This paper tests the hypothesis that language comprehenders update their beliefs about the statistics of their language throughout the lifespan, and that this belief update allows comprehenders to combine probabilistic linguistic cues according to their reliability. We conduct a multi-day sentence comprehension study in which the reliability of a probabilistic cue to syntactic structure is manipulated between subjects. We find that as the reliability of one cue to syntactic structure decreases, comprehenders come to rely more on a second cue to syntactic structure. The results are consonant with rational models of cue integration in speech perception and in non-linguistic domains, thus suggesting a unifying computational principle governing the way humans use information across both perceptual and higher-level cognitive tasks.