Processing coordinate structures

Coordination often involves syntactically like categories. Based on the results of four reading time studies, it is argued here that the syntactic like-category restriction is not grammatical. Coordination of unlike categories can be just as acceptable as coordination of like categories. However, syntactically like category coordination is processed faster than coordination of unlike categories even when the two sentence types are judged to be fully acceptable. Further, parallelism of conjuncts facilitates processing regardless of whether it is parallelism in the category of the conjuncts (a property which the grammar might regulate) or parallelism in the internal structure of the conjuncts (a property which the grammar does not regulate, on anyone's view). Parallelism did not facilitate processing when the structure of a subject and object were manipulated, implying that parallelism effects are largely limited to the conjuncts of a coordinate structure and not due simply to the repetition of a phrase with a particular shape.