Male facial attractiveness: evidence for hormone-mediated adaptive design
Introduction
In a fraction of a second, the brain of a human male or female can ascertain the physical attractiveness of another person's face Johnston & Oliver-Rodriguez, 1997, Oliver-Rodriguez et al., 1999. This remarkable feat appears to depend on the delicate interplay between physical markers on the face, perhaps fitness indicators (Miller, 2000), and exquisitely sensitive brains that generate feelings, perhaps fitness monitors (Johnston, 1999), in response to such signals. Support for this model comes from studies that have used various image-processing techniques to systematically manipulate the features and proportions of female facial images and observe the behavioral and/or emotional responses of men and women exposed to such images. The current experiment attempts to evaluate and refine this model by examining how the attractiveness of male and female faces varies with both their displayed hormone markers, and the hormonal state of female viewers, as indicated by their menstrual phase.
Early studies suggested that the most attractive female face was the average face in a population Langlois & Roggman, 1990, Langlois et al., 1994, Langlois et al., 1991. Several experimenters, however, have concluded that the image-processing technique used in these studies may be flawed, and proposed that although the average face is attractive, it is not the most attractive face in a population Alley & Cunningham, 1991, Johnston, 2000. Strong support for the “non-average” hypothesis comes from Perrett, May, and Yoshikawa (1994), who demonstrated that an average face made by combining random faces is judged to be less attractive than the average of attractive faces drawn from the same sample of faces. Indeed, there is now substantial evidence indicating that attractive female faces are not average, but differ from the average in a systematic manner. More specifically, they possess a shorter, narrower, lower jaw, fuller lips, and larger eyes than an average face Cunningham et al., 1995, Johnston & Franklin, 1993, Perrett et al., 1994. These specific markers have been shown to be effective across cultures Cunningham et al., 1995, Perrett et al., 1994, and electrophysiological studies have revealed that they elicit emotional responses in male, but not female, viewers of female faces (Johnston & Oliver-Rodriguez, 1997). Because pubertal bone growth (brow ridges and lower jaw) is stimulated by androgens (Tanner, 1978), and lip fullness parallels estrogen-dependent fat deposits elsewhere on the female body (Farkas, 1981), Johnston and Franklin (1993) have hypothesized that an attractive female face may be displaying hormone markers (high estrogen/low androgen) that serve as reliable indicators of fecundity.
In contrast to the research on female facial attractiveness, studies examining the importance of hormone markers on male faces have produced apparently incompatible results. For example, although a number of experimenters have demonstrated that women favor a “masculinized” male face possessing a large jaw and prominent brow ridges and cheekbones Grammer & Thornhill, 1994, Scheib et al., 1999, other studies have reported that both British and Japanese females prefer a more “feminized” male face with a shorter-than-average lower jaw Penton-Voak et al., 1999, Perrett et al., 1998. Still others have found that a mixture of mature features (large lower jaw, prominent cheekbones, and thick eyebrows) and neotenous features (large eyes and small nose) is the most desirable configuration of male faces (Cunningham, Barbee, & Pike, 1990). Some of these inconsistencies across studies may result from the different procedures used to generate the male facial stimuli. For example, both studies reporting a preference for feminized male faces used a caricaturing technique to create their masculinized and feminized facial images. Producing a masculinized male face using this procedure involves an algorithm that further exaggerates the differences between an average female and an average male face. This mathematical procedure is based on the assumption that extreme masculine and feminine faces can be produced by a linear extrapolation of the differences between these average faces. However, given that the majority of consistent male–female differences are the result of bone growth, which depends on a complex interaction between androgens, estrogens, and growth hormone, this linear growth assumption may not be valid Grumbach, 2000, Tanner, 1978.
A second consequence of caricaturing is its potential effects on modifying facial symmetry. Average male and female faces are inevitably low in fluctuating asymmetries (traits that differ among individuals but are symmetrical at a population level), so morphs generated between such faces (feminized males and masculinized females) will invariably be quite symmetrical. It is also inevitable, however, that any small deviation from perfect symmetry that exists in either the average male or average female face will be substantially exaggerated in caricatured faces (masculinized males and feminized females). Caricaturing, therefore, is likely to generate feminized male facial images that are more symmetrical than masculinized male images. Because facial symmetry may influence attractiveness Gangestad et al., 1994, Grammer & Thornhill, 1994, Mealey et al., 1999, Perrett et al., 1999), any observed preference for a feminized or a masculinized male face may be confounded by differences in the symmetry of these faces. The current study attempts to improve on this research methodology by examining preferences for male (and female) faces that have been masculinized (or feminized) on the basis of perceived masculinity (or femininity), rather than employing the caricaturing technique.
