  During communication, the interpretation of utterances is sensitive to a
listener's probabilistic prior beliefs, something which is captured by one
currently influential model of pragmatics, the Rational Speech Act (RSA)
framework. In this paper we focus on cases when this sensitivity to priors
leads to counterintuitive predictions of the framework. Our domain of interest
is exhaustivity effects, whereby a sentence such as "Mary came" is understood
to mean that only Mary came. We show that in the baseline RSA model, under
certain conditions, anti-exhaustive readings are predicted (e.g., "Mary came"
would be used to convey that both Mary and Peter came). The specific question
we ask is the following: should exhaustive interpretations be derived as purely
pragmatic inferences (as in the classical Gricean view, endorsed in the
baseline RSA model), or should they rather be generated by an encapsulated
semantic mechanism (as argued in some of the recent formal literature)? To
answer this question, we provide a detailed theoretical analysis of different
RSA models and evaluate them against data obtained in a new study which tested
the effects of prior beliefs on both production and comprehension, improving on
previous empirical work. We found no anti-exhaustivity effects, but observed
that message choice is sensitive to priors, as predicted by the RSA framework
overall. The best models turn out to be those which include an encapsulated
exhaustivity mechanism (as other studies concluded on the basis of very
different data). We conclude that, on the one hand, in the division of labor
between semantics and pragmatics, semantics plays a larger role than is often
thought, but, on the other hand, the tradeoff between informativity and cost
which characterizes all RSA models does play a central role for genuine
pragmatic effects.
