Exercise 11.3 The most important data-based evidence for movement in
relative clauses with zero relative pronouns is that the island-constraints
hold in just the same way for them as for relative clauses with overt
relative pronouns. If the island constraints are constraints on movement,
then something must be moving in the zero-relative pronoun case. Because of
this parallelism, all modern theories of syntax treat the zero case as
structurally identical to the overt relative pronoun case.
An added conceptual advantage of the movement analysis of the zero case
is that it allows a unified treatment of all relative clauses, treating
them like the other movement constructions, wh- questions and
topicalizations.