  Given a sentence "Abby told Brittney that she upset Courtney", one would
struggle to understand who "she" refers to, and ask for clarification. However,
if the word "upset" were replaced with "hugged", "she" unambiguously refers to
Abby. We study if modern coreference resolution models are sensitive to such
pronominal ambiguity. To this end, we construct AmbiCoref, a diagnostic corpus
of minimal sentence pairs with ambiguous and unambiguous referents. Our
examples generalize psycholinguistic studies of human perception of ambiguity
around particular arrangements of verbs and their arguments. Analysis shows
that (1) humans are less sure of referents in ambiguous AmbiCoref examples than
unambiguous ones, and (2) most coreference models show little difference in
output between ambiguous and unambiguous pairs. We release AmbiCoref as a
diagnostic corpus for testing whether models treat ambiguity similarly to
humans.
