A common feature of discourse coherence is hierarchical organization: more generally, central relations (characterizing the overall topic or goal) dominate complementary or modifying relations. In this hierarchy, higher levels tend to be marked by stronger prosodic cues than lower levels. This study seeks to understand how such a system emerges in human communication – what is present at the outset, and what takes time to develop. Specifically, we investigate whether the conceptualization of hierarchical organization precedes overt linguistic structuring, and whether distinct types and strengths of prosodic marking at different hierarchical levels can be discerned in the process of emergence. The only empirical evidence for such an investigation comes from sign languages, because they can arise de novo at any time. Sign languages offer the additional advantage of directly linking instantiations of linguistic structure to articulations of different visually perceived bodily articulators. Our study of a young sign language, Israeli Sign Language (ISL), finds that conceptual hierarchical structuring of discourse arises very early. However, the organization of bodily articulators to linguistically mark hierarchical information takes time to emerge: Younger ISL signers use smaller, less salient articulators at lower levels of the hierarchy compared to older signers.

The current study investigates the conceptual hierarchy of humans−animals−plants−non-animate objects by using novel hybrids. Three experiments were conducted. In Experiment 1, twenty-one participants were presented with a grammatically asymmetrical phrase, in which the two components are associated with different linguistic properties, (e.g., a man with a horse’s head) followed by a visual hybrid, and were asked to judge whether the phrase described the hybrid. In Experiment 2, thirty participants were presented with a visual hybrid and were asked to categorize it according to one of its visually presented components in a forced-choice judgment task. In Experiment 3, twenty-nine participants were presented with a visual hybrid that followed a grammatically symmetrical phrase, in which both components carry similar grammatical properties (e.g., half-human half-horse), and were asked to judge whether the phrase described the hybrid. A conceptual hierarchy effect was found in Experiment 1 but not in the other two experiments. These findings show that the hierarchy effect occurs only in verbal tasks that involve asymmetrical grammatical constructions. We suggest that the pragmatic tendency to map the hierarchically higher concept onto the higher grammatical function applies to asymmetrical constructions but not to symmetrical constructions.