Monozygotic twin sisters differ in voice quality: Acoustic analysis of the diphthong [‘oj] produced by Brazilian native speakers

Abstract

Identical twins challenge individual voice recognition applied in communication, criminal investigation, and health technologies. The similarities between siblings are due to the genetic heritage that produced either their vocal tract or the psychological processes acting since birth. To contribute to a precise individual identification by voice, we tested the hypothesis that even MZ siblings would differ in the spectral distribution of formants and sound quality. We recorded 86 same-sex twin pairs (71 MZ, 15 DZ 30 ± 12 years old) saying the Portuguese verse of “Hi, my name is (Pedro or Ana).” We trimmed the diphthong Ój (“Hi”) and measured f0, F1-F4 formants, jitter, shimmer, and Harmonic-to-noise ratio (HNR). As the male sampling was small, we compared only female twins using the Wilcoxon-matched test, the intrapair Correlation Coefficient (ICC), and GLMM models. On average, [Ój] lasted 0.24 ± 0.07 s, and f0 ranged from 469 to 510 kHz according to sex. We found no differences between the MZ or DZ sisters in any acoustic parameter, besides F1 for MZ ones. Also, MZ sisters were more alike (F1, jitter, HNR, weak; F3, F4 moderate) than DZ (only F1 high ICC). The interaction zygosity × age affected intrapair similarity, the older the sisters, the greater their differences. Short voice segments, like the diphthong [Ój], may not be good for differentiating identical twins but provide insights to discussion; F2, F4, and shimmer should be investigated in future studies considering males, younger participants, and other vocal expressions.