Adult and child L2 acquisition of syntax-semantics interface phenomena must be compared with monolingual L1 acquisition of the same phenomena in order to assess the possible effects of interference and transfer. However, this "L1A touchstone" can also be misleading because non-grammatical mechanisms that interact with such interface phenomena may be independently undergoing development in child cognition. This can affect linguistic performance, and perhaps also the acquisition of linguistic competence. The purpose of this paper is to investigate one such cognitive factor that has recently been hypothesized to affect monolingual children's comprehension of the specific reading of indefinite subjects and objects. The hypothesis is based on experimental research with Dutch children. We will examine English children, focusing on the indefinite subject in the single-clause English sentence. (Contains 3 tables, 2 figures and 6 notes.)