This article offers commentary that the Multiple Grammar (MG) language acquisition theory proposed by Luiz Amaral and Tom Roeper (A&R) in the present issue lacks elaboration of the psychological mechanisms at work in second language acquisition. Topics discussed include optionality in a speaker's grammar and the rules of verb position in declarative sentences bilingual speakers must be aware of. The author concludes that A&R's Multiple Grammars approach cannot generate rigorous predictions about learner behavior if it does not specify how learners distinguish between the competing L1 and L2 rules.