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Order 546087: Are there vocal cues to human developmental stability? Relationships between facial fluctuating asymmetry and voice attractiveness.

FirstDraftHandoutS171 hill_et_al_2016_ehb

Type of paperArticle Review
SubjectPsychology
Number of pages3
Format of citationAPA
Number of cited resources1
Type of serviceWriting

Do not include abstract and reference section. Include an introduction, a methods section, a results section, a discussion section, a critical assessment section

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Original Article

Are there vocal cues to human developmental stability? Relationships
between facial fluctuating asymmetry and voice attractiveness

Alexander K. Hill a,1, Rodrigo A. Cárdenas b, John R. Wheatley a, Lisa L.M. Welling a,2, Robert P. Burriss a,3,
Peter Claes c, Coren L. Apicella d, Michael A. McDaniel e, Anthony C. Little f, Mark D. Shriver a, David A. Puts a,g,⁎
a Department of Anthropology, The Pennsylvania State University, University Park, PA 16802, USA
b Department of Psychology, The Pennsylvania State University, University Park, PA 16802, USA
c KU Leuven, ESAT/PSI—UZ Leuven, MIRC—iMinds, Medical IT Department, Belgium
d Department of Psychology, University of Pennsylvania, Philadelphia, PA 19104, USA
e Department of Management, Virginia Commonwealth University, Richmond, VA 23284, USA
f Department of Psychology, University of Bath, Bath, UK
g Center for Brain, Behavior, and Cognition, The Pennsylvania State University, University Park, PA 16802, USA

a b s t r a c ta r t i c l e i n f o

Article history:
Initial receipt 26 November 2015
Final revision received 25 October 2016
Available online xxxx

Keywords:
3D images
Fluctuating asymmetry
Geometric morphometrics
Hunter-gatherers
Mate choice
Sexual selection

Fluctuating asymmetry (FA), deviation from perfect bilateral symmetry, is thought to reflect an organism’s rela-
tive inability to maintain stable morphological development in the face of environmental and genetic stressors.
Previous research has documented negative relationships between FA and attractiveness judgments in humans,
but scant research has explored relationships between the human voice and this putative marker of genetic qual-
ity in either sex. Only one study (and in women only) has explored relationships between vocal attractiveness
and asymmetry of the face, a feature-rich trait space central in prior work on human genetic quality and mate
choice. We therefore examined this relationship in three studies comprising 231 men and 240 women from
two Western samples as well as Hadza hunter–gatherers of Tanzania. Voice recordings were collected and
rated for attractiveness, and FA was computed from two-dimensional facial images as well as, for a subset of
men, three-dimensional facial scans. Through meta-analysis of our results and those of prior studies, we found
a negative association between FA and vocal attractiveness that was highly robust and statistically significant
whether we included effect sizes from previously published work, or only those from the present research,
and regardless of the inclusion of any individual sample or method of assessing FA (e.g., facial or limb FA).
Weighted mean correlations between FA and vocal attractiveness across studies were −.23 for men and −.29
for women. This research thus offers strong support for the hypothesis that voices provide cues to genetic quality
in humans.

1. Introduction

Mate choice is predicated, in part, on heritable fitness benefits
(Petrie, 1994), and organisms are attentive to biomarkers that putative-
ly indicate whether a potential mate possesses genes that would con-
tribute additively to offspring fitness (Andersson, 1994)—that is,
whether a mate is high in genetic quality (Hunt, Bussiere, Jennions, &
Brooks, 2004). One such biomarker is fluctuating asymmetry (FA;
e.g., Møller, 1992; Swaddle, 1996; Thornhill & Gangestad, 1999a; Van

Valen, 1962, 1973). FA is exhibited in bilaterally symmetrical traits if
the difference between the left and right sides has a population normal
distribution with a mean of zero (Palmer & Strobeck, 1986). Substantial
evidence from humans (e.g., Gangestad, Thornhill, & Yeo, 1994; Livshits
& Kobyliansky, 1991) as well as an array of vertebrates and insects
(Parsons, 1990) indicates that the degree to which bilaterally symmet-
rical traits deviate from perfect symmetry reflects developmental insta-
bility resulting from harmful mutations and environmental
perturbations. Experimental exposure to common environmental stressors,
for example, can increase asymmetry in laboratory rats (e.g., Mooney,
Siegel, & Gest, 1985; Sciulli, Doyle, Kelley, Siegel, & Siegel, 1979).

In humans, individuals with known genetic or chromosomal abnor-
malities have greater dental FA than do controls (e.g., Barden, 1980;
Peretz et al., 1988), and FA displays a negative relationship with genetic
heterozygosity (Livshits & Kobyliansky, 1991), which can increase fitness
by lowering the risk of expressing deleterious recessive alleles and by in-
creasing acquired immunity through the major histocompatibility

Evolution and Human Behavior xxx (2016) xxx–xxx

⁎ Corresponding author. Department of Anthropology, The Pennsylvania State Universi-
ty, 218 Carpenter Building, University Park, PA 16802, USA. Tel.: +1 814 867 0453.

E-mail address: dap27@psu.edu (D.A. Puts).
1 Present address: Department of Anthropology, University of Washington, Seattle, WA

98195, USA.
2 Present address: Department of Psychology, Oakland University, Rochester, MI 48309,

USA.
3 Present address: Department of Psychology, University of Basel, Basel, Switzerland.

http://dx.doi.org/10.1016/j.evolhumbehav.2016.10.008
1090-5138/© 2016 Elsevier Inc. All rights reserved.

Contents lists available at ScienceDirect

Evolution and Human Behavior

journal homepage: www.ehbonline.org

Please cite this article as: Hill, A.K., et al., Are there vocal cues to human developmental stability? Relationships between facial fluctuating
asymmetry and voice attractiveness, Evolution and Human Behavior (2016), http://dx.doi.org/10.1016/j.evolhumbehav.2016.10.008

http://dx.doi.org/10.1016/j.evolhumbehav.2016.10.008

mailto:dap27@psu.edu

http://dx.doi.org/10.1016/j.evolhumbehav.2016.10.008

http://www.sciencedirect.com/science/journal/

http://dx.doi.org/10.1016/j.evolhumbehav.2016.10.008

complex (MHC; reviewed in Kempenaers, 2007). Lower FA in ear height,
wrist breadth, and digit length has predicted reduced resting metabolic
rate (Manning, Koukourakis & Brodie, 1997) and better mental health
(Martin, Manning, & Dowrick, 1999) in men, and lower facial FA shows
a similar relationship with perceptions of physical health (Jones et al.,
2001). Moreover, FA is positively correlated with female body mass, age,
and age at first childbirth, all indications of reduced fecundity (Manning,
Scutt, Whitehouse & Leinster, 1997). A recent meta-analysis also found
a small negative relationship between FA and general intelligence
(Banks, Batchelor, & McDaniel, 2010). Together, these results suggest
that bilateral symmetry may reflect the fit between an organism’s geno-
type and environment during development.

As a putative indicator of a genotype’s resilience to environmental
insults, FA is related to mate choice across a variety of nonhuman spe-
cies. For example, among field crickets (Gryllus campestris), males suc-
cessful in obtaining mates display lower FA than do less successful
males (Simmons, 1995). Likewise, female zebra finches (Taeniopygia
guttata) prefer to be near males wearing symmetrically colored experi-
mental leg bands (Swaddle, 1996).

In humans, several studies have reported significant correlations be-
tween FA and perceptions of facial attractiveness (e.g., Abend, Pflüger,
Koppensteiner, Coquerelle, & Grammer, 2015; Grammer & Thornhill,
1994; Komori, Kawamura, & Ishihara, 2009; Perrett et al., 1999; Rhodes
et al., 2001). In addition, women who view themselves as attractive
show a greater preference for symmetry in male faces (Little, Burt,
Penton-Voak, & Perrett, 2001), suggesting that symmetrical mates are
valued and are more attainable by women of higher mate value. Some
studies (e.g., Little & Jones, 2012; Little, Jones, Burt & Perrett, 2007),
but not others (e.g., Peters, Simmons, & Rhodes, 2009), have found
that women in the fertile phase of the ovulatory cycle exhibit a greater
preference for symmetrical faces and bodies, again suggesting an associ-
ation between symmetry and genetic quality, since genetic benefits can
be obtained only when conception is possible. There is also evidence
that humans mate assortatively for facial symmetry, indicating that
symmetry or correlated traits may influence mate choice (Burriss,
Roberts, Welling, Puts, & Little, 2011).

While bilateral symmetry appears to be important in predicting
mating decisions, preferences for symmetrical mates may not always
reflect preferences for symmetry per se. Bilateral symmetry has been
found to predict attractiveness even in the absence of visual cues to
asymmetry itself, and thus asymmetry must be related to other pheno-
typic cues. For example, women in the fertile phase of the ovulatory
cycle have shown a preference for symmetrical men when exposed ex-
clusively to the men’s body odor (Gangestad & Thornhill, 1998;
Thornhill & Gangestad, 1999b). Another study found a female prefer-
ence for low FA in men even when study participants were shown
only the left or right half of each male face, preventing them from
assessing symmetry directly (Scheib, Gangestad, & Thornhill, 1999).

Developmental stability may also be conveyed vocally. Hughes,
Harrison, and Gallup (2002) found both sexes to exhibit a negative rela-
tionship between upper limb FA (particularly finger length FA) and at-
tractiveness assessed from vocal stimuli alone. Abend et al. (2015)
similarly obtained a negative relationship between women’s vocal at-
tractiveness and measures of FA from their upper and lower limbs as
well as the head and face. These studies join other research demonstrat-
ing the salience of voice as an influence on mate choice in humans
(reviewed in Feinberg, 2008; Pisanski & Feinberg, 2013; Puts, Jones, &
DeBruine, 2012).

Moreover, vocal and facial attractiveness may covary (e.g., Saxton,
Burriss, Murray, Rowland, & Roberts, 2009) in part because both the
face and vocal folds are bilaterally symmetrical, and both may therefore
reflect deleterious environmental influences during development. For
example, asymmetric shape, size, or tension in the vocal folds could re-
sult in audible changes in the voice, such as hoarseness (Eysholdt,
Rosanowski, & Hoppe, 2003), and indeed, Hughes, Pastizzo, and Gallup
(2008) found that limb FA in men predicted a measure of vocal

hoarseness. Developmental instability may also affect the size and
shape of vocal structures, as well as tension on the vocal folds or the po-
sitioning of the larynx in the vocal tract, which would influence vocal
pitch and timbre, respectively (for fuller consideration of potential
links between FA and vocal attractiveness, please see Discussion).

Some research indicates that women’s vocal attractiveness is related
to fertility and reproductive potential (Bryant & Haselton, 2009;
Pipitone & Gallup, 2008, 2011; Puts et al., 2013; Wheatley et al., 2014),
though scant research has explored relationships between vocal attrac-
tiveness and FA in either sex. To our knowledge, Abend et al. (2015) is
the only study conducted to date investigating relationships between
vocal attractiveness and facial FA. Furthermore, not all research on FA
supports the prediction that individuals will tend to prefer mates
lower in FA. For example, some research on humans (e.g., Langlois,
Roggman, & Musselman, 1994; Swaddle & Cuthill, 1995), as well as non-
human species (e.g., Oakes & Barnard, 1994; Tomkins & Simmons,
1998), has failed to show preferences for mates with low FA, while
other research has found a statistically significant relationship between
FA and attractiveness only in particular situations (Springer et al., 2007).

There are likely several reasons for these discrepancies. First, FA is a
weak index of developmental instability, such that associations be-
tween FA and measures of mate quality are difficult to detect using
modest sample sizes (Van Dongen & Gangestad, 2011). Second, rela-
tionships with FA may be complicated by directional asymmetry (DA),
which characterizes traits for which the difference between the left
and right sides has a population normal distribution with a mean that
deviates from zero (Møller, 1994). Directional asymmetry, such as oc-
curs in testes size in many bird species (Møller, 1994) and in the
lower limbs of humans (Auerbach & Ruff, 2006), may constitute part
of an organism’s normal development (Graham, Emlen, Freeman,
Leamy, & Kieser, 1998; Martin, Puts, & Breedlove, 2008). Since the pres-
ence of DA may complicate measures of FA, it is critical to take DA into
consideration in research examining FA. While some studies have
done so (e.g., Abend et al., 2015; Brown et al., 2008; Manning, 1995),
others have not (e.g., Gangestad & Thornhill, 1997; Hughes et al.,
2002; Thornhill, Gangestad, & Comer, 1995). Finally, prior research has
disproportionately sampled from Western societies, especially individ-
uals of European genetic ancestry. Not only do such individuals differ
in facial morphology from those of other populations (Farkas, Katic, &
Forrest, 2005), but Westerners may also have greater access to modern
medical and cosmetic technologies that can buffer them from environ-
mental stressors, weakening relationships between FA and measures
of attractiveness (Apicella, 2011).

In the present paper, we tested whether somatic FA predicts vocal
attractiveness by collecting data in three studies that were designed in
part to address the limitations of prior research while enabling us to ex-
plore the robustness of these relationships across methodologies and
populations. We measured FA from the face due to the centrality of fa-
cial symmetry in prior work as well as the status of the face as a
feature-rich space that is developmentally distinct from the upper
limb, measured by Hughes et al. (2002) and the upper and lower limb
included in Abend et al. (2015). In Studies 1 and 2, we measured FA
from two-dimensional (2D) facial photographs, following most prior
studies of facial asymmetry and attractiveness (Jones et al., 2001).
Study 1 data were collected from U.S. undergraduate students, whereas
Study 2 data were collected from the Hadza, a Tanzanian forager popu-
lation less protected by modern medicine from environmental insults
than are samples drawn from the West. Compared to Western samples,
Hadza facial and vocal characteristics should thus more strongly reflect
individuals’ constitutional resilience to environmental stressors, such as
pathogens. Finally, in Study 3, we explored facial FA and vocal attrac-
tiveness among U.S. undergraduates using the 2D methods employed
in the first two studies as well as three-dimensional (3D) images mea-
sured via spatially dense geometric morphometrics, which provides
far greater shape information than does 2D imagery (Abend et al.,
2015; Brown et al., 2008).

2 A.K. Hill et al. / Evolution and Human Behavior xxx (2016) xxx–xxx

http://dx.doi.org/10.1016/j.evolhumbehav.2016.10.008

2. Study 1

2.1. Methods

2.1.1. Participants
One hundred twenty-four male (mean age = 20.08 ± 1.76 years)

and 201 female (mean age = 19.97 ± 1.55 years) students from Mich-
igan State University participated as part of a larger study of siblings
(see, e.g., Puts et al., 2013; Wheatley et al., 2014). If same-sex siblings
participated, we randomly selected one from each sibling group (gener-
ally pairs). In addition, only students self-identifying as “White” were
used, as facial morphology varies by ethnicity (e.g., Farkas et al., 2005),
and we did not have sufficient numbers of non-White participants to
permit a meaningful analysis stratified by ancestry group. Both male
and female participants provided voice recordings and 2D facial photo-
graphs at two data collection sessions scheduled one week apart, with
all procedures approved by the Michigan State University institutional
review board.

2.1.2. Face measurement
Two-dimensional facial photographs were taken of participants at

each session, with only photographs from session 1 used unless there
was no session 1 photograph for a participant, in which case the session
2 photograph was used (the methodology followed to take these photo-
graphs was identical between the sessions). Participants were instructed
to assume a neutral expression after having removed any jewelry, eye-
glasses, or makeup (with wet wipes provided at the time of data collec-
tion). While not all male participants were clean-shaven, care was taken
to ensure high-quality facial landmarks for all photographs (see below).
Facial photographs were taken with a tripod-mounted Canon PowerShot
S10 digital camera at a distance of approximately 1 m, a height adjusted to
the participant, and using constant lighting across participants. All facial
images were normalized on interpupillary distance and rotated so that
both pupils lay on the same horizontal plane.

Each facial photograph was landmarked by two research assistants
(RAs) with 14 x,y-coordinates in the software ImageJ, after which land-
mark coordinates were averaged between RAs. Horizontal asymmetry
measurements were obtained from landmarks as in Grammer and
Thornhill (1994) by first determining midpoints for six pairs of bilateral
landmarks (Fig. 1). If a face is perfectly symmetrical about the midsagittal
plane, then all midpoints will fall on the vertical line representing this
plane, and the horizontal differences between these midpoints will be
zero. Thus, to measure horizontal asymmetry, horizontal (i.e., x-
coordinate) distances between midpoints were computed for all 15 non-
redundant pairs of midpoints and then summed. Vertical asymmetry was
computed from the sum of the vertical (i.e., y-coordinate) distances be-
tween the 6 landmark pairs (Scheib et al., 1999). As in Scheib et al.
(1999), the horizontal and vertical asymmetry measures were finally
summed to produce an overall measure of facial asymmetry.

Inter-measurer reliability was assessed by calculating for each facial
distance relative error magnitude (REM) values, which incorporate
measurement size so as to arrive at a more precise assessment of consis-
tency (Weinberg, Scott, Neiswanger, Brandon, & Marazita, 2004). The
mean for the x-coordinates was 1.67 (SD = 1.08) and the mean for
the y-coordinates was 1.21 (SD = .74), indicating strong agreement.
DA was calculated for each sex and each facial trait separately and
then used to correct for DA when calculating individual FA values. DA
for each facial trait j was calculated as

DAj ¼ ∑
n

i¼1

Lij−Rij
Lij þ Rij

! ”
=

5=n

where L and R represent the left and right sides of the trait for each par-
ticipant i, and n represents the number of participants for whom data
were available. Thus, for each trait in each sex, DA was the average

signed left–right difference in distance across participants expressed
as a proportion of the size of the trait. We were then able, in accordance
with methods used by Gangestad et al. (1994) and Abend et al. (2015),
to calculate FA for each trait j in each individual i as

FAij ¼
Lij−Rij

Lij þ Rij
! ”

2
4
3

5−DA j

######

FA was computed exclusively from front-on photographs (Table S1,
available on the journal’s website at www.ehbonline.org), and we elim-
inated the facial image of one male participant whose facial FA exceeded
the mean by 4.0 SD, possibly due to head tilt. See Palmer and Strobeck
(1986) for a discussion of variation in methods of calculating FA.

2.1.3. Voice recording
ParticipantswererecordedspeakingthefirstsixsentencesoftheRainbow

Passage (Fairbanks, 1960) into a Shure SM58 vocal cardioid microphone in a
quiet recording booth. A curved wire projecting from the microphone stand
kept each participant’s mouth approximately 9.5 cm from the microphone.
Voices were recorded via a Sound Devices USBPre 2 preamplifier onto a com-
puter using GoldWave software in mono at a sampling rate of 44,100 Hz and
16-bit quantization, and saved as uncompressed WAV files.

2.1.4. Vocal and facial attractiveness ratings
Unfamiliar members of the opposite sex (n = 569, mean age =

19.4 years for male raters; n = 558, mean age = 19.1 years for female
raters) recruited from The Pennsylvania State University rated voice re-
cordings and facial images on attractiveness for a short-term relation-
ship made on a 7-point Likert scale (anchors: 1 = not at all attractive,
7 = very attractive). This portion of the study was approved by The
Pennsylvania State University’s institutional review board. Raters first
read the definition of a short-term relationship given by Little, Cohen,
Jones and Belsky (2007, p. 970): “You are looking for the type of person
who would be attractive in a short-term relationship. This implies that

Fig. 1. Landmarks used to compute horizontal and vertical facial asymmetry from two-
dimensional images. Left and right pupils were landmarked to standardize interpupillary
distance and register faces horizontally.

3A.K. Hill et al. / Evolution and Human Behavior xxx (2016) xxx–xxx

http://www.ehbonline.org/#_blank

http://dx.doi.org/10.1016/j.evolhumbehav.2016.10.008

the relationship may not last a long time. Examples of this type of rela-
tionship would include a single date accepted on the spur of the mo-
ment, an affair within a long-term relationship, and possibility of a
one-night stand.” We chose to use ratings of short-term attractiveness
made by members of the opposite sex because short-term relationships
represent the mating context in which genetic quality is most salient
relative to other mate choice criteria, such as investment (Kenrick,
Sadalla, Groth, & Trost, 1990). At private workstations, each rater then
assessed stimulus sets comprising approximately 25 facial photographs
presented on a computer screen and 25 recordings of the first sentence
of the Rainbow Passage presented through Sennheiser HD 280 Profes-
sional headphones. Recordings and photographs were randomly allo-
cated to a set, each of which was rated by at least 15 raters (mean =
18.9). The order in which participants completed the rating tasks
(faces or voices first) was random across participants, as was the
order in which stimuli were presented within each task. We calculated
mean short-term sexual attractiveness scores for each participant using
the first 15 ratings per stimulus (Table S1, available on the journal’s
website at www.ehbonline.org).

2.2. Results

In both men (n = 124, r = −.21, p = .017) and women (n = 201,
r = −.24, p = .001), there was a significant negative relationship be-
tween vocal attractiveness and facial FA (Tables 1 and 3, Fig. 2). We
also explored the possibility that the relationship between facial FA
and vocal attractiveness could be attributable to correlations between
(1) vocal and facial attractiveness (men: n = 118, r = .04, p = .685;
women: n = 189, r = .21, p = .005) and (2) facial attractiveness and fa-
cial FA (men: n = 118, r = −.04, p = .708; women: n = 189, r = −.15,
p = .043). Attractiveness is an important mediator of mating success
(Apicella & Feinberg, 2009; Rhodes, Simmons, & Peters, 2005), and facial
attractiveness has also correlated with both symmetry (Grammer &
Thornhill, 1994; Perrett et al., 1999) and vocal attractiveness (Abend
et al., 2015; Collins & Missing, 2003; Lander, 2008; Wheatley et al.,
2014). Hence, it is possible that facial symmetry might increase vocal at-
tractiveness indirectly by increasing facial attractiveness. For instance,
people with more symmetrical and thus more attractive faces may conse-
quently feel more confident and therefore speak more attractively. How-
ever, after controlling for facial attractiveness in multiple regression
models predicting vocal attractiveness, we found that the relationships
between facial FA and vocal attractiveness remained in both men
(β = −.23, p = .014; model: F(2115) = 3.23, R = .23, p = .043) and
women (β = −.25, p b .001; model: F(2186) = 10.75, R = .32, p b .0001).

3. Study 2

3.1. Methods

3.1.1. Participants
Twenty-eight male (mean age = 29.32 ± 5.84 years) and 39 female

(mean age = 29.03 ± 6.37 years) participants were recruited from the ap-
proximately 1000 Hadza hunter–gatherers living in remote savannah–

woodland habitats of Northern Tanzania. The Hadza have a sexual division
of labor in which women collect fruits and nuts and dig for tubers, while
men collect honey and hunt animals (Marlowe, 2004). The Hadza are
among an increasingly small number of human populations still subsisting
by hunting and gathering and are thought to possess a social organization
similar to that exhibited by early anatomically modern humans. For a de-
scription of the Hadza and hunter–gatherers in general, see Apicella and
Crittenden (2016).

Adult participants were recruited from eight Hadza camps visited
opportunistically in the summer of 2006. Only individuals between
the ages of 18 and 40 were used in the present study, and one male
and one female were removed from the dataset because they possessed
facial FA more than 3.2 SD from the mean FA for their sex. The research-
er located the first Hadza camp by driving in the area of Hadzaland, with
subsequent camps selected in a snowball sampling fashion based on in-
formation provided by participants about the proximity of the next clos-
est camp. Verbal consent was obtained for all participants, and the study
was approved by Harvard University’s ethics internal review board.

