Original articleDominance and the evolution of sexual dimorphism in human voice pitch
Introduction
The human voice is highly sexually dimorphic. Pitch, the most perceptually salient feature of human voice (Banse & Scherer, 1996), is about half as high in men as it is in women (Titze, 2000). This dimorphism is due not merely to sex differences in body size; relative to both height and body volume, voice pitch is lower in men than it is in women and prepubescent children of both sexes (Titze, 2000). Sexual selection (Darwin, 1871) is the primary evolutionary cause of sex differences, and Collins (2000) suggested that sex differences in the human voice evolved though sexual selection via female mate choice. Some studies have shown correlations between female mate preferences and male voice pitch (Collins, 2000, Oguchi & Kikuchi, 1997), whereas others have examined the effects of experimental pitch manipulation on female preferences (Feinberg et al., in press, Feinberg et al., 2005, Puts, 2005). Puts, 2005 and Feinberg, Jones, Law Smith, et al., (in press) demonstrated menstrual cycle variation in women's preferences for masculine voices. Normally-cycling women's preferences for low, masculine voices increased with conception risk (Feinberg et al., 2005, Puts, 2005), and women preferred lower male voices mainly for short-term, sexual relationships (Puts, 2005). Taken together, these findings suggest that female mate choice may have influenced the evolution of male voice.
However, another type of sexual selection, intrasexual selection via male dominance competition, may also have been an important selection pressure on the voices of ancestral males. Dominance entails access to mates and resources that is relatively unchallenged by competitors. In most animals, dominance is achieved through aggression or threats of aggression, here termed physical dominance. In humans, dominance may also be achieved through skillful leadership and persuasion (Henrich & Gil-White, 2001), hereafter called social dominance. Among nonhuman animals, low voice pitch is associated with physical dominance (Morton, 1977, Morton & Page, 1992), and in humans, voice pitch is associated with interpersonal power and deference relations (Benjamin, 1981, Benjamin, 1992, Gregory, 1994, Gregory et al., 1993).
The reasons that the acoustic features of voice may have evolved as dominance signals can be clarified by examining their proximate causes. The frequency of vocal fold vibration during phonation is called the fundamental frequency, or F0, and closely determines what is perceived as pitch. The determinants of F0 are apparent from the equationwhere L is the vocal fold length, σ is the longitudinal stress on the vocal folds, and ρ is the vocal fold tissue density (Titze, 2000). Thus, voice pitch is inversely proportional to vocal fold length and directly proportional to the square root of tension on the vocal folds. Longer vocal folds with less tension on them lead to lower voice pitch. The perception of pitch is also affected by formant frequencies (Higashikawa et al., 1996, Wolfe & Ratusnik, 1988). The term pitch will thus be used hereafter to refer to the perception of voice “highness” or “lowness,” which is influenced by both fundamental and formant frequencies. Formant frequencies, and, in particular, the spacing between them (formant dispersion), are largely determined by the length of the supralaryngeal vocal tract (Fant, 1960, Fitch & Hauser, 1995).
Vocal fold length and tension and vocal tract length have direct relationships to traits associated with dominance. Vocal tract length is related to body size (Fitch & Giedd, 1999), and both fundamental and formant frequencies influence perceptions of speaker size (Collins, 2000, Smith et al., 2005). In addition, substantial evidence supports an association between vocal anatomy and androgens. Under the influence of pubertal androgens, the vocal folds (Hollien, Green, & Massey, 1994) and supralaryngeal vocal tracts (Fitch & Giedd, 1999) of human males lengthen faster than the overall rate of body growth, resulting in drops in F0, formant position, and formant dispersion. Fundamental frequency continues to correlate negatively with endogenous androgen levels in young adult men (Dabbs & Mallinger, 1999) and decreases with exogenous androgen treatment (Need, Durbridge, & Nordin, 1993). In addition to its association with low voice pitch, androgen is positively related to physical aggressiveness (Archer, 1991, Harris, 1999, Ramirez, 2003) and physical prowess (e.g., Clark & Henderson, 2003). More generally, it has been suggested that masculine traits such as low voice pitch, whose development or maintenance depends on high androgen levels, may be honest signals of a competitor's health and vigor (Folstad & Karter, 1992, Zahavi & Zahavi, 1997).
Habitual voice pitch varies across individuals, but pitch is also modulated between and within interactions. If low voice pitch is generally a reliable signal of physical dominance, then the relationship between voice pitch and dominance should be reciprocal. That is, not only should pitch affect perceptions of dominance, but dominance relationships should influence how pitch is modulated during competitive interactions. Indeed, pitch modulation is related to dominance and submissiveness across nonhuman animal species (Morton, 1977, Morton & Page, 1992). Morton (1977) suggested that lowering pitch may be analogous to piloerection in that it increases the apparent size of an animal and thus the threat it poses.
