  Language is not only used to transmit neutral information; we often seek to
persuade by arguing in favor of a particular view. Persuasion raises a number
of challenges for classical accounts of belief updating, as information cannot
be taken at face value. How should listeners account for a speaker's "hidden
agenda" when incorporating new information? Here, we extend recent
probabilistic models of recursive social reasoning to allow for persuasive
goals and show that our model provides a pragmatic account for why weakly
favorable arguments may backfire, a phenomenon known as the weak evidence
effect. Critically, this model predicts a systematic relationship between
belief updates and expectations about the information source: weak evidence
should only backfire when speakers are expected to act under persuasive goals
and prefer the strongest evidence. We introduce a simple experimental paradigm
called the Stick Contest to measure the extent to which the weak evidence
effect depends on speaker expectations, and show that a pragmatic listener
model accounts for the empirical data better than alternative models. Our
findings suggest further avenues for rational models of social reasoning to
illuminate classical decision-making phenomena.
