Does knowledge of language transfer across language modalities? For example, can speakers who have had no sign language experience spontaneously project grammatical principles of English to American Sign Language (ASL) "signs"? To address this question, here, we explore a grammatical illusion. Using spoken language, we first show that a single word with doubling (e.g., "trafraf") can elicit conflicting linguistic responses, depending on the level of linguistic analysis (phonology vs. morphology). We next show that speakers with no command of a sign language extend these same principles to novel ASL signs. Remarkably, the morphological analysis of ASL "signs" depends on the morphology of participants' "spoken" language. Speakers of Malayalam (a language with rich reduplicative morphology) prefer XX signs when doubling signals morphological plurality, whereas no such preference is seen in speakers of Mandarin (a language with no productive plural morphology). Our conclusions open up the possibility that some linguistic principles are amodal and abstract.