Assistant Professor Mark Changizi of the Rensselaer Polytechnic Institute said, "The dictionary's large-scale organization has been driven over time by the way humans mentally systematize words and their meanings."

Changizi said dictionaries are organized like an inverted pyramid, with a larger number of more complex words in the upper levels that are defined by a smaller number of more basic words in the next lower level.

"The presence of around seven levels of definition will reduce the overall size of the dictionary, so that it is about 30 percent of the size it would be if there were only two hierarchical levels," Changizi said.

He said his calculations show dictionaries are optimally organized to minimize the amount of space required to define the lexicon, in the same way that cultural forces "shaped the organization of our lexicon so as to require as little mental space and energy as possible."

The study appears in the online edition of the Journal of Cognitive Systems Research.