  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.
