In this article I provide evidence that despite frequently cited differences between child first language (L1) and adult second language (L2) speakers in overt behavior (performance) during grammatical development, the nature, source, and limits of implicit knowledge (competence) in native and second language grammars are equivalent (i.e., they share a common epistemology; Schwartz (1986)). Evidence for this claim comes from the intuitions of L1 English-speaking classroom learners of French with respect to two properties of the French nominal system, both of which are posited to be the surface manifestations of a single parametric option for noun movement within a D(eterminer) P(hrase). These include (i) the distinction between result and process nominals in the licensing of postnominal genitives and (ii) the distinction between prenominal and postnominal adjective position in the context of unique versus nonunique noun referents. An analysis of the results of an acceptability judgment task administered to 100 university-level learners, 27 native French speakers, and 30 native English-speaker controls demonstrated that particular interpretive asymmetries associated with these properties, although underdetermined in the input and not syntactically instantiated in English, nonetheless appeared in the interlanguage grammars of the study participants. Moreover, changes in learner response patterns by level were highly suggestive of a developmental path involving an initial English-like parse of the test sentences followed by a parametric shift at the 3rd-year level, leading to increasingly more native-like intuitions at later levels wherein both properties cluster together. Such results call into question the need for theories of L2 acquisition positing selective transfer, selective impairment, or both at the level of syntactic representation.