RESUMO
A comparison of voice onset time (VOT) production in younger and older female monozygotic twins revealed greater similarity for the younger than the older female twin pairs. Based on findings from this study and a review of previous twin literature, it is hypothesized that 'source' characteristics of speech (voice properties) are under more constraints by genetics, while 'filter' characteristics of speech (VOT, formant frequencies) are freer to diverge due to environmental factors such as different linguistic environments. Since the older twins separated to two distinct linguistic areas of the United States at the age of 25 years, while the 2 younger twins continue to share domicile at 21 years, the findings of this study appear to support this preliminary hypothesis.