Models of the human sentence processing mechanism have traditionally appealed to innate architectural restrictions to explain observed patterns of behavior. Recently, a number of proposcds have instead emphasized the role of linguistic experience in guiding sentence interpretation, suggesting that various frequency measures play a crucial role in ambiguity resolution. What has been lacking thus far is a detailed analysis of the linguistic and computational properties that could explain why those particular aspects of experience are effective in shaping behavior. In this paper, we present a linguistic analysis that reveals restrictions on the representational ability of the sentence processor, explaining its sensitivity to particular factors in the linguistic environment. The proposal receives strong support from a large-scale corpus analysis.