Behavioral Ecology doi:10.1093/beheco/arr134 Advance Access publication 3 August 2011
Original Article
Integrating fundamental and formant frequencies in women’s preferences for men’s voices
Prior work has emphasized independent effects of voice pitch and apparent vocal tract length on women’s preferences for men’s voices. An interaction between the effects of pitch and apparent vocal tract length on male vocal attractiveness might be expected, however, given interactions between the effects of voice pitch and apparent vocal tract length on perceptions of body size, age, and sex. Here, we present empirical evidence for such an interaction. Women preferred low pitch in men’s voices more when associated with large vocal tracts than when associated with small vocal tracts and preferred large vocal tracts in men’s voices more when associated with low pitch than when associated with high pitch. Collectively, these findings demonstrate integration of different vocal cues in women’s mate preferences, potentially allowing women to better assess the quality of potential mates. Key words: attractiveness, cue integration, formant, pitch, vocal, vocal tract. [Behav Ecol 22:1320–1325 (2011)]
INTRODUCTION he source–filter theory of vocal production (Fant 1960) assumes a functionally independent sound source (i.e., vocal fold vibration) and filter (i.e., vocal tract resonances). Vocal fundamental frequency and corresponding harmonics reflect the rate of vocal fold vibration, and certain harmonic bands are filtered by the natural resonances of the vocal tract, resulting in formant frequencies (Titze 1994). The percept of these fundamental frequencies is commonly referred to as voice pitch (Titze 1994). Although ‘‘fundamental’’ frequencies reflect the rate of vocal fold vibrations, ‘‘formant’’ frequencies reflect the supralaryngeal vocal tract (henceforth, vocal tract) resonances and are unrelated to the rate of vocal fold vibration (Titze 1994). Vocal tract length and its acoustic correlates are sexually dimorphic and positively associated with height, weight, and testosterone levels among men (Fitch and Giedd 1999; Bruckert et al. 2006). Despite a significantly larger descent of the larynx at puberty among men than women, height can explain more than 70% of the variance in vocal tract length (Fitch and Giedd 1999). Thus, vocal tract length and its acoustic correlates are strong indices of height. The lowering of voice pitch is caused by an influx of testosterone binding to receptors on the vocal cords, causing a cascade of growth hormones that lengthen and thicken the vocal cords (Butler et al. 1989; Harries et al. 1997; Abitbol et al. 1999; Dabbs and Mallinger 1999; King et al. 2001; Evans et al. 2008). This process occurs naturally in boys during pu-
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Address correspondence to D.R. Feinberg. E-mail: feinberg@ mcmaster.ca. D. R. Feinberg and B. C. Jones contributed equally to this work. Received 18 August 2010; revised 17 June 2011; accepted 27 June 2011. The Author 2011. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved. For permissions, please e-mail:
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berty (Butler et al. 1989) but can also occur for both sexes, given enough circulating testosterone (Van Borsel et al. 2000; King et al. 2001). Indeed, although people misattribute large body size to voices with low pitch (Collins 2000; Feinberg et al. 2005; Smith et al. 2005, 2007; Rendall et al. 2007), many studies have found that adult male height is unrelated to voice pitch (Lass and Brown 1978; Cohen et al. 1980; Graddol and Swann 1983; Kunzel 1989; van Dommelen and Moxness 1995; Collins 2000; Evans et al. 2006; Gonzalez 2007). Studies on voice preferences have emphasized the role of men’s voice pitch (i.e., the perception of fundamental frequency and/or corresponding harmonics) on women’s attractiveness ratings of men’s voices. For example, correlational studies have found that women prefer lower pitched voices to higher pitched voices (Collins 2000). Consistent with these findings, experimental studies have found that lowering men’s voice pitch using pitch-manipulating software generally increases attractiveness ratings (Feinberg et al. 2005; Feinberg 2008; Feinberg, DeBruine, Jones, and Little 2008; Vukovic et al. 2008; Jones, Boothroyd, et al. 2010; Jones, Feinberg, et al. 2010). Furthermore, among men, voice pitch is inversely related to reproductive success (Apicella