I should like to acknowledge help from Sue Blackwell, Annabel Cormack, Jennifer Ganger, * Ted Gibson, Chris Golston, So Hiranuma, Steve Harlow, Richard Lewis, Caroline Liberg, John Limber, Bruce Nevin, Michael Niv, Neal Pearlmutter, Colin Phillips, Karin Stromswold, Patrick Sturt, David Wharton. UCL Working Papers in Linguistics 8 (1996) The difficulty of (so-called) self-embedded structures * RICHARD HUDSON Abstract Why is it impossible to process The rat the cat the dog chased ate died? The standard explanations all focus on its syntactic structure, but the present paper offers an alternative explanation in terms of semantic structure. The syntactic account cannot explain why some sentences which are syntactically similar are much easier to process. The difficult sentences seem to be those in which an embedded clause which modifies a noun has its own subject modified by another clause whose subject is a common noun (not a pronoun). The explanation offered here is that the senses of the modified nouns are not sufficiently distinct to be maintained as separate concepts in short-term memory, so the various concepts stored 'interfere' with one another. 1 Introduction Why is sentence (1) so hard to process? (1) The rat the cat the dog chased ate died. The example was used by Miller and Chomsky (1963:286) to illustrate the effects of limited working memory on the processing of complex syntactic structures. Their conclusion relates the difficulties to the 'parenthetical embedding' in the syntactic structure: