Male facial appearance signals physical strength to women

AMERICAN JOURNAL OF HUMAN BIOLOGY 19:82–87 (2007)

Original Research Article

Male Facial Appearance Signals Physical Strength to Women BERNHARD FINK,1* NICK NEAVE,2 AND HANNA SEYDEL1 Department of Sociobiology/Anthropology, University of Go¨ttingen, D-37073 Go¨ttingen, Germany 2 Human Cognitive Neuroscience Unit, School of Psychology and Sport Sciences, Northumbria University, Newcastle upon Tyne NE1 8ST, UK

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ABSTRACT Previous studies showed that male faces with extreme features that are likely to be associated with testosterone (T) are perceived as dominant and masculine. Women were reported to prefer masculinized male faces, as they may consider T markers to be an ‘‘honest’’ indication of good health. However, it is also likely that female preferences for certain male faces arise from the fact that dominant- and masculine-looking males are signaling characteristics which may be beneficial in intrasexual conflict, and thereby also indicate potential achievers of high status, an important factor in female mate selection. Although numerous studies were built on this assumption, nothing is known about the relationship between perceived facial dominance and physical strength in men. We measured hand-grip strength, as a measure of overall physical strength, in a sample of 32 male students, and recorded age, body weight, and height. Seventy-nine women rated facial images of these men for dominance, masculinity, and attractiveness. After controlling for age and body weight, hand-grip strength was found to correlate significantly positively with all three measures. The present data thus support the supposition that a male’s physical strength is also signaled via facial characteristics of dominance and masculinity, which are considered attractive by women. Am. J. Hum. Biol. 19:82–87, 2007. ' 2006 Wiley-Liss, Inc.

Among diverse animal species, males are often selected to compete among each other for access to females, this being known as intrasexual selection. Such selection can be expected to lead to the evolution of sex-dependent differences in strength and speed. In humans, there is a robust sex difference in strength, such that men are usually stronger than women, even after adjustment for weight and/or body mass (Miller et al., 1993; Musselman and Brouwer, 2005). The gonadal sex-steroid testosterone (T) is thought to be an important determinant of this sex difference, because it has potent anabolic effects on the musculoskeletal system, including an increase in lean body mass, a dose-related hypertrophy of muscle fibers, and an increase in muscle strength (Evans, 2004). For example, in a placebo-controlled, double-blind, randomized trial, Bhasin et al. (1996) noted that supraphysiological doses of T led to significant increases in arm and leg muscle size and strength, and an increase in fat-free body mass. In particular, androgenic-anabolic steroids (AAS; synthetic derivatives of T) are known to exert strong effects on the human body that may be beneficial in athletic per-

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formance (Hartgens and Kuipers, 2004). However, the results of intervention studies are mixed, so that some studies reported no effect of T on male strength (e.g., Snyder et al., 1999), while others found significant increases in strength following T supplementation (e.g., Wang et al., 2000; Sih et al., 1997). Hansen et al. (1999) reported that the development of strength in young (peripubescent) male elite and nonelite soccer players was significantly related to changes in serum T concentrations. In elderly men not receiving testosterone supplementation, positive correlations were reported between bioavailable T and physical performance and strength (Schaap et al., 2005). One possibility is that T may influence strength in men at two different stages in development, i.e., T may cause organizational *Correspondence to: Dr. Bernhard Fink, Department of Sociobiology and Anthropology, Institute for Zoology and Anthropology, University of Go¨ttingen, Berliner Strasse 28, D-37073 Go¨ttingen, Germany. E-mail: bernhard.fi[email protected] Received 2 July 2006; Revision received 4 August 2006; Accepted 8 August 2006 Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/ajhb.20583