3.1.2. Face measurement
Adult male and female participants had their faces photographed out-

side during the day with a Sony Cyber-shot 18 megapixel digital camera.
Participants were asked to maintain a neutral expression and look directly
into the camera. Horizontal asymmetry was estimated from x,y-
coordinates of 6 bilateral points made by a single measurer following tech-
niques used in previous studies (Grammer & Thornhill, 1994; Penton-Voak
et al., 2001; Scheib et al., 1999). Symmetry was computed by taking left and
right deviation from the midline, calculated from interpupillary distance,
and then summing the absolute value of the individual scores (Table S1,
available on the journal’s website at www.ehbonline.org). DA was calculat-
ed in the manner used in Study 1 and subtracted from the FA measures. For
women, the resulting values were natural log-transformed to correct skew.

3.1.3. Vocal attractiveness ratings
Voice recordings were made privately inside a Land Rover vehicle.

Participants were asked to speak the Swahili word “hujambo,” which
loosely translates to “hello,” into a Sennheiser MKH-60 microphone. Re-
cordings were encoded in mono directly onto a computer hard disk
using Sonic Foundry’s Sound Forge at 44,100 Hz sampling rate and 16-
bit quantization, and saved as uncompressed WAV files. The recordings
were used as stimuli for vocal attractiveness ratings, which, as in Study
1, were made on a 7-point Likert scale (anchors: 1 = not at all attractive,
7 = very attractive) by unfamiliar members of the opposite sex (n = 29,
mean age = 19.38 years for male raters; n = 26, mean age =
20.38 years for female raters) recruited from The Pennsylvania State
University (Table S1, available on the journal’s website at www.
ehbonline.org). Raters listened to all opposite-sex recordings through
Sennheiser HD 280 Professional headphones, and the order in which
stimuli were presented was randomized for each rater.

3.2. Results

In contrast with the results of Study 1, we found no significant rela-
tionship between vocal attractiveness and facial FA in either men (n =
28, r = −.16, p = .431; Table 1) or women (n = 39, r = −.08, p = .626;
Table 3), though the sample size of Hadza participants was much small-
er, and in both sexes the non-significant correlation coefficients were in
the same direction as in Study 1.

4. Study 3

4.1. Methods

4.1.1. Participants
Seventy-nine self-identified White men (mean age = 19.92 ± 1.51)

from The Pennsylvania State University participated as part of two

Table 1
Meta-analysis of FA and vocal attractiveness in men.

Study name r Lower limit Upper limit z p n

Study 1 −0.21 −0.38 −0.04 −2.39 0.017 124
Study 2 −0.16 −0.50 0.23 −0.78 0.435 28
Study 3 (2D facial FA) −0.10 −0.32 0.12 −0.89 0.372 79
Study 3 (3D facial FA) −0.17 −0.42 0.11 −1.20 0.229 52
Hughes et al. (2002) −0.36 −0.58 −0.10 −2.62 0.009 50
Overall1 −0.21 −0.32 −0.09 −3.41 0.001 281
Overall2 −0.23 −0.35 −0.11 −3.64 b0.001 254
1 Using 2D facial FA from Study 3.
2 Using 3D facial FA from Study 3.

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broader studies, one on long-term romantic relationships (supplemen-
tary online material 2, sample A), the other on interpersonal dynamics
(supplementary online material 2, sample B). Data-collection proce-
dures were approved by the University’s institutional review board.

4.1.2. Face measurement
Consistent with methods used in Studies 1 and 2, two-dimensional

facial photographs were taken using an Olympus Evolt E-300 digital
camera. Participants sat in front of a light-colored background; removed
any facial jewelry, eyeglasses, or hats; used a headband to remove any
hair obstructing the face; and looked directly into the camera lens
while maintaining a neutral facial expression. The resultant photo-
graphs were used to obtain facial FA per the methods of Study 1
(Table S1, available on the journal’s website at www.ehbonline.org).
Also as in Study 1, inter-measurer reliability was assessed by calculating
REM values (Weinberg et al., 2004) for each facial distance. The mean
for the x-coordinates was .87 (SD = 1.05), and the mean for the y-
coordinates was .79 (SD = .67), indicating strong agreement.

In addition to standard two-dimensional photographs, three-
dimensional facial images were collected for a subset of participants
(n = 52, mean age = 19.87 ± 1.76) using the 3dMD face system
(3dMD, Atlanta, GA). Participants were asked to close their mouths
and maintain a neutral facial expression while providing their images,
which were then exported from the 3dMD Patient software in OBJ file

format and imported into a scan-cleaning program for cropping and
trimming, as well as removal of hair, ears, and any dissociated polygons.

We subsequently partitioned facial shape into patterns of symmetry
and asymmetry following the work of Claes, Walters and Clement
(2012). First, an anthropometric mask was non-rigidly mapped onto
the original 3D facial scans, as well as onto the reflected scan
representing the mirror image of the original (Fig. 3), which was con-
structed by changing the sign of the x-coordinate (Claes, 2007; Claes,
Walters, & Clement, 2012; Claes, Walters, Vandermeulen, & Clement,
2011; Klingenberg & McIntyre, 1998; Mardia, Bookstein, & Moreton,
2000). This established homologous spatially dense quasi-landmark
configurations for all original and reflected 3D images (Claes et al.,
2011). Secondly, following Mardia et al. (2000), a generalized Procrus-
tes superimposition was performed (Rohlf & Slice, 1990), eliminating
differences in position, orientation, and scale between the original and
reflected configurations. Finally, the overall consensus configuration
was perfectly symmetrical, and a single shape could be decomposed
into its asymmetrical and bilaterally symmetrical parts (Mardia et al.,
2000). The average of an original image and its reflected configuration
constitutes the symmetrical component, whereas the difference be-
tween both configurations constitutes the asymmetrical component
(Kimmerle & Jantz, 2005; Klingenberg, Barluenga, & Meyer, 2002;
Fig. 3). Because the consensus configuration is, by construction, sym-
metrical with respect to a plane, it is used to analyze variation of

Fig. 2. Relationship between vocal attractiveness and facial FA among (a) men (n = 124, r = −.21, p = .017) and (b) women (n = 201, r = −.24, p = .001) from Study 1.