et al. 2007), mating success (Puts 2005), and perceptions of dominance (Puts et al. 2007; Watkins et al. 2010; Wolff and Puts 2010; Vukovic et al. forthcoming ). Preferences for low pitch in men’s voices are generally stronger among women than among men, suggesting that voice pitch may be important for mate preferences in particular (Jones, Boothroyd, et al. 2010). In contrast to findings for voice pitch, correlates of vocal tract length do not predict women’s attractiveness ratings of men’s voices (Collins 2000; Feinberg et al. 2005), although taller and heavier women have stronger preferences for large apparent vocal tracts in men’s voices than do shorter and thinner women (Feinberg et al. 2005). Recent studies have also shown that acoustic correlates of vocal tract length are positively related to the perceived dominance of men’s voices
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D. R. Feinberg,a B. C. Jones,b L. M. DeBruine,b J. J. M. O’Connor,a C. C. Tigue,a and D. J. Boraka Department of Psychology, Neuroscience & Behaviour, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4K1, Canada and bDepartment of Psychology, Neuroscience & Behaviour, School of Psychology, Aberdeen University, Aberdeen AB24 3EN, UK
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METHODS Stimuli Following standard established methods for both human and nonhuman studies (for reviews, see Feinberg et al. 2005; Feinberg 2008), 6 men’s voices were manipulated in both
apparent vocal tract length and pitch using Praat acoustic phonetics software (Boersma and Weenink 2009). Following Feinberg et al. (2005), vocal tract qualities were altered by first resampling the sound by 615% of the original sampling rate to raise or lower all frequencies by 15%. Then, we overrode the new sampling rate with the original sampling rate such that duration was restored to its original values while leaving all frequencies raised or lowered. Finally, voice pitch (fundamental frequency and the resultant harmonic spectrum) was manipulated to be 60.5 equivalent rectangular bandwidths of the unmanipulated original. This process produced 4 versions of the original voice: 1) raised pitch and shortened apparent vocal tract, 2) raised pitch and lengthened apparent vocal tract, 3) lowered pitch and shortened apparent vocal tract, and 4) lowered pitch and lengthened apparent vocal tract. We used 6 original voices here because many other studies of behavioral responses to manipulated vocal cues in human (Feinberg et al. 2005, 2006; Feinberg, DeBruine, Jones, and Little 2008; Feinberg, DeBruine, Jones, and Perrett 2008; Vukovic et al. 2008; Jones, Feinberg, et al. 2010) and nonhuman (i.e., red deer, Charlton et al. 2008) studies have used similar numbers of voices. Furthermore, in human studies, research using 4–6 voices (Feinberg et al. 2005, 2006; Feinberg, DeBruine, Jones, and Perrett 2008; Vukovic et al. 2008; Jones, Feinberg, et al. 2010) found equivalent effects to those using more than 100 (Puts 2005; Feinberg, DeBruine, Jones, and Perrett 2008), suggesting that small numbers of voices produce results that generalize to larger samples. Procedure Eighty-three women (Internet: n ¼ 40, mean age ¼ 20.39 years, 6standard error [SE] ¼ 0.39; lab: n ¼ 43, mean age ¼ 18.73 years, 6SE ¼ 0.27) completed a voice perception test (Pitch Test) that assessed whether preferences for lowered versus raised fundamental frequency in recordings of male voices vary according to the formant frequency of the voices judged. Each woman was played 12 pairs of male voices, where each pair consisted of a version with lowered pitch and a version of the same voice with raised pitch. Half of these pairs of voices consisted of voices that both had increased apparent vocal tract length and the other half consisted of voices that both had decreased apparent vocal tract length. Participants were instructed to indicate which voice in each pair was more attractive. The order in which pairs of voices were played and the order in which the 2 versions in each pair were played were both fully randomized. Eighty-three different women (Internet: n ¼ 40, mean age ¼ 20.69 years, 6SE ¼ 0.31; lab: n ¼ 43, mean age ¼ 19.62 years, 6SE ¼ 0.15) completed a voice perception test (Vocal Tract Test) that assessed whether preferences for masculinized versus feminized formants in male voices vary according to the fundamental frequency of the voices judged. In this test, each woman was played 12 pairs of male