First formant difference for /i/ and /u/: a cross-linguistic study and an explanation

The value of the first formant of high back and high front vowels (/u/ and /i/) has been determined for near minimal pairs in a 30-language sample. It is found that for 29 out of 30 languages the average of the first formant is higher for high back vowels than for high front vowels, and that for 26 out of 28 languages the majority of minimal pairs has a high back vowel with a higher first formant than that of the high front vowel. A trend towards smaller differences was found in women, but this is not significant in the present data set.
Two factors may explain this observation. Firstly, the human vocal tract can only vary the position of gradual (and not abrupt) transitions of cross-sectional area. Secondly, there is a narrow tube just above the glottis (the epilarynx tube). Both factors cause the first formant of high back vowels to be raised, but neither is sufficiently important to explain the observed differences on its own.