Do null subjects really transfer?

Evidence for null subject transfer in early L2 acquisition has come almost exclusively from grammaticality judgment (GJ) tasks, in which early learners show about 30-40% acceptance of ungrammatical English sentences with null subjects. This contrasts curiously with early production data, in which the proportion of null subjects is generally low. The current study reexamines null-subject transfer in L2 English acquisition by adult speakers of Spanish. Three tasks are used: a production task, a GJ task, and a novel comprehension task designed to assess learners' interpretations of null-subject sentences. The same learners who accept null-subject sentences in the GJ task are nonetheless shown to be native-like in the comprehension task, even at the earliest stages of learning. This finding suggests that they do not have referential pro at their disposal to interpret sentences without overt subjects as declaratives, and raises the theoretical question of whether referential pro is subject to transfer, or whether it constitutes a principled limitation to transfer in L2 acquisition.