The observation that there is a cross-linguistic preference for corner vowels (that is, vowels in the corners of the vowel triangle).
The Quantal theory of speech explains dispersion by referring to stability in the mapping from articulation to acoustics. The corner vowels are acoustically the most stable vowels (i.e. the vowels that have the most room for sloppy production).
In Lindblom's theory of adaptive dispersion (1990), the corner vowels are most common in the languages of the world exactly because they are the corner vowels. In other words, the vowels that can be most reliably distinguished from each other are those that are maximally distinct. Adaptive dispersion takes into account the role of perception.
- Lindblom, B. 1990. Explaining phonetic variation: A sketch of the H & H theory, In W. J. Hardcastle & A. Marchal: "Speech Production and, 403-439, Kluwer Academic Publishers, Dordrecht