  Secondly, Chomskyans may produce other versions of the poverty of the stimulus argument. For instance, Crain 1991 constructs a poverty of  the stimulus argument concerning children's acquisition of knowledge  of certain constraints on movement. However, while Crain's argument  carefully documents children's conformity to the relevant grammatical rules, its nativist conclusion still relies on unsubstantiated  intuitions as to the non-occurrence of relevant forms or evidence in  the pld. It is thus inconclusive. (Cf. Crain 1991; Crain's  experiments and their implications are discussed in Cowie  1999 ; Cf. also Crain and Pietrowski  2001, 2002).