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Abstract: Common features in sign languages that have no history of contact present a puzzle. The current article brings together findings from three studies of Lengua de Señas Nicaragüense (LSN) to identify processes that underpin the inception and changes in language as it emerges. We use an apparent time approach to capture language change in three domains: pointing and deixis, nonmanual markers with Wh-questions, and spatial differentiation for locatives and arguments. By comparing the language of Nicaraguan homesigners, three successive cohorts of LSN signers, and the signing of hearing children of deaf LSN signers (Codas), we propose that language change is characterized by two complementary processes, in balance: emergence and convergence. Emergence is the constructive process by which forms arise, such as new lexical items, new patterns of word order, and new mappings between forms and functions, such as a pointing sign taking on a pronominal function (study 1). We argue that vertical transmission from a more experienced user to a less experienced learner drives emergence, often via linguistic reanalysis in which the learner assigns a different function to a form than the one used in the grammar of the experienced user. Complementing the process of emergence, convergence is the reductive process by which competing forms and structures are condensed to a smaller set to achieve parity across a group of language users, a process exemplified by the selection of the brow furrow and head tilt to accompany Wh-questions (study 2). We argue that horizontal peer interactions are key to convergence processes; the language of learners who do not experience such peer interactions, such as homesigners and Codas, shows less convergence (study 3). Taken together, these studies illustrate how the developmental characteristics of the learner intersect with the characteristics of their language (internal ecology), environments, and interactions (external ecology) to drive the processes of language change. We conclude that both vertical transmission and horizontal peer interactions are key to the emergence of new languages, enabling the mechanisms responsible for the typological similarities observed across unrelated languages. 

Two strong hypotheses bridge between typological studies of the frequency of different marking patterns and psychological studies of how these patterns are acquired. The word-order hypothesis proposes that order is a cognitively-salient cue, available to children before linguistic cues and thus appears early in language emergence. The agent-first hypothesis proposes that agents hold a privileged role in event representations and shape emerging languages. A recent study of Lengua de Señas Nicaragüense (LSN) found no preference for consistent or agent-initial word order. Instead LSN-signers used other linguistic devices. We looked at homesigners, each representing a different origin point for language emergence. We found no support for the agent-first hypothesis: most homesigners produced more patient-initial responses than agent-initial responses. We found no support for the word-order hypothesis: only one homesigner produced the same order on more than half the trials. Instead homesigners used a variety of other devices for marking participant roles. 

A key question in developmental psychology and linguistics is whether language merely expresses pre-existing concepts or provides new cognitive tools. De Villiers and colleagues suggest that abstract transitive relationships can only be reliably encoded via language, supporting this claim with evidence that infants and adults struggle with transitive role differentiation without language. This study examines adult Nicaraguan homesigners, an ideal case study because they lack conventional language but create personal communication systems. Homesigners and English-speaking five-year-olds were tested in an imitation task, revealing that both groups could accurately encode transitive relationships, with no significant differences between groups or event types. In the critical two-participant condition, performance was well above chance for both groups. These results suggest that the ability to encode and generalize transitive relationships exists independently of conventional language.