Fig. 3. Overall asymmetry is calculated as the Procrustes distance between original and reflected images of each individual. The heatmap (third image from left) shows the regional
variation in asymmetry with red color highlighting regions with higher levels of asymmetry and blue color highlighting regions with lower levels of asymmetry. The reader is
encouraged to consult Claes, Walters, Shriver, et al. (2012) for a more detailed description.

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symmetry in populations and provides an estimate of the midsagittal
plane with a frame of reference that has clear anatomical meaning
(Klingenberg et al., 2002). As a result, Procrustes distances in the tan-
gent shape space between mapped 3D images and their reflections
after superimposition were used as measures of facial asymmetry. Be-
cause the asymmetry shape-space is centered around the average
asymmetry in the sample (i.e., the average asymmetry is subtracted
from the asymmetry measurements), the DA component was eliminat-
ed in computing facial asymmetry.

4.1.3. Vocal attractiveness ratings
Voice recordings were made from the first six sentences of the Rain-

bow Passage (Fairbanks, 1960), collected according to the protocol used
in Study 1. Vocal attractiveness was assessed by 167 female students
(mean age = 18.80 ± .95 years) recruited from The Pennsylvania
State University (Table S1, available on the journal’s website at www.
ehbonline.org), who listened to only the first sentence of the Rainbow
Passage (to avoid fatigue) on Sennheiser HD 280 Professional head-
phones. Voice stimuli were split randomly into two sets so that each lis-
tener rated half of the stimuli, but all stimuli were evaluated an equal
number of times. Presentation order of stimuli was randomized for
each listener. Recordings from sample A were rated on attractiveness
using a 7-point Likert scale (anchors: 1 = very unattractive, 7 = very at-
tractive). Recordings from sample B were rated on how attractive the
men are for both a “short-term, purely sexual relationship, such as a
one-night stand” and a “long-term, committed relationship, such as
marriage,” using a 10-point Likert scale (anchors: 1 = not at all, 10 =
more than anyone else I know), and these scores were averaged to pro-
duce a measure of overall vocal attractiveness. Ratings were standard-
ized within each sample to place them on the same scale.

4.2. Results

As in Study 2, the correlations between vocal attractiveness and FA
were in the predicted direction but were not statistically significant
(2D: n = 79, r = −.10, p = .373; 3D Procrustes: n = 52, r = −.17,
p = .227; Table 1). Interestingly, measures of FA made from 2D and
3D images were not significantly correlated (n = 52, r = −.08, p =
.581), though this may be attributable to the appreciably larger amount
of spatial information captured in 3D meshes, which, in theory, repre-
sent a methodological improvement over 2D photographs (see also Dis-
cussion). As a result, 3D images have been able to confirm the oft-
replicated negative relationship between FA and attractiveness in both
sexes obtained using 2D imagery (Brown et al., 2008). Despite this,
however, the strength of the relationship between vocal attractiveness
and FA in Study 3 did not differ between 2D and 3D measurements
(n = 52, z = .19, p = .425).

5. Meta-analysis

In addition to examining independently the three studies reported
here, we meta-analyzed the data across the studies, as well as the re-
sults of Hughes et al. (2002) and Abend et al. (2015), to produce sum-
mary estimates of the relationship between FA and vocal
attractiveness. Effect sizes were obtained for males from all studies re-
ported herein plus Hughes et al. (2002), and for females from Studies
1 and 2 as well as Hughes et al. (2002) and Abend et al. (2015). This re-
sulted in total samples of 281 men (utilizing 2D facial FA measurements
from Study 3) or 254 men (utilizing 3D facial FA measurements from
Study 3), as well as 328 women.

Because of methodological differences in sampling, variable mea-
surement (see Palmer & Strobeck, 1997 for a discussion of the effect of
measurement error on FA), and data treatment across studies, a random
effects model was used in the analyses, which allows for the presence of
moderator variables (e.g., differences in the precision with which FA in
different parts of the phenotype indexes developmental instability).

Correlation coefficients between FA and vocal attractiveness served as
the effect size. Variance estimates based on a small number of studies
are at best speculative, and the accuracy of sampling variance estimates,
though unbiased, improves with increasing numbers of samples. Both
within-sample Q estimates and I2 estimates (Higgins, Thompson,
Deeks, & Altman, 2003) suggest that all variances across studies are
best attributed to random sampling error. Better estimates of variance
across studies must await the accumulation of additional research. The
data were analyzed in Comprehensive Meta-Analysis software version
3 (Borenstein, Hedges, Higgins, & Rothstein, 2005), and males and fe-
males were treated separately. This software is in the Hedges and
Olkin (1985) tradition and performs calculations using the Fisher r-to-
z transformation and then reports the results in the untransformed cor-
relation coefficient metric. Moreover, in order to determine whether the
removal of a study would affect the meta-analytic results, we conducted
for each sex an “omit-one-study” analysis.

The point estimate for the population correlation between FA and
vocal attractiveness in men was r = −.23 and −.21 using 3D and 2D fa-
cial FA from Study 3, respectively (Table 1, Fig. 4a). Excluding individual
samples in the omit-one-study analysis resulted in effect sizes ranging
from r = −.25 to r = −.20 using 3D facial FA from Study 3 (Table 2,
Fig. 4c), and r = −.24 to r = −.17 using 2D facial FA from Study 3
(Table S2, Fig. S1a, available on the journal’s website at www.
ehbonline.org). In all analyses, the 95% CI excluded a correlation of zero.

