistically-grounded pedagogy and assessment.