The Effect of the Number of Vowels on the Size of Vowel Spaces: Adaptive Dispersion Theory's Key Prediction

In this paper, the validity of the key prediction of Adaptive Dispersion Theory was tested. According to this prediction, vowels of a given language are organized in such a way that they are sufficiently distinct on the perceptual level. It implies that more crowded inventories, which also have wider F1 dimension, occupy greater vowel spaces than inventories with fewer vowels. To investigate this hypothesis, the size of Persian vowel space with 6 peripheral vowels was compared with that of Fereydounkenari variety of Mazandarani dialect with 5 peripheral vowels, using the methodology introduced by Padgett and Tabain (2005) and three size indices proposed by Becker-Krystal (2010). Through rigorous statistical analysis, the sizes of vowel spaces were found to be positively correlated with the number of vowels in the inventory; that is, Persian has clearly greater vowel spaces in all groups, i.e., male and female stressed and unstressed contexts, than Mazandarani, but the number of peripheral vowels did not affect the F1 dimension. For an in-depth analysis, the three area indices were also calculated for back and front subspaces in those groups. The results indicated that only Persian male and female unstressed groups in back subspaces have greater F1span than Mazandarani dialect. With few exceptions, then, the prediction of Adaptive Dispersion Theory was validated.