Beyond Input: Language Learners Produce Novel Relative Clause Types Without Exposure.

Syntax famously consists of abstract hierarchical representations. Less famously, most theories of syntax also assume a higher level of abstract representation: one that abstracts over the hierarchical representations. The existence of such representations would imply that, under certain circumstances, speakers should be able to produce syntactic structures they have never been exposed to. We test this prediction directly. In particular, different types of relative clauses have different surface word orders. These may be represented in two ways: with many individual representations, or with one general representation. If the latter, then learning one type of relative clause amounts to learning them all. We teach participants a novel grammar for only some relative clause types (e.g. just subject relative clauses) and test their knowledge of other types (e.g. object relative clauses). Across experiments, participants consistently produced untrained types, providing the first experimental evidence for this higher level of abstract syntactic knowledge.