voices, where each pair consisted of a version with increased apparent vocal tract length and a version of the same voice with decreased apparent vocal tract length. Half of these pairs of voices consisted of voices that both had lowered pitch and the other half consisted of voices that both had raised pitch. As in the Pitch Test, participants indicated which voice in each pair was more attractive. Both the order in which pairs of voices were played and the order in which the two versions in each pair were played were fully randomized. The paradigm we used for the Pitch Test and the Vocal Tract Test is identical to the paradigm that has been used in many previous studies of the effects of manipulated vocal characteristics on attractiveness judgments (for review, see Feinberg 2008; Jones, Boothroyd, et al. 2010). In both samples, women
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(Puts et al. 2007; Wolff and Puts 2010). Height explains more than 70% of the variance in vocal tract length (Fitch and Giedd 1999) and is positively related to indices of physical dominance, such as strength (Sell et al. 2009). Additionally, male height is correlated with indices of social dominance, such as job success and academic performance (Hensley and Cooper 1987; Hensley 1993; Gautschi and Hangartner 2006; Case and Paxson 2008), as well as reproductive success (Pawlowski et al. 2000; Mueller and Mazur 2001) and partner attractiveness (Feingold 1982). Thus, female preferences for large male vocal tracts are potentially adaptive. Research has demonstrated interactions between the effects of fundamental frequencies and apparent vocal tract length on perceptions of size, sex, and age. Lowering the pitch of voices with large vocal tracts has greater effects on perceptions of size, sex, and age than does lowering the pitch of voices with relatively small vocal tracts (Smith et al. 2005, 2007). Similarly, increasing the apparent vocal tract of voices that already have low pitch has greater effects of perceptions of size, sex, and age than does increasing the apparent vocal tract of voices that have relatively high pitch (Smith et al. 2005, 2007). No study, however, has investigated interactions between the effects of fundamental frequency and apparent vocal tract length on judgments of vocal attractiveness. If people integrate information from voice pitch and vocal tract length when judging others’ size, age, and masculinity, these same acoustic features may also interact when women judge the attractiveness of men’s voices. Given that female preferences for low fundamental frequency and large apparent vocal tract length are potentially adaptive, it is possible that women integrate information from these 2 cues when judging the attractiveness of men’s voices. Indeed, Vukovic et al. (2010) recently suggested (but did not test) that the strength of women’s preferences for low pitch in men’s voices might be modulated by other vocal cues, such as vocal tract length. Additionally, recent research has shown that female tree frogs’ ability to discriminate attractive vocalizations is enhanced when multiple acoustic components are available (Richardson and Lengagne 2010). Furthermore, Charlton et al. (2008) demonstrated that fundamental frequency may enhance the efficacy of formant frequencies as body size cues in Cervus elaphus (red deer), providing further evidence for integration of information from multiple vocal cues in nonhuman species. To test for interactions between women’s preferences for fundamental and formant frequencies, we manipulated men’s voices to have lowered pitch and larger apparent vocal tract length (i.e., lowered formant frequencies), lowered pitch and smaller apparent vocal tract length, raised pitch and larger apparent vocal tract length, and raised pitch and smaller apparent vocal tract length. In 2 separate experiments, we 1) compared women’s preferences for low versus high voice pitch when associated with larger or smaller vocal tracts and 2) women’s preferences for larger versus smaller vocal tracts when associated with lower or higher pitch. We predicted that women would prefer lower pitch more when associated with larger apparent vocal tracts than with smaller apparent vocal tracts. We also predicted that women would prefer larger vocal tracts when associated with lower pitch than when associated with higher pitch. Such findings would reveal integration of fundamental and formant frequencies in women’s mate preferences.
