Who "it" is influences what "it" does: Discourse effects on children's syntactic parsing.

Children often interpret first noun phrases (NP1s) as agents, which improves comprehension of actives but hinders passives. While children sometimes withhold the agent-first bias, the reasons remain unclear. The current study tests the hypothesis that children default to the agent-first bias as a "best guess" of role assignment when they face uncertainty about sentence properties. Thus, rather than always relying on early-arriving cues, children can attend to different sentence cues across communicative contexts. To test this account, we manipulated interpretive uncertainty by varying cues to the discourse status of initial arguments (referring to new vs. given entities) and measured interpretation accuracy for active (where the agent-first bias predicted verb morphology) and passive sentences (where the two conflicted). Across three experiments, we found that children relied on the agent-first bias more when new discourse entities were signaled by definite NP1s, reference to unmentioned entities, and novel words. This, in turn, led to higher accuracy for actives relative to passives. In contrast, when given entities were implied through pronoun NP1s, reference to mentioned entities, and known words, children avoided the agent-first bias and instead assigned roles using more reliable but later-arriving verb morphology. This led to similar comprehension accuracy across constructions. These findings suggest that children simultaneously interpret relations between sentences (e.g., discourse continuity) and within sentences (e.g., role assignment), such that commitments to the former can influence parsing cues for the latter.