Our findings suggest that lower male pitch and formants may be valid cues of some components of fighting ability in men

Low fundamental and formant frequencies predict fighting ability among male mixed martial arts fighters. Toe Aung, Stefan Goetz, John Adams, Clint McKenna, Catherine Hess, Stiven Roytman, Joey T. Cheng, Samuele Zilioli & David Puts. Scientific Reports volume 11, Article number: 905 (2021). Jan 13 2021. https://www.nature.com/articles/s41598-020-79408-6

h/t David Schmitt (20) David Schmitt on Twitter: ""lower male pitch and formants may be valid cues of some components of fighting ability in men" https://t.co/4dRGZT7XyH"

Abstract: Human voice pitch is highly sexually dimorphic and eminently quantifiable, making it an ideal phenotype for studying the influence of sexual selection. In both traditional and industrial populations, lower pitch in men predicts mating success, reproductive success, and social status and shapes social perceptions, especially those related to physical formidability. Due to practical and ethical constraints however, scant evidence tests the central question of whether male voice pitch and other acoustic measures indicate actual fighting ability in humans. To address this, we examined pitch, pitch variability, and formant position of 475 mixed martial arts (MMA) fighters from an elite fighting league, with each fighter’s acoustic measures assessed from multiple voice recordings extracted from audio or video interviews available online (YouTube, Google Video, podcasts), totaling 1312 voice recording samples. In four regression models each predicting a separate measure of fighting ability (win percentages, number of fights, Elo ratings, and retirement status), no acoustic measure significantly predicted fighting ability above and beyond covariates. However, after fight statistics, fight history, height, weight, and age were used to extract underlying dimensions of fighting ability via factor analysis, pitch and formant position negatively predicted “Fighting Experience” and “Size” factor scores in a multivariate regression model, explaining 3–8% of the variance. Our findings suggest that lower male pitch and formants may be valid cues of some components of fighting ability in men.

Discussion

Sexual dimorphism in fo likely arose in the common ancestor of the catarrhine primates after their divergence from the New World monkeys6 approximately 43.5mya44 and appears to have been subsequently elaborated or reduced depending on the form and degree of male mating competition6. Relatively low male fo may have evolved as a means of exaggerating the appearance of size to same-sex competitors and/or potential mates6,18, but there is considerable debate regarding whether male fo is purely deceptive20,21,45 or provides any reliable information about formidability in men11,17,33.

To shed light on this debate, we investigated whether fo is associated with fighting ability among a large sample of male MMA fighters. Results of our pre-registered analyses were generally in the direction of lower fo predicting greater fighting ability but were mixed in terms of statistical significance. When we addressed the limitations of these analyses by creating more precise measures of fighting ability and accounting for the contributions of the distinct dimensions revealed through principal axis factoring, we found that lower fo was associated with greater fighting ability in all analyses. In the statistical model that most precisely measured fighting ability by including principal axis factors related to fighting experience, fighting success, and body size, fo predicted fighting ability generally and specifically components of fighting ability related to experience and size, but not within-weight class fighting success. Overall, these results suggest that low fo is an honest cue of formidability in men (Table 3).

Table 3 Component axis analysis for measures related to fighting ability.

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Effect sizes were small, however. Even when fighting ability was measured most precisely, fo explained only 1–3% of the variance (Table 4). On the one hand, the strength of these associations accords with theoretical predictions derived from the fact that signaling is multimodal and multi-component17. On the other hand, it is important to emphasize that relationships between fo and fighting success among MMA fighters may underestimate those in the general population due to range restriction on fighting ability. Among elephant seals, the body length of males who occupied the center of harems correlated with neither maximum harem size nor tenure length on the beach during the breeding season46. However, when all males were analyzed together, including those that were peripheral to or outside of the harem, male body length explained 17% of the variation in tenure length on the beach. The degree to which fo correlates with fighting ability in the general population has yet to be tested for obvious ethical reasons.

Table 4 Results of a multi-variate regression model.

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Voice pitch and other acoustic variables are modulated across social contexts, including those related to perceived fighting ability relative to a competitor13,47, relative dominance and prestige48, authority36, current aggressive intent49, emergent rank16, and volitional exaggeration50,51. On the one hand, if voice pitch is modulated in relation to self-perceived relative formidability and status, as this prior research indicates, then it is possible that any voice modulation of fighters during interviews could have strengthened relationships between acoustic parameters and measures of fighting ability in the present study. On the other hand, if voice modulation is less patterned or less dependent upon perceived relative formidability, then this would introduce noise in measuring individual differences in voices, which would tend to weaken relationships between acoustic parameters and true fighting ability. In these data based on naturalistic observations, although we cannot rule out that some fighters may have modulated their voice pitch or other vocal parameters during interviews to sound stronger, we sought to strengthen our measures of individual differences in acoustic traits by sampling across multiple interview occasions. Indeed, we found that acoustic measures were consistent across fighters, even between pre- and post-fight conditions in the subset of our sample for which this information was available (see Supplemental Procedures for details), and we attempted to capture more stable individual acoustic differences rather than measurements specific to particular recording conditions by using the unbiased linear estimates across recordings for each fighter in all analyses.

