Abstract
We present statistical analyses of the large-scale structure of 3 types of semantic networks: word
associations, WordNet, and Roget’s Thesaurus. We show that they have a small-world structure, characterized
by sparse connectivity, short average path lengths between words, and strong local clustering.
In addition, the distributions of the number of connections follow power laws that indicate a
scale-free pattern of connectivity, with most nodes having relatively few connections joined together
through a small number of hubs with many connections. These regularities have also been found in
certain other complex natural networks, such as the World Wide Web, but they are not consistent with
many conventional models of semantic organization, based on inheritance hierarchies, arbitrarily
structured networks, or high-dimensional vector spaces. We propose that these structures reflect the
mechanisms by which semantic networks grow. We describe a simple model for semantic growth, in
which each new word or concept is connected to an existing network by differentiating the connectivity
pattern of an existing node. This model generates appropriate small-world statistics and power-law
connectivity distributions, and it also suggests one possible mechanistic basis for the effects of learning
history variables (age of acquisition, usage frequency) on behavioral performance in semantic
processing tasks.