F1-F3 data from the study by Perry, Ohde, & Ashmead (2001) who reported data for boys and girls at the ages of 4, 8, 12 and 16 years. The corner vowels are the averages from 20 subjects per age group (10 male, 10 female), with 5 repetitions per vowel. Sex differences in the acoustic space are evident at age 4 and increase with age. The overlap in the vowel quadrilaterals for the two sexes decreases and by age 16 there is no overlap. Note a sex difference in F1 for the low vowels across all age groups, with males having lower F1 values.