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RESULTS Pitch Test Responses were first analyzed using a 2 3 2 repeated measures analysis of variance (ANOVA) (within subject factor: vocal tract length [small vocal tract, large vocal tract]; between subject factor: sample [Internet sample, lab sample]). This analysis revealed that women chose male voices with lowered pitch as more attractive than male voices with raised pitch more often when judging pairs of voices that both had increased apparent vocal tract length (mean ¼ 0.64; SE ¼ 60.03) than when judging pairs of voices that both had decreased apparent vocal tract length (mean ¼ 0.45, SE ¼ 60.036; F1,82 ¼ 26.21, P , 0.001). There was no interaction between apparent vocal tract length and sample type (F1,82 ¼ 0.99, P ¼ 0.322), however, there was a main effect of sample, where preferences for voices with lowered pitch were stronger in the laboratory than on the Internet (F1,82 ¼ 7.73, P ¼ 0.007). Comparing the proportion of trials on which the versions with lowered pitch were chosen to what would be expected by chance alone (i.e., 0.5), using one-sample t-tests, showed that women chose the male voices with lowered pitch as the more attractive more often than chance when both voices in each pair had increased apparent vocal tract length (Combined samples: t83 ¼ 4.40, P , 0.001; Internet sample: t39 ¼ 2.18, P ¼ 0.035; mean ¼ 0.58, SE ¼ 60.36; Lab sample: t43 ¼ 3.90, P , 0.001; mean ¼ 0.69, SE ¼ 60.05). By contrast, a corresponding analysis showed that women did not choose the male voices with lowered pitch as the more attractive more often than chance when both voices in each pair had decreased apparent vocal tract length (Combined sample: t83 ¼ 21.47, P ¼ 0.145; Internet sample: t39 ¼ 23.28, P ¼ 0.002; mean ¼ 0.35, SE ¼ 60.46; Lab sample: t43 ¼ 0.72, P ¼ 0.477; mean ¼ 0.54; SE ¼ 60.05). Figure 1 summarizes these results. Vocal Tract Test Responses were initially analyzed using a 2 3 2 repeated measures ANOVA (within subject factor: pitch [raised, lowered]; between subject factor: sample [Internet sample, lab
Figure 1 illustrates preferences for lowered pitch when associated with both lengthened and shortened apparent vocal tract length and preferences for lengthened apparent vocal tract length when associated with both high and low pitch. Error bars represent the standard error, and chance value is 0.5.
sample]). This analysis revealed that women chose male voices with increased apparent vocal tract length as more attractive than male voices with decreased apparent vocal tract length more often when judging pairs of voices that both had lowered pitch (mean ¼ 0.72, SE ¼ 60.028) than when judging pairs of voices that both had raised pitch (mean ¼ 0.59, SE ¼ 60.031; F1,81 ¼ 22.47, P , 0.001). There was no interaction between voice pitch and sample type (F1,82 ¼ 0.36, P ¼ 0.55), and there was no main effect of sample type (F1,82 ¼ 0.008, P ¼ 0.93). Comparing the proportion of trials on which the versions with lowered pitch were chosen to what would be expected by chance alone (i.e., 0.5), using one-sample t-tests, showed that women chose the male voices with increased apparent vocal tract length as the more attractive more often than chance when both voices in each pair had lowered pitch (Combined: t82 ¼ 7.9, P , 0.001; Internet: t39 ¼ 5.45, P , 0.001; mean ¼ 0.73, SE ¼ 60.04; Lab: t42 ¼ 5.7, P , 0.001; mean ¼ 0.71; SE ¼ 60.038). A corresponding analysis showed that women chose the male voices with increased apparent vocal tract length as the more attractive more often than chance when both voices in each pair had raised pitch (Combined: t82 ¼ 2.86, P ¼ 0.005; Internet: t39 ¼ 1.69, P ¼ 0.10; mean ¼ 0.58, SE ¼ 60.05; Lab: t42 ¼ 2.34, P ¼ 0.024; mean ¼ 0.6, SE ¼ 60.043), although this effect was not significant for the Internet sample alone. Figure 1 summarizes these results. DISCUSSION We observed interactions between the effects of pitch and vocal tract length on women’s preferences for men’s voices. Preferences for lower versus higher pitch were stronger when judging the attractiveness of voices with larger apparent vocal tracts than when judging the attractiveness of the same voices with smaller apparent vocal tracts. Similarly, preferences for larger versus smaller apparent vocal tracts were stronger when judging the attractiveness of voices with lower pitch than when judging the attractiveness of the same voices with higher pitch. Female preferences for low pitch and large vocal tracts are potentially adaptive, as male voice pitch is negatively related to underlying testosterone levels and reproductive success (Apicella et al. 2007) and because height, which is tied to vocal tract length (Fitch and Giedd 1999), is positively related to indices of both physical dominance (Sell et al. 2009) and
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were randomly allocated to either the Pitch Test or the Vocal Tract Test. The experiment was run both online and in the laboratory. Previous research has shown that online studies of preferences for manipulated characteristics in voice recordings produce patterns of results that are virtually identical to those observed in lab-based studies (for review, see Feinberg 2008). We conducted the experiment both online and in the laboratory to confirm that the integration of apparent vocal tract length and voice pitch in women’s voice preferences did not differ between samples. For each woman who completed the Pitch Test, we calculated the proportion of trials on which she chose the voice with lowered pitch as more attractive than the version with raised pitch. This measure was calculated separately for pairs of voices where both versions had increased apparent vocal tract length and for pairs of voices where both versions had decreased apparent vocal tract length. For each woman who completed the Vocal Tract Test, we calculated the proportion of trials on which she chose the voice with increased apparent vocal tract length. This measure was calculated separately for pairs of voices where both versions had lowered pitch and for pairs of voices where both versions had raised pitch. Two-tailed P values are reported for all analyses.