For women, the point estimate for the population correlation be-
tween FA and vocal attractiveness was r = −.28 and −.29 using limb
FA and 3D facial FA from Abend et al. (2015), respectively (Table 3,
Fig. 4b). Excluding individual samples in the omit-one-study analysis
resulted in effect sizes ranging from r = −.32 to r = −.25 (Tables 4
and S3, Fig. S1b, available on the journal’s website at www.ehbonline.
org). In all analyses, the 95% CI excluded a correlation of zero. We also
meta-analyzed only the two studies reported in the present paper that
included women and obtained a population correlation of r = −.21
(95% CI: −.33 to −.09, p b .001).

6. Discussion

In the present research, we found relationships between vocal attrac-
tiveness and fluctuating asymmetry in both sexes. Though analyses
should be re-conducted as more data accumulate, our meta-analyses
gave point estimates for population correlations between vocal attractive-
ness and FA of −.23 for men and −.29 for women, using 3D facial FA from
Study 3 and Abend et al. (2015). All effect sizes across samples were in the
same (negative) direction, although they ranged in magnitude, with
greater deviations from the population estimate unsurprisingly tending
to derive from smaller samples. Our estimate indicates that 146 male
and 91 female observations would be required to find a statistically signif-
icant effect (two-tailed power analyses for α = .05 and β = .20). As such,
conclusions based on the meta-analysis that cumulated across studies are
likely to be more robust than conclusions drawn from under-powered
primary studies. Importantly, the negative association between FA and
vocal attractiveness was highly robust and manifest in every meta-
analysis of the data. The correlation was statistically significant regardless
of the inclusion of any individual study or sample, applied to FA measured
from the face as well as from the body, and to both short-term and overall
attractiveness, and was present whether we included effect sizes from
previously published work or only those from the present research.

Our findings thus provide important new evidence supporting the
salience of the human voice in conveying information on mate quality.
Furthermore, we are able to exclude several potential alternative inter-
pretations. First, it is possible that behavioral tendencies may spuriously
link vocal attractiveness with measurements of facial symmetry. For ex-
ample, shyness might result in both a wavering, less attractive voice, as
well as a tendency not to face the camera directly, which would affect
symmetry measurements. However, in taking 2D images, we were care-
ful to ensure that participants faced the camera fully, subsequently

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verifying this through visual inspection of the photographs and removal
of those in which head turn was apparent. More importantly, the rela-
tionship between vocal attractiveness and FA was in the predicted di-
rection, albeit non-significantly in Study 3, when FA was measured
from 3D imagery, which is not influenced by the spatial orientation of
the head. A pertinent question is how developmental stability can be
indexed by FA measured from both 2D and 3D images if the two types
of measurements were not correlated in Study 3. It is possible that,
while developmental instability influences FA in both two and three di-
mensions, it does so differently such that the relationship between the
two is not necessarily positive. For instance, increased asymmetry in
the coronal plane, which is measured in 2D images, might be related
to decreased asymmetry in the sagittal plane, which is measured in
3D but not 2D images. That FA from both types of image reliability

indexes developmental stability accords with results of Tigue, Pisanski,
O’Connor, Fraccaro, and Feinberg (2012), who found that, while 3D im-
ages elicited more favorable assessments of attractiveness than did 2D
images, attractiveness ratings from the two types of stimuli were none-
theless correlated. This notwithstanding, we encourage future re-
searchers to employ 3D imagery to capture appreciably more spatial
information and to characterize more precisely subtle shape variation
present in facial morphology.

Second, it is possible that developmental instability only indirectly
influences vocal attractiveness by increasing confidence associated
with having a symmetrical, attractive face. However, this does not ap-
pear to be the case. In Study 1, we were able to statistically control for
facial attractiveness and found that the relationships between facial FA
and vocal attractiveness remained undiminished in both sexes after
doing so. Moreover, these findings indicate that voices and faces provide
partly unique information about mate quality, in that voices provided
information about developmental instability that was not encompassed
by facial attractiveness. Vocal and facial attractiveness correlated mod-
erately in Study 1 women, suggesting that women’s voices and faces
provide partly redundant information about mate quality. Previous re-
search has shown similar levels of agreement across these domains in
women (e.g., Abend et al., 2015; Collins & Missing, 2003; Feinberg
et al., 2005; Lander, 2008; Saxton et al., 2009; Wheatley et al., 2014),
and auditory and visual cues considered in isolation may be less

Fig. 4. Correlations and 95% confidence intervals for relationships between vocal attractiveness and facial FA. Primary meta-analytic results are shown in panels (a) for men and (b) for
women. Overall effects refer to results of random effect models. In panel (a), superscripts 1 and 2 denote results using 2D and 3D facial FA, respectively, from Study 3. In panel (b),
superscripts 1 and 2 denote results using limb FA and 3D facial FA, respectively, from Abend et al. (2015). Results of “omit-one-study” analyses are illustrated for (c) men and
(d) women, with FA in Study 3 and Abend et al. (2015) computed from 3D facial images.

Table 2
“Omit-one-study” analysis of FA and vocal attractiveness in men using 3D facial FA from
Study 3.

Study omitted r Lower limit Upper limit z p n

Study 1 −0.25 −0.40 −0.07 −2.75 0.006 130
Study 2 −0.24 −0.36 −0.11 −3.57 b0.001 226
Study 3 (3D facial FA) −0.24 −0.37 −0.11 −3.47 0.001 202
Hughes et al. (2002) −0.20 −0.33 −0.06 −2.77 0.006 204

Table 3
Meta-analysis of FA and vocal attractiveness in women.

Study name r Lower limit Upper limit z p n

Study 1 −0.24 −0.36 −0.10 −3.40 0.001 201
Study 2 −0.08 −0.39 0.24 −0.48 0.630 39
Abend et al. (2015; limb FA) −0.44 −0.66 −0.16 −2.98 0.003 42
Abend et al. (2015; 3D facial FA) −0.45 −0.66 −0.16 −2.99 0.003 42
Hughes et al. (2002) −0.41 −0.62 −0.13 −2.84 0.005 46
Overall1 −0.28 −0.41 −0.14 −3.87 b0.001 328
Overall2 −0.29 −0.42 −0.14 −3.86 b0.001 328
1 Using limb FA from Abend et al. (2015).
2 Using 3D facial FA from Abend et al. (2015).

