The vowels spoken by the boys/men have lower formant values: all vowels are more towards the top-right of the vowel space than is the case with the girls/women. This is because men, during and especially after puberty, have larger resonance cavities than women. Because of the larger cavities, the resonance frequencies (i.e., formants) are some 15% lower for men than for women. The centroids tend to cluster in small groups within which there will be insufficient contrast in vowel quality. There is a high-back cluster /u, ʊ, o/, a low back cluster /ɔ, ɑ, ʌ/, a front vowel cluster /i, ɪ, e, ε/. Low front /æ/ is a singleton and will not be confused with other vowels. This patterning is seen for all four groups alike, although there would seem to be some distance between subclusters /i, ɪ/ and /e, ε/ for the female speakers. What is especially revealing is that there are no vowels in the center portion of the space. It would appear that monolinguals and bilinguals basically have the same structure in their vowel system of American English. The structure of the EFL vowel system averaged over de four speaker groups is shown in Figure 3/A. For the sake of visual comparison, panel B shows the location of the 11 centroids for the American speakers (averaged over genders).