In humans, voice pitch varies with emotional state (Williams & Stevens, 1972) and actors' portrayals of emotional states (Banse & Scherer, 1996), and considerable agreement about speakers' emotional states exists among listeners (Johnson, Emde, Scherer, & Klinnert, 1986), whose judgments depend partly on voice pitch (Sobin & Alpert, 1999). In particular, pitch modulation appears to convey information about the relative emotional engagement, or activation, of a speaker (Russell, Bachorowski, & Fernandez-Dols, 2003). Emotional activation raises F0 by increasing tension on the vocal fold mucosa (σ, in Eq. (1)), mainly via contraction of the cricothyroid muscles and consequent lengthening of the vocal folds (Titze, 2000). Thus, raised F0 is associated with disparate high-activation emotions such as hot anger, elation, and panic fear, whereas lowered F0 is associated with sadness, boredom, and contempt (low-activation emotions) (Banse & Scherer, 1996, Ekman et al., 1976, Sobin & Alpert, 1999, Williams & Stevens, 1969, Wittels et al., 2002).
The positive relationship between emotional stress and voice pitch in humans may reflect homology with nonhuman animals (Ohala, 1983, Ohala, 1984). Across animal species, interaction with relatively dominant conspecifics may increase activation (nervousness), inhibiting displays of dominance and raising vocalization pitch, whereas interaction with submissive associates may induce less activation, disinhibiting dominance displays and lowering pitch.
Surprisingly, no study has examined whether perceived dominance affects speakers' voice pitch during competitive human interactions. Several studies have explored whether voice pitch affects dominance ratings made by listeners, but these studies have obtained mixed results. Aronovich (1976) found no correlation between speakers' average F0 and dominance ratings made by listeners, whereas Tusing & Dillard (2000) found a significant positive correlation between F0 and dominance ratings (i.e., lower F0 voices were rated as less dominant). However, these results are difficult to interpret because neither study was experimental, so neither can address whether any acoustic sexual dimorphism, by itself, affects perceptions of dominance. Other features of vocalizations, such as speech content, intonation, speed, and inflection, may have covaried with pitch and substantially influenced ratings.
A recent experimental study investigated the effects of voice manipulation on dominance ratings. Feinberg, Jones, Little, et al. (2005) manipulated voice pitch/masculinity by shifting both fundamental and formant frequencies. Female participants rated masculinized male and female voices as more dominant than feminized voices. However, because this experiment examined the effects of voice on females' perceptions of male dominance, it could not address whether voice pitch mediates dominance competition among males.
In the present study, we experimentally test the hypothesis that intrasexual selection influenced the evolution of human voice pitch sexual dimorphism through its role in male dominance competition. Specifically, we test three predictions of this hypothesis: voice pitch (1) signals dominance to other males, (2) is modulated in response to circumstantial dominance, and (3) increases mating success.
In testing Prediction 1 (voice pitch signals dominance to other males), we examine the effects of experimental voice manipulation on dominance ratings made by male listeners. If our hypothesis is correct, more masculine voices should lead to higher dominance ratings. Because voice pitch may generally advertise health, strength, vigor, or other traits that contribute to physical competitive ability, pitch manipulation is predicted to affect ratings of physical dominance more than ratings of social dominance.
We test Prediction 2 (voice pitch is modulated in response to relative dominance) by examining whether males who rate their dominance as high tend to lower their pitch more during competition than males who rate themselves as less dominant. We predict that pitch modulation will be more closely related to physical dominance than to social dominance.
Finally, we test Prediction 3 (voice pitch increases mating success) by examining whether men who spontaneously speak at a lower pitch report more sexual partners in the past year.
Section snippets
Subjects
One hundred ninety-seven self-identified heterosexual male undergraduates from the University of Pittsburgh took part as “participants” or “raters” in this human subjects board-approved study. The mean age of participants (n=111) was 18.9 years (range=18-24, S.D.=1.2). The mean age of raters (n=86) was 20.0 years (range=18-28, S.D.=2.1). Participants were native English-speaking nonsmoking males who reported being uninvolved in committed relationships.
Participants: voice pitch in competition for mates
Upon arrival at the voice laboratory and
Dominance ratings of unmodified competitive recordings
For comparison with prior research (Aronovich, 1976, Tusing & Dillard, 2000), we initially examined the correlations between dominance (physical and social) and voice pitch (measured by F0) for the unmodified competitive recordings. Although both relationships were in the predicted direction (lower voices rated as more dominant), neither correlation attained statistical significance (Pearson correlations: r=−.176, n=111, p=.064 for physical dominance; r=−.150, n=111, p=.106 for social
Discussion
Preliminary analyses showed a marginally significant negative correlation between ratings of physical (but not social) dominance and the F0 of subjects' unmodified competitive recordings. Experimental manipulation of these same recordings, however, produced strong negative relationships between voice pitch and both types of dominance, with pitch manipulation affecting perceptions of physical dominance more than it affected perceptions of social dominance. This paper is the second (after
Acknowledgments
We thank Lisa Brevard, Rachel Chandler, Christina Jerzyk, Jerome Lee, Nicole Li, Rebecca Prosser, John Putz, Melinda Putz, Linda Snyder, and Amy Stetten for their generous assistance during various phases of experiment preparation and data collection and analysis.
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Current address: Neuroscience Program, Michigan State University, East Lansing, MI 48824, USA.