  Referring is one of the most basic and prevalent uses of language. How do
speakers choose from the wealth of referring expressions at their disposal?
Rational theories of language use have come under attack for decades for not
being able to account for the seemingly irrational overinformativeness
ubiquitous in referring expressions. Here we present a novel production model
of referring expressions within the Rational Speech Act framework that treats
speakers as agents that rationally trade off cost and informativeness of
utterances. Crucially, we relax the assumption that informativeness is computed
with respect to a deterministic Boolean semantics, in favor of a
non-deterministic continuous semantics. This innovation allows us to capture a
large number of seemingly disparate phenomena within one unified framework: the
basic asymmetry in speakers' propensity to overmodify with color rather than
size; the increase in overmodification in complex scenes; the increase in
overmodification with atypical features; and the increase in specificity in
nominal reference as a function of typicality. These findings cast a new light
on the production of referring expressions: rather than being wastefully
overinformative, reference is usefully redundant.
