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Abstract The linguistic input children receive has a massive and immediate effect on their language acquisition. This fact makes it difficult to discover the biases that children bring to language learning simply because their input is likely to obscure those biases. In this article, I turn to children who lack linguistic input to aid in this discovery: deaf children whose hearing losses prevent their acquisition of spoken language and whose hearing parents have not yet exposed them to sign language. These children lack input from a conventional language model, yet create gestures, called homesigns, to communicate with hearing individuals. Homesigns have many, although not all, of the properties of human language. These properties offer the clearest window onto the linguistic structures that children seek as they either learn or, in the case of homesigners, construct language. 

The noun–verb distinction has long been considered a fundamental property of human language, and has been found in some form even in the earliest stages of language emergence, including homesign and the early generations of emerging sign languages. We present two experimental studies that use silent gesture to investigate how noun–verb distinctions develop in the manual modality through two key processes: (i) improvising using novel signals by individuals, and (ii) using those signals in the interaction between communicators. We operationalise communicative interaction in two ways: a setting in which members of the dyad were in separate booths and were given a comprehension test after each stimulus vs. a more naturalistic face-to-face conversation without comprehension checks. There were few differences between the two conditions, highlighting the robustness of the paradigm. Our findings from both experiments reflect patterns found in naturally emerging sign languages. Some formal distinctions arise in the earliest stages of improvisation and do not require interaction to develop. However, the full range of formal distinctions between nouns and verbs found in naturally emerging language did not appear with either improvisation or interaction, suggesting that transmitting the language to a new generation of learners might be necessary for these properties to emerge. 

Logical properties such as negation, implication, and symmetry, despite the fact that they are foundational and threaded through the vocabulary and syntax of known natural languages, pose a special problem for language learning. Their meanings are much harder to identify and isolate in the child’s everyday interaction with referents in the world than concrete things (like spoons and horses) and happenings and acts (like running and jumping) that are much more easily identified, and thus more easily linked to their linguistic labels (spoon, horse, run, jump). Here we concentrate attention on the category of symmetry [a relation R is symmetrical if and only if (iff) for all x, y: if R ( x, y), then R (y, x)], expressed in English by such terms as similar, marry, cousin, and near. After a brief introduction to how symmetry is expressed in English and other well-studied languages, we discuss the appearance and maturation of this category in Nicaraguan Sign Language (NSL). NSL is an emerging language used as the primary, daily means of communication among a population of deaf individuals who could not acquire the surrounding spoken language because they could not hear it, and who were not exposed to a preexisting sign language because there was none available in their community. Remarkably, these individuals treat symmetry, in both semantic and syntactic regards, much as do learners exposed to a previously established language. These findings point to deep human biases in the structures underpinning and constituting human language. 

In language evolution, formation of conceptual categories preceded formation of linguistic semantic categories (Hurford, 2007). The mapping from concepts to semantics is non-isomorphic, however, as particular languages categorize conceptual space in divergent ways (e.g. English put in is acceptable for both tight-fit and loose-fit relations, while Korean kkita encodes tight-fit relationships only; Choi & Bowerman, 1991). Despite this variation, are there crosslinguistic patterns in how words lexicalize conceptual space? We address this question analyzing how child homesigners from four different cultures describe instrumental events (e.g. cutting bread with a knife). Homesigners are congenitally deaf individuals who have not been taught a signed language. Despite growing up without structured linguistic input, these individuals use a gestural system ("homesign") to communicate (Goldin-Meadow, 2003). We find that homesign descriptions of instrumental events reflect categories present in adult English, Spanish and Mandarin, suggesting crosslinguistic biases for how verbs encode the conceptual space of events, biases which may have been present over the course of language evolution.