The recent availability of large on-line parsed corpora makes it possible to test theories of psycholinguistic complexity by comparing the frequency distributions of closely related constructions. In this paper, we use this technique to test the psycholinguistic theory proposed by Gibson et al. (1993), which includes two independently motivated attachment principles: Recency Preference and Predicate Proximity. In order to test this theory, we examined two general classes of attachment ambiguities from the parsed Wall Street Journal corpus from the Penn Treebank: 1) ambiguities which involve three prospective noun phrase attachment sites; and 2) ambiguities which involve three prospective verb phrase attachment sites. Given three prospective noun phrase (NP) sites in English, the theory most naturally predicts a complexity ordering of NP3 (easiest, most recent), NP1, NP2, but a ranking of VP3, VP2, VP1 for verb phrase attachments. Our corpus analyses support both of these predictions.