Children use correlations between the syntax of a clause and the meaning of its predicate to draw
inferences about word meanings. On one proposal, these inferences are underwritten by a
structural similarity between syntactic and semantic representations: learners expect that the
number of clause arguments exactly matches the number of participant roles in the event concept
under which its referent is viewed. We argue against this proposal, and in favor of a theory
rooted in syntactic and semantic contents— in mappings from syntactic positions to thematic
relations. We (i) provide evidence that infants view certain scenes under a concept with three
participant relations (a girl taking a truck from a boy), and (ii) show that toddlers do not expect
these representations to align numerically with clauses used to describe those scenes: they
readily accept two-argument descriptions (“she pimmed the truck!”). This argues against
syntactic bootstrapping theories underwritten by mappings between structural features of
syntactic and semantic representations. Instead, our findings support bootstrapping based on
grammatical and thematic content. Children’s earliest inferences may rely on the assumption that
the syntactic asymmetry between subject and object correlates with a difference in how their
referents relate to the event described by the sentence.