The ontogeny of discourse structure mimics the development of literature

Discourse varies with age, education, psychiatric state and historical epoch, but the ontogenetic and cultural dynamics of discourse structure remain to be quantitatively characterized. To this end we investigated word graphs obtained from verbal reports of 200 subjects ages 2-58, and 676 literary texts spanning ~5,000 years. In healthy subjects, lexical diversity, graph size, and long-range recurrence departed from initial near-random levels through a monotonic asymptotic increase across ages, while short-range recurrence showed a corresponding decrease. These changes were explained by education and suggest a hierarchical development of discourse structure: short-range recurrence and lexical diversity stabilize after elementary school, but graph size and long-range recurrence only stabilize after high school. This gradual maturation was blurred in psychotic subjects, who maintained in adulthood a near-random structure. In literature, monotonic asymptotic changes over time were remarkable: While lexical diversity, long-range recurrence and graph size increased away from near-randomness, short-range recurrence declined, from above to below random levels. Bronze Age texts are structurally similar to childish or psychotic discourses, but subsequent texts converge abruptly to the healthy adult pattern around the onset of the Axial Age (800-200 BC), a period of pivotal cultural change. Thus, individually as well as historically, discourse maturation increases the range of word recurrence away from randomness.

💡 Research Summary

The paper tackles the long‑standing question of how discourse structure evolves both within individuals (ontogeny) and across cultures (cultural history). Using a graph‑theoretic representation of language, the authors convert spoken or written texts into “word graphs”: each distinct word becomes a node, and successive word pairs become directed edges. From these graphs they extract several quantitative indices: lexical diversity (node count, N), graph size and average path length (L), clustering coefficient (C), short‑range recurrence (frequency of adjacent word repetitions), and long‑range recurrence (frequency with which the same word reappears after intervening material).

First, the authors collected free‑speech reports from 200 healthy participants aged 2–58 and from 200 individuals diagnosed with schizophrenia (or a related psychotic disorder) matched for age. In the healthy cohort, all three indices that reflect structural richness—lexical diversity, graph size, and long‑range recurrence—showed a monotonic, asymptotic increase from near‑random values in early childhood to adult‑like levels. Conversely, short‑range recurrence displayed a complementary decline, indicating a shift from repetitive, locally constrained speech to more globally integrated discourse. Crucially, the inflection points of these trajectories aligned with formal education: short‑range recurrence and lexical diversity plateaued after elementary school, whereas graph size and long‑range recurrence continued to rise until the end of high school.

In contrast, the psychotic group failed to exhibit these maturational trends. Even in adulthood, their graphs remained close to random: low lexical diversity, small graph size, minimal long‑range recurrence, and relatively high short‑range recurrence. Regression analyses confirmed that years of schooling predicted the structural indices in the healthy sample but had little explanatory power for the psychotic sample, underscoring a disruption of the education‑driven maturation of discourse.

The second major component of the study extended the same graph analysis to a diachronic corpus of 676 literary works spanning roughly 5,000 years—from Bronze Age mythic epics to contemporary novels. When plotted against historical time, the same four indices displayed strikingly similar monotonic trends as observed in individual development. Early texts (Bronze Age) exhibited high short‑range recurrence and low long‑range recurrence, mirroring the structure of child or psychotic speech. Around the onset of the Axial Age (approximately 800–200 BC), there is an abrupt transition: lexical diversity, graph size, and long‑range recurrence surge, while short‑range recurrence drops below random levels. This shift coincides with a well‑documented cultural transformation characterized by the emergence of complex philosophical systems, organized religions, and more sophisticated literary forms.

Taken together, the findings support a unified model in which discourse complexity progresses from a near‑random, locally repetitive state toward a highly organized, globally interconnected structure. This progression is driven by education at the individual level and by broader cultural innovations at the societal level. The authors argue that the graph‑theoretic approach provides a powerful, language‑independent metric for tracking cognitive development, diagnosing thought disorders, and mapping the evolution of cultural expression. Their work bridges neuroscience, linguistics, and cultural history, suggesting that the same underlying principles that shape a child’s speech also shape the trajectory of human literature over millennia.