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voice pitch and apparent vocal tract may be used by the sender to exaggerate perceived size, and hence dominance, during intrasexual competition (Fitch and Hauser 1995; Fitch 1999), which occurs across several mammalian and avian species (Fitch and Kelley 2000; Fitch and Reby 2001; Harris et al. 2006; Puts et al. 2006; Frey et al. 2007; Sanvito et al. 2007; Vannoni and McElligott 2008; Taylor et al. 2010). Thus, it is possible that low pitch enhances the effects of large vocal tracts on attractiveness ratings by exaggerating the perceived size and/or dominance of the target individual (Collins 2000; Feinberg et al. 2005; Smith et al. 2005, 2007; Puts et al. 2006). Large vocal tracts, and hence large bodies, may enhance the attractiveness of men’s low-pitched voices. By using multiple cues to different underlying qualities, perceivers may gain a better picture of the organism’s overall fitness (Candolin 2003; Smith et al. 2009; Connallon 2010). Indeed, both low voice pitch and large body size are associated with increased reproductive success (Pawlowski et al. 2000; Mueller and Mazur 2001; Apicella et al. 2007). Thus, preferences for these traits are potentially adaptive and integrating information from both may improve the accuracy of assessments of others’ mate quality. Prior research has identified that female preferences for male voices lowered in pitch are stronger when women are not breast-feeding (Apicella and Feinberg 2009), are naturally cycling (Feinberg, DeBruine, Jones, and Little 2008), among women who consider themselves to be relatively attractive (Vukovic et al. 2008) and among women who possess attractive vocal characteristics themselves (Vukovic et al. 2010). Furthermore, male voices digitally lowered simultaneously in both pitch and increased in apparent vocal tract length are preferred to those raised simultaneously in both pitch and decreased in apparent vocal tract length (Feinberg et al. 2005). These preferences are also enhanced during the late follicular phase of the menstrual cycle (Puts 2005; Feinberg et al. 2006) and when women rate men’s attractiveness as short-term rather than long-term partners (Puts 2005). Despite these individual differences, main effects of voice pitch are generally found (see Feinberg 2008, for review). Furthermore, Feinberg et al. (2005) revealed preferences for large apparent vocal tracts when accounting for women’s own height and/or weight. Future research should investigate the role of individual differences in preferences for apparent vocal tract length and voice pitch. Importantly, our results differ from those reported by Feinberg et al. (2005) in that Feinberg et al. (2005) did not test for interactions between fundamental and formant frequencies, whereas testing for this interaction was the main focus of the current research. In our Internet sample, women preferred raised pitch when associated with shortened vocal tracts. By contrast, our laboratory data showed no preference for high or low voice pitch when associated with short vocal tracts. That women in the Internet sample preferred raised pitch when associated with shortened vocal tracts was surprising. One potential explanation for this unexpected finding is that low voice pitch sounds incongruous when paired with small vocal tracts and is, thus, unattractive. Although explaining this effect goes beyond our data, it may be an interesting topic for investigation in future research. In summary, this is the first evidence for integration of male voice pitch and apparent vocal tract length in women’s mate preferences. Neither low pitch nor large vocal tracts were consistently found to be the most attractive option. Similar results have been found when considering individual differences in women’s preferences for masculinized vocal traits (Feinberg et al. 2005, 2006; Puts 2005, 2006; Feinberg, DeBruine, Jones, and Little 2008; Vukovic et al. 2008; Apicella and Feinberg 2009), suggesting that although general preferences for
