Potentially recursive structures emerge quickly when a new language community forms

Abstract

Human languages can express an infinite number of thoughts despite having a finite set of words and rules. This is due, in part, to recursive structures, which allow us to embed one instance of a rule inside another. We investigated the origins of recursion by studying the development of Nicaraguan Sign Language (LSN), which emerged in the last 40 years and is not derived from any existing language. Before this, deaf individuals in Nicaragua lacked access to language models and each individual created their own gestural system, called homesign. We tested four groups: homesigners, who represent the point of origin, and the first three generations of LSN signers, who represent consecutive stages in the language's development. We used a task that was designed to elicit sentences with relative clauses, a device that allows for the recursive embedding of sentences inside of sentences (e.g., [the girl [who was drawing] removed the picture]). Signers in all three LSN cohorts consistently produced utterances that appeared to have embedded predicates (girl draw remove picture) which served the function of a relative clause (picking out the correct member of a set, based on previously mentioned information). Furthermore, in these utterances, the first verb was shorter than the second and shorter than the same verb in parallel unembedded structures. In contrast, homesigners produced similar utterances in embedded and unembedded contexts. They did not reintroduce previously mentioned information or produce reduced verb forms in the embedded context. These results demonstrate that syntactic embedding that is potentially recursive can emerge very early in a language. These embedded predicates, however, may not be widespread, or systematically marked, in homesign systems. This raises the possibility that the emergence of recursive linguistic structure is a consequence of interaction within a language community. These findings pave the way for future work which investigates the syntactic form of these embedded predicates and explores whether multiple levels of embedding are possible.

Keywords: Language emergence; Language evolution; Lengua de Señas Nicaragüense; Recursion; Sign language.

Figure 1.

Proportion of identifier+action trials in which signers produced utterances with the content and function of relative clauses in Experiment 1. Signers first established the set, then produced an utterance with two verbs, one picking out a member of the set, the candidate relative clause, and the other predicating something new of the member, the candidate main verb. Error bars represent +/− SEM.

Figure 2.

Examples of stimuli. In the identifier+action condition, videoclips first depicted three similar-looking characters, each engaged in a different action. One of the three characters then carried out a second action. The target sentence contains a nominal with an embedded verb before the main verb (the girl who was drawing removed a picture). In the sequential action condition, a single character engaged in an action, followed by a second, different action (a girl drew and then removed a picture). In the repeated action condition, a single character engaged in an action, and then carried out the same action with a different object (a girl drew on a piece of paper, and then drew on an easel pad).

Figure 3.

Experiment 2 results. Evidence for embedding in LSN signers. Signers in all cohorts used verbal embedding (repeating the identifying verb) or the sequential verb strategy (no repetition of the identifying verb) in the identifier+action (i+a) condition, in which the doer of the second verb had to be selected from a set of possibilities. However, they did not use verbal embedding in the sequential action (sa) condition, when disambiguation was unnecessary (and infelicitous).

Figure 4.

Experiment 2 results. To detect formal evidence of embedding, we analyzed utterances containing two different verbs (e.g., DRAW and REMOVE). We then calculated the ratio of the duration of the first verb, DRAW, to the second verb, REMOVE. For all LSN groups, the first verb was the same duration or shorter than the second verb in the identifier+action condition (orange bars), but not in the sequential action condition (purple bars) [χ2(1) = 19.97, p<.001 for comparison of random effects model and condition model, χ2(2) =.90, p=.639 for comparison of condition model and condition + cohort model]. This systematic shortening that is restricted to the identifier+action condition suggests that the first verb, DRAW, is embedded, and functions as a relative clause that picks out the doer by referring to her earlier action (“the girl who drew”). Error bars represent +/− SEM.

Figure 5.

Percentage of identifier+action trials in which homesigners described both the identifying and the new action of the target character.

Figure 6.

Experiment 3 results. Homesigners, unlike the LSN signers, did not repeat the first verb (the identifying action) in the identifier+action (i+a) condition and thus their responses in this condition were more similar to the responses produced in the sequential action (sa) control condition. Trials coded as ‘Other’ did not include verbs for both actions.

Figure 7.

The Critical Category End Both Actions category (purple bars) shows responses in which the participant described more than one character with the target character appearing last followed by a verb phrase expressing the second action. The Only Critical Character Both Action category (orange bars) shows responses in which the participant described only the actor and action of the second event (avoiding the need for disambiguation). Responses in the Only Critical Action category (teal bars) included producing a noun phrase (“the man with the ball”) followed by a verb phrase expressing the second action.