Internal head constraint

Relative clauses in English (and in an overwhelming number of languages) obey a `no internal head' constraint. This constraint is exemplified in the contrast between ((278)) and ((279)).

(277)0(277

(278)

the person [who_i [_C Muriel likes _i]]  (278)0(278

(279)

*the person [[which person]_i [_C Muriel likes _i]] 

We know of no good way to rule ((279)) out, while still ruling ((280)) in.

(279)0(279

(280)

the person [[whose mother]_i [_C Muriel likes _i]] 

Dayal (1996) suggests that `full' NPs such as which person and whose mother are R-expressions while who and whose are pronouns. R-expressions, unlike pronouns, are subject to Condition C. ((278)) is, then, ruled out as a violation of Condition C since the person and which person are co-indexed and the person c-commands which person. If we accept Dayal's argument, we have a principled reason for allowing overgeneration of relative clauses that violate the internal head constraint, the reason being that the XTAG grammar does generate binding theory violations.