MALE FACIAL APPEARANCE SIGNALS STRENGTH TO WOMEN

effects during fetal development which are permanent, but T may also produce shortterm activational effects on strength in adults. Evidence for the former comes from Fink et al. (in press), who reported that digit ratio (2D:4D), a putative marker for prenatal T exposure (Manning, 2002a; McIntyre, 2005), correlated with measures of hand-grip strength in men. The lower (more masculine) ratio was associated with stronger grip strength. With regard to facial appearance, Fink et al. (2005) found that some facial characteristics, which are considered typically ‘‘male’’ (e.g., large jaw, and prominent brow ridges and cheekbones), correspond to low (masculinized) 2D: 4D ratios, whereas some typically ‘‘female’’ features (e.g., small mouth and full lips) correspond to high (feminized) 2D:4D ratios. Although evidence is accumulating from previous research for links between facial characteristics and sex steroids, it is still unknown how and at what stages of ontogeny they affect changes in facial shape. Furthermore, most studies investigated associations between T and male facial characteristics (Penton-Voak and Chen, 2004; Schaefer et al., 2005), while data on the relationship between female facial characteristics and estrogen (E) remain speculative, since they were studied only at the perceptual level. Neave et al. (2003) reported that low 2D:4D ratios in males were significantly related to female perceptions of male facial dominance and masculinity, but not to attractiveness, based on facial photographs. This is consistent with the supposition by Swaddle and Reirson (2002) that features developed under the influence of T do not directly account for attractiveness, but rather for male dominance and masculinity. However, Neave et al. (2003) did not find substantial correlations between circulating levels of T and dominance, masculinity, or attractiveness, leading them to speculate that any association between these features operates only at an early stage in life. However, Penton-Voak and Chen (2004) investigated the relationship between circulating T levels and facial appearance by constructing composite faces from pictures of males with high and low T. Females rated these composites for ‘‘masculinity’’ and ‘‘attractiveness,’’ and the ‘‘high-T’’ composites were judged as being more masculine. Across a wide variety of species, behaviors intended to achieve, maintain, and enhance status are observed primarily among high-T individuals (Josephs et al., 2003; Kraus et al.,

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1999). If perceived facial dominance indeed relates to the potential for high status in male dominance hierarchies, then in the human resource-based mating system (Buss, 1989; Feingold, 1992; Kenrick and Keefe, 1992), it may signal a fitness-relevant quality (Dewsbury, 1982; Ellis, 1995; Mueller, 1993). It is particularly important for humans to distinguish between dominant behavior, which aims at achieving and maintaining high status and greater control of resources over a conspecific, and aggressive behavior, which aims at inflicting physical injury on a conspecific. Mazur and Booth (1998) concluded that high levels of T were linked with dominance and competitiveness in human males. Specifically, they suggested that T rises in the face of a challenge and activates behaviors intended to dominate and enhance status. In the present study, we assessed possible relationships between hand-grip strength and perceptions of male facial masculinity, dominance, and attractiveness. We chose hand-grip strength as a potential marker for dominance because it was found to correlate with the strength of other muscle groups and shows strong sex differences, which are robust across ethnic groups (Nicolay and Walker, 2005; Rantanen et al., 1994, 2000; Xiao et al., 2005). Fink et al. (in press) found that prenatal T (as measured via 2D:4D ratio) may have an early organizing effect on physical strength in men, and low 2D:4D ratios (meaning high prenatal T) were reported to correspond to ‘‘male-like’’ facial characteristics. Moreover, Neave et al. (2003) demonstrated that men with low 2D:4D ratios are also perceived as more dominant and masculine. We therefore hypothesized that physically stronger men (i.e., those with higher hand-grip strength) should also be perceived as more dominant and masculine, traits which are subsequently perceived as attractive by women. Such a finding would have implications for human mating decisions, in that women have a preference for male facial characteristics (likely ‘‘organized’’ early in life) that signal physical strength and thereby status. MATERIALS AND METHODS Participants Our sample consisted of 32 males aged between 18–32 years (mean age, 23.55 years; SD, 3.47 years), recruited from the student population in Go¨ttingen, Germany. All participants were white Caucasians, and claimed to

