Clahsen and Felser (CF) analyze the performance of monolingual children and adult second language (L2) learners in off-line and on-line tasks and compare their performance with that of adult monolinguals. They conclude that child first language (L1) processing is basically the same as adult L1 processing (the contiguity assumption), with differences in performance being due to cognitive developmental limitations. They argue that differences in L2 performance, however, are more qualitative and not explained by shortage of working memory (WM) resources, differences in processing speed, transfer of L1 processing routines, or incomplete acquisition of the target grammar. They propose a shallow structure hypothesis (SSH) to explain the differences reported in sentence processing. According to this, the syntactic representations computed by L2 learners during comprehension are shallower and less detailed than those computed by native speakers and involve more direct form-function mappings.