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informative about mate quality than are stimuli combining these and
other traits (Campanella & Belin, 2007). In Study 1 men, vocal and facial
attractiveness did not correlate significantly. Although ours is the larg-
est study of which we are aware to test this correlation in men, our re-
sults corroborate most (e.g., Lander, 2008; Saxton et al., 2009; Wells,
Baguley, Sergeant, & Dunn, 2013), but not all (Saxton, Caryl, & Roberts,
2006; Skrinda et al., 2014), previous studies exploring this correlation.
Further research is required to determine the extent to which men’s fa-
cial and vocal attractiveness correlate, which components of mate qual-
ity may underlie any such correlation, and how these relationships may
vary across samples.

Finally, our results provide further evidence that human preferences
for symmetrical mates are not merely consequences of symmetrical
stimuli being more easily processed by the visual system (Enquist &
Johnstone, 1997). It is now becoming clear that people prefer not only
the faces (Jones, DeBruine, & Little, 2007; Little & Jones, 2003; Scheib
et al., 1999) and odors (Gangestad & Thornhill, 1998; Thornhill &
Gangestad, 1999b) but also the voices (Abend et al., 2015; Hughes
et al., 2002, 2008; present study) of mates with low levels of FA, even
when symmetry cannot be directly assessed visually.

The present paper thus adds to an efflorescing literature exploring
relationships between FA and attractiveness in humans. While some
contributions to this literature have questioned relationships between
FA and fitness-relevant outcomes (e.g., Clarke, 1998), a recent meta-
analysis of 293 estimates of the relationship between FA and measures
of health and quality taken from 94 studies on humans suggests a robust
correlation (Van Dongen & Gangestad, 2011).

Much previous research on FA and mate preferences has focused on
putative genetic benefits provided by symmetrical partners. Heritability
estimates for FA vary across species and across traits within species
(Leamy, 1997; Møller & Thornhill, 1997; Polak et al., 2003), but it is pos-
sible that individuals derive non-genetic benefits from symmetrical
mates, as well (Gangestad & Thornhill, 2013). For example, the meta-
analysis of Van Dongen and Gangestad (2011) estimated correlations
of 0.11 and 0.20 between FA and reduced intelligence and schizophre-
nia/schizotypy, respectively, both of which may have influenced invest-
ment capacity ancestrally. However, other research suggests that
symmetrical males may allocate relatively greater reproductive effort
to mating than to investment in mates or offspring (e.g., Gangestad &
Simpson, 2000; Gangestad & Thornhill, 1997, 2013), suggesting that ge-
netic benefits to offspring may have driven the evolution of preferences
for low-FA partners, at least in women.

7. Future directions

Future studies should investigate how developmental instability re-
lates to the anatomy of the vocal tract, and which vocal anatomical char-
acteristics and acoustic features of the voice mediate relationships
between somatic asymmetry and vocal attractiveness (see, for example,
Hughes et al., 2008). While such an exploration is beyond the scope of
the present paper, some informed speculation is possible. One possibil-
ity is that developmental instability affects vocal attractiveness by
influencing the symmetry of the vocal apparatus, for example the
vocal folds or the shape of the supralaryngeal vocal tract. Anatomical
asymmetries or asymmetric tension between the two vocal folds will
cause the vocal folds to vibrate differently or at different rates, produc-
ing perturbations in periodicity as well as incomplete closure and

leading to perceptible vocal changes, such as hoarseness, breathiness,
and roughness (Eysholdt et al., 2003). Facial asymmetries, such as
those caused by cleft lip and palate, can affect aspects of articulation
and resonance, as well. Because vocalization is integral to human com-
munication, there may have been strong selection for the vocal appara-
tus to be buffered against such perturbations, but research suggests
pronounced asymmetries in the human vocal folds (e.g., Hirano,
Yukizane, Kurita, & Hibi, 1989). This is consistent with the idea that
traits shaped by intersexual selection tend to display greater FA than
do non-sexually selected traits (Møller & Hoglund, 1991). Developmen-
tal instability may influence other aspects of the size and shape of vocal
structures (e.g., the length, thickness, or density of the vocal folds), as
well as neurological characteristics that could affect vocal behavior,
such as tension on the vocal folds or the positioning of the larynx in
the vocal tract, which would influence vocal pitch and timbre, respec-
tively. Functional magnetic resonance imaging of vocal anatomy during
phonation, precise acoustic measurements, and large samples may be
particularly useful in exploring these possibilities.

Funding

This work was supported by the National Institutes of Mental Health
[grant number T32 MH70343-05], the National Science Foundation
[REG grant to C.L.A. and GRFP grant to J.R.W.], and The Pennsylvania
State University and Harvard University.

Supplementary materials

Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.evolhumbehav.2016.10.008.

Acknowledgments

We thank Arslan Zaidi for assistance designing Fig. 3, Tobias
Kordsmeyer for insights on measuring facial FA, Matt Winn for input
on the Discussion, and Lisa DeBruine and two anonymous reviewers
for comments on previous drafts of this manuscript.

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http://dx.doi.org/10.1016/j.evolhumbehav.2016.10.008

Are there vocal cues to human developmental stability? Relationships between facial fluctuating asymmetry and voice attract…

1. Introduction
2. Study 1
2.1. Methods
2.1.1. Participants
2.1.2. Face measurement
2.1.3. Voice recording
2.1.4. Vocal and facial attractiveness ratings
2.2. Results
3. Study 2
3.1. Methods
3.1.1. Participants
3.1.2. Face measurement
3.1.3. Vocal attractiveness ratings
3.2. Results
4. Study 3
4.1. Methods
4.1.1. Participants
4.1.2. Face measurement
4.1.3. Vocal attractiveness ratings
4.2. Results
5. Meta-analysis
6. Discussion
7. Future directions
Funding
Supplementary materials
Acknowledgments
References

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