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be heterosexual and right-handed. Moreover, none of the participants reported considerable health problems that may have affected strength measurement. Ethical approval was granted from the local ethics committee, and informed written consent was obtained from participants. Data recording Hand-grip strength was measured in kilograms force (kgf), using a dynamometer with the handle adjusted to the second position (Jamar dynamometer, Sammons Preston). Participants were asked to perform a maximum force trial for each hand. If an error was noted, the test was repeated after a 5-min rest. Studies reported equivocal results for comparison of dominant and nondominant hand strengths. However, such a difference (if present) seems to be fairly low in nonclinical samples (Armstrong and Oldham, 1999). In the present study, we decided to calculate the arithmetical mean of right- and left-hand measures (rather than right-left differences) for statistical analyses of face perception and strength relations. We noted participant age (years), body weight (kg), and height (cm), as these variables were considered potential correlates of grip strength. Color digital images of each participant’s face were taken at high resolution (1,600
1,200 pixels, uncompressed TIFF file format) under standardized light conditions in frontal view. Participants were asked to remove any facial adornment, look directly into the camera, and present a neutral facial expression.

our data were normally distributed. Cronbach’s alpha coefficient was calculated for determining the reliability between raters’ judgments of attributes. Pearson correlation (r) was used for assessing the relationship between hand-grip strength, age, and physical measurements, and partial correlations (rp) were used (if applicable) for controlling potential confounds. Our prediction was that perceived facial dominance, masculinity, and attractiveness would be positively correlated with hand-grip strength, so we applied onetailed tests on these relationships. RESULTS The K-S-test indicted that all variables were normally distributed (age: Z ¼ 0.837, P ¼ 0.486; weight: Z ¼ 1.167, P ¼ 0.131; height: Z ¼ 0.517, P ¼ 0.952; hand-grip strength: Z ¼ 0.618, P ¼ 0.839; dominance: Z ¼ 0.533, P ¼ 0.939; masculinity: Z ¼ 0.717, P ¼ 0.683; attractiveness: Z ¼ 0.567, P ¼ 0.905). All face ratings had a Cronbach’s alpha >0.9, indicating high consistency of judgment between raters (dominance, a ¼ 0.96; masculinity, a ¼ 0.96; attractiveness, a ¼ 0.96). Descriptive statistics Body height of male participants ranged from 160–189 cm (mean, 178.56; SD, 6.04), weight from 56.8–115.8 kg (mean, 76.43; SD, 14.80), and hand-grip strength (arithmetical mean of right- and left-hand measures) from 33.50–63.50 kgf (mean, 50.83; SD, 7.83). Correlations

Face ratings Seventy-nine female raters, mainly college students from the local university (Go¨ttingen) aged 19–32 years (mean age, 23.33 years; SD, 2.73 years), rated each face for perceived dominance, masculinity, and attractiveness. Faces were randomly presented via computer, and remained on-screen until an appropriate response was made using a seven-point Likerttype scale (1 ¼ extremely subordinate/feminine/unattractive, 7 ¼ extremely dominant/ masculine/attractive). At the end of the session, the female raters were asked if they recognized any of the faces (none did). Statistical analysis A one-sample Kolmogorov-Smirnov (K-S) goodness-of-fit test was used to test whether

American Journal of Human Biology DOI 10.1002/ajhb

There were no significant correlations between physical measures and mean hand-grip strength (age: r ¼ 0.031, P ¼ 0.866; weight: r ¼ 0.224, P ¼ 0.277; height: r ¼ 0.135, P ¼ 0.460). In keeping with previous reports (e.g., Neave et al., 2003), intercorrelations between perceived dominance, masculinity, and attractiveness were high and significant (all r > 0.62, all P < 0.01). Ratings of perceived dominance and masculinity were significantly positively correlated with age (r ¼ 0.414, P ¼ 0.018), and a significant negative correlation was detected between perceived attractiveness and weight (r ¼ 0.477, P ¼ 0.006). We therefore decided to use partial correlations (rp, controlling for the influence of age and weight) for assessment of associations between hand-grip strength and perceived attributes.