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social dominance (Hensley and Cooper 1987; Hensley 1993; Gautschi and Hangartner 2006; Case and Paxson 2008), as well as measures of partner attractiveness (Feingold 1982) and reproductive success (Pawlowski et al. 2000; Mueller and Mazur 2001). Among men, both voice pitch and formant frequency dispersion (an acoustic correlate of vocal tract length) are correlated negatively with adult testosterone levels (Dabbs and Mallinger 1999; Collins 2000; Bruckert et al. 2006; Evans et al. 2008) and, consequently, are, at least partly, redundant testosterone cues. As we observed an interaction among preferences for low pitch and large apparent vocal tracts, voice pitch and apparent vocal tract length may serve as cue amplifiers rather than backup messages (Candolin 2003). If voice pitch and apparent vocal tract length were backup cues, we would have expected independent main effects of these cues on voice preferences. Cue amplifiers, however, enhance the attractiveness of another cue (Candolin, 2003), which, in our experiments, would be evident as interactions between the effects of pitch and apparent vocal tract length on preferences. Thus, our results support a cue amplification system whereby low pitch amplifies preferences for large vocal tracts and large vocal tracts amplify preferences for low pitch. Another possible explanation as to why women preferred large vocal tracts in men’s voices more when associated with low pitch is that lowering pitch increases the salience of the formant structure (Charlton et al. 2008). It should be noted, however, that in one of our samples (Internet), women preferred shortened vocal tracts when they were associated with high pitch. This result is difficult to explain solely in terms of greater salience of the formant structure when pitch is lowered. Nonetheless, more direct empirical tests of this and other possibilities may provide valuable insights into the proximate mechanisms that underpin the integration of different acoustic properties in voice preferences, and we emphasize such research as an important topic for future research. Our findings are consistent with research on the interaction between the effects of perceived health and sexual dimorphism on face preferences (Smith et al. 2009), which showed that preferences for exaggerated sex-typical shape cues were stronger when judging the attractiveness of healthy looking male and female faces than when judging the attractiveness of unhealthy looking male and female faces. These findings show that integration of different cues of mate quality are not unique to the vocal modality but may be a general property of mate preferences in multiple modalities. Although our findings are consistent with interactions between the effects of different cues of quality on face preferences, our findings contrast with recent research on red deer, which found no evidence for interactions between the effects of fundamental and formant frequencies on mate preferences (Charlton et al. 2008). Among men, voice pitch and formant frequency dispersion are negatively correlated with perceived body size (Fitch and Giedd 1999; Collins 2000; Feinberg et al. 2005; Smith et al. 2005, 2007; Evans et al. 2006). However, voice pitch is unrelated to actual body size among adults of one sex (Lass and Brown 1978; Cohen et al. 1980; Graddol and Swann 1983; Kunzel 1989; Collins 2000; Evans et al. 2006; Gonzalez 2007; van Dommelen and Moxness 1995). Given that voice pitch is not a reliable size cue, voice pitch and apparent vocal tract length are not true backup cues to body size (Candolin 2003). Instead of using vocal tract size cues to override inaccurate body size cues from voice pitch, people tend to combine voice pitch and apparent vocal tract length additively when making body size judgments, further obscuring the relationship between perceived and physical body size (Smith et al. 2005). This may, however, be potentially adaptive for the sender, as
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men with low pitch and large vocal tracts may be adaptive, they may not be the optimal preferences in all situations. Such findings stress the importance of using multiple cues when assessing mate quality (Candolin 2003). FUNDING D.R.F. is funded by Social Science and Humanities Research Council of Canada (410-2009-2924) Canada Foundation for Innovation (17515), and Ministry of Research and Innovation of Ontario. We thank P. K. Chu, S. Haller, S. Mitges, S. Pandaya, and C. Schandl for their aid in data collection.
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REFERENCES
Behavioral Ecology
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Voice attractiveness and cue integration
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Downloaded from http://beheco.oxfordjournals.org/ at McMaster University Library on April 15, 2012
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