Only one previous study of which we are aware28 examined links between fighting success and fo or other acoustic measures and found that acoustic measures did not predict win percentage among male MMA fighters. Likewise, in our bivariate correlation analysis, we did not find a significant relationship between win percentage and vocal fo (Table 1), although lower fo predicted a greater number of UFC matches. Our ability in subsequent analyses to detect relationships between fo and fighting ability where a previous study did not28 may have been due in part to our examination of a larger sample of fighters (474 vs. 29), along with longer total voice samples from each fighter (approximately 85 s vs. 8 s).

Our use of unstandardized, spontaneous speech samples also offered enhanced ecological validity47, and although this approach adds noise to the measurement of individual differences in acoustic parameters, previous research shows strong correspondence across speech and/or vocalization types for both fundamental52 and formant10 frequencies. Moreover, the best linear unbiased composite estimates of acoustic measures from multiple voice recordings of each fighter, along with relatively long (> 85 s) average total speech samples from individual fighters, provided reliable estimates of individual differences in acoustic features.

Perhaps most importantly, we extracted component variables via factor analysis to produce more comprehensive measures of fighting ability than can be offered by individual measures such as Elo rating or win percentage. For example, win percentage alone is an imperfect indicator of fighting success because fighters with fewer fights and losses can achieve higher win rates53,54 and because win percentage does not account for the strength of opponents as Elo ratings do. Our factor analysis addressed such limitations and produced three unique components of fighting ability, which we termed Fighting Experience, Fighting Success, and Size. Fighting Experience reflects the component of flighting ability most strongly related to total number of fights but also strongly related to years active in the UFC and age. Given that Elo ratings also loaded positively onto this component, it may represent the component of fighting skill attributable to experience. Fighting Success reflects a component of fighting ability related to a history of winning in one’s weight class that is relatively unrelated to experience or size, perhaps tapping characteristics such as speed, agility, and strength for one’s size. The extracted Size factor reflects the component of fighting ability related to height and weight. Larger size is generally associated with lower fo across mammals55 and is a major determinant of dominance and social status across species56,57,58,59,60. Physical size is such a strong determinant of outcomes in combat sports such as wrestling, boxing, and martial arts that weight classes are needed to prevent dangerous lopsided contests, and fighters are willing to sacrifice energy and hydration by cutting weight to fight smaller opponents. Parallel to findings in previous studies32,61, fighters’ fo predicted their height and weight, as well as their Size factor scores.

Our orthogonal factor analysis extracted independent components of fighting ability, maximizing the variance among items and facilitating interpretation. A structural equation model further confirmed the structure of our proposed components of fighting ability, but allowing residual correlations between latent variables produces a better model fit. These analyses highlight the importance of considering fighting ability across related measures instead of using a single measure of fighting experience, fighting success, or size. Because fighting ability measures (e.g., win percentage and number of fights) reflect weight division-specific measures, results from analyses that do not incorporate the influence of fighter’s weight (Table 1) may be less valid than others (Table 4).

Although not the focus of the present paper, other sexually dimorphic acoustic parameters were associated with fighting ability. In correlation analyses, lower fo-SD and Pf predicted a greater number of UFC fight matches, and lower Pf predicted higher Elo ratings (Table 1). In addition, Pf predicted overall measures of fighting ability in the multivariate regression model (Fig. 2) and independently predicted Fighting Experience and Size (Fig. S2). Our findings extend those from previous studies showing that Pf predicts objective measures of threat potential, such as size32, strength, and physical aggression10, as well as perceived fighting ability14,62, and suggest that Pf communicates size-independent information about vocalizers’ physical formidability.

A potential statistical issue concerns the use of uncorrected p-values associated with multiple tests. However, the adjustment of p-values, such as Bonferroni correction, not only decreases the rates of Type I error, but also increases the rates of Type II error and has been highly criticized63. Additional analyses used in this study, such as multi-level modeling64, multi-variate regression, and structural equation modeling, that consider dependent variables simultaneously should reduce concerns associated with multiple comparisons.

In general, our findings are consistent with the broader perspective that men’s anatomy, behavior, and psychology have been shaped by an evolutionary history of contest competition27,65, the use of force or threat of force to exclude same-sex competitors from mates59. Contest competition should favor psychological mechanisms to attend to and assess the formidability and threat potential of competitors19. People can accurately assess physical strength from body19 and face66 images. For example, sexually dimorphic facial cues predict fighting ability among MMA fighters33,67 but see68, and using these cues participants can predict fighting outcomes above chance69. The voice appears to be another aspect of the phenotype that indicates formidability10,13,19; we showed that fo was associated with both body size and fighting experience (independent of body size) among male MMA fighters. Although associations between measures of fighting ability and fo and other sexually dimorphic acoustic parameters were small, they comport with the notion that fo is but one of many components of a vocal acoustic signal, and that voices represent one of many signals of men’s formidability17. A deep, resonant voice clearly indicates status as a physically mature or maturing male. Our findings support the further possibility that attention to differences in voice pitch among men may be functional as well, as fo and other sexually dimorphic components of men’s voices appear to provide information about differences in men’s threat potential.