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When controlling for these potential confounds, and in accordance with our hypothesis, significant positive correlations were found between mean hand-grip strength, perceived masculinity (rp ¼ 0.370, P ¼ 0.022), dominance (rp ¼ 0.371, P ¼ 0.022), and attractiveness (rp ¼ 0.336, P ¼ 0.035) (Fig. 1). DISCUSSION The aim of this study was to ascertain possible relationships between hand-grip strength and women’s perceptions of males via their facial appearance. Previous studies demonstrated that male faces displaying secondary markers for T are perceived as more dominant and masculine (but not necessarily attractive) by female raters (e.g., Neave et al., 2003; Penton-Voak and Chen, 2004; Swaddle and Reierson, 2002). Women may prefer masculinized male faces, as they may consider T markers to be an ‘‘honest’’ indication of good health/immunocompetence (Swaddle and Reirson, 2002). However, such preferences may also indicate potential success in intramale dominance encounters, and thereby a male’s potential to achieve and maintain high status. Our data support the hypothesis that male faces may signal aspects of masculinity and dominance (in this case, physical strength), as the faces of men who were physically stronger (i.e., those with higher hand-grip strength) were also perceived as significantly more dominant and masculine by female raters. In addition, we found a significant positive relationship between grip strength and ratings of attractiveness. Such associations remained when we controlled for age and weight. Swaddle and Reirson (2002) proposed that male facial features developed under the influence of T increase perceived dominance but not attractiveness in human males. Neave et al. (2003) found that low 2D:4D ratios (indicating higher prenatal T) in males were significantly correlated with female perceptions of male dominance and masculinity, but also not with attractiveness. These studies suggest that features developed under the influence of T do not directly account for attractiveness but rather for male dominance and masculinity, which thereby indicate status. Our data revealed positive correlations of all three attrib-

Fig. 1. Scatterplots with regression lines visualizing associations between hand-grip strength and women’s ratings of perceived male facial dominance (a), masculinity (b), and attractiveness (c).

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utes with hand-grip strength, indicating that women are able to evaluate certain facial characteristics that signal physical strength. It is known that androgens have an anabolic effect on the musculoskeletal system, resulting in an increase of muscle strength (Bhasin et al., 1996; Evans, 2004; Herbst and Bhasin, 2004), and generally affect human body composition (Bhasin, 2003), though most of these studies were conducted in clinical populations, and thus may not apply to ‘‘normal’’ males. Recent data on associations of digit ratio with physical performance and strength suggest that that prenatal T may have an early organizing effect on physical abilities in men (Fink et al., in press). 2D:4D was shown to correlate with male performance in soccer (Manning and Taylor, 2001), skiing (Manning, 2002b), running (Manning, 2002a), gym-based physical fitness (Hoenekopp et al., 2006), and also hand-grip strength (Fink et al., in press). Manning and Taylor (2001) argued that sports and athletic disciplines can be seen as proxies for male competition, and in such domains, higher T is supposed to be advantageous for males. Given that previous studies found significant associations between 2D:4D and face shape (Fink et al., 2005), and also 2D:4D and hand-grip strength (Fink et al., in press), we thus argue for an organizational effect of prenatal T, which is likely to affect both male physical strength and facial appearance. Our data suggest that dominant- and masculinelooking male faces signal physical strength to women, which is also rated attractive by women. It remains speculative as to which facial features actually signal physical strength, and whether these features are correlated with T. However, on the basis of the available studies, it it is likely that prenatal T plays a central role in moderating the organization of certain facial characteristics and body composition in men, which is subsequently relevant for women’s mating decisions (see also Mueller and Mazur, 1997). ACKNOWLEDGMENTS We thank John T. Manning and two anonymous referees for providing valuable comments on an earlier version of the manuscript. LITERATURE CITED Armstrong CA, Oldham JA. 1999. A comparison of dominant and non-dominant hand strengths. J Hand Surg [Br] 24:421–425.

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