all filler sentences had a subject (abbreviated D N) and a verb (abbreviated V), with the verb being optionally transitive and followed by a direct object (D N). The subject, object, or both could be modified by a prepositional phrase (P D N). The subject and object could be either singular or plural, with the optional auxiliary (Aux) agreeing in number with the subject. There were 30 nouns (N; 60 with plural forms), 2 auxiliaries (Aux; *was/were* and *has/had*), 1 determiner (D; *the*), 14 verbs (V), and 4 prepositions (P). An example filler sentence is given in \[syn\_filler\], and an example RC sentence is given in \[syn\_rc\].

. \[syn\_exp\] . The nephew near the children was seen by the players next to the lawyer.\[syn\_filler\] . The gymnast has met the hostage of the women that was eating. \[syn\_rc\]

We trained RNN LMs on our synthetic language using the same parameters as the English LMs given in Section \[lm\_details\], with 120,000 unique sentences in the training corpus. The resultant RNN LMs were tested on 300 sentences with ambiguous RC attachment, and we measured the surprisal at the RC auxiliary verb (*was/were*), following the methodology given in Section \[measures\].

To determine how 