Low fluctuating asymmetry (FA) is thought to reflect an ability to resist the harmful effects of mutations, parasites, and/or toxins during development (Møller & Swaddle, 1997). Because such resistance is partially heritable (Møller & Thornhill, 1997), there may be important fitness benefits for females who exhibit a preference for such mates. A variety of experimental findings support this hypothesis. Across species, symmetrical males have significantly greater mating success (Møller & Thornhill, 1998), and symmetrical males have been shown to be more desirable and have more sexual opportunities than asymmetrical men Gangestad & Thornhill, 1997, Thornhill & Gangestad, 1994. In populations lacking widespread contraception and modern medicine, these enhanced sexual opportunities can be translated into more offspring and better health (Waynforth, 1998). However, despite the correlation between symmetry and attractiveness, it appears that human females may not use or even perceive fluctuating asymmetries when judging the attractiveness of male faces (Scheib et al., 1999).
Scheib et al. (1999) found that the measured FA of male faces was not only correlated with their attractive ratings, but was also correlated with the attractiveness ratings of a right or left half-face, circumstances where all direct cues to bilateral symmetry are absent. Clearly, their experimental participants did not require explicit cues of bilateral symmetry to make accurate judgments of attractiveness. It appears that the salient cues for attractiveness are apparent on each half of a male's face and their presence is correlated with computed FA, but not perceived symmetry. An examination of the attractive faces led the authors to conclude that the specific features correlated with attractiveness and FA were a longer lower jaw and prominent cheekbones. Keating (1985) also found that the shape of the lower jaw was an important attribute of male facial attractiveness. Using an Identi–Kit methodology, she examined the effects of eye size, lip fullness, brow thickness, and jaw shape on both dominance and attractiveness ratings. As she had predicted, the combination of masculine features (square jaw, narrow eyes, thick eyebrows, and thin lips) enhanced the dominance ratings of male faces, but only a subset of these attributes (square jaw and thin lips) resulted in significantly higher attractiveness ratings. It appears that some high testosterone markers (square jaw) and low estrogen markers (thin lips) influence both the dominance and attractiveness of male faces, but dominance and attractiveness are not identical attributes. Given these prior results, the current study attempts to clarify the role of hormone markers in the perception of dominance and attractiveness, in both male and female facial images.
Some of the discrepancy in findings among male attractiveness studies may be a consequence of differences in the participant populations. One potential source of variance is the hormonal status of female participants. Penton-Voak et al. (1999) have shown that females' preferences for male faces changed as a function of the viewer's menstrual phase at the time of testing. Specifically, females tested during the 9 days prior to ovulation (high conception risk group) preferred a less feminized male face than females tested outside of this window (low conception risk group). In a more recent study, using 139 participants who responded to a magazine survey, Penton-Voak and Perrett (2000) reported that women in the high conception risk group were significantly more likely to prefer a masculine face than those in the low conception risk group. The authors interpret their findings as evidence for a conditional mate choice strategy whereby females in the high conception risk group are exhibiting a preference for male facial cues that signal adaptive heritable genetic characteristics, such as immunocompetence. However, these menstrual studies have not shown that the observed change in preference over the menstrual cycle is specific to attractive male faces. Do females' preferences for female faces also change? If so, then the observed effect may simply reflect a general change in mood over the menstrual cycle (Dalton, 1982), rather than a specific adaptation. To explore the generality of the menstrual effect, the current study examines how a variety of different facial preferences (attractive male, attractive female, dominant male, dominant female, etc.) are, or are not, modified by the hormonal state of female viewers. An effect on male facial attractiveness alone would provide strong evidence for complex adaptive design (Williams, 1966). This is the hypothesis that is evaluated in the current study.
Section snippets
Participants
The participants were 42 female volunteers between 18 and 35 years of age (M=22). These women were recruited from the undergraduate population at New Mexico State University in Las Cruces (USA) and the Ludwig–Boltzmann Institute for Urban Ethology in Vienna (Austria) (n=40 and n=2, respectively). Participation in the experiment was limited to volunteers who stated that they were (1) heterosexual, (2) not currently pregnant or breastfeeding a child, and (3) not currently taking any steroid
Results
For each target face, the participants' selections made during the follicular phase (pre-ovulation) were compared to those made during the luteal phase (post-ovulatory) of their menstrual cycle. A series of paired t tests revealed that that there was no significant difference in any face choice as a function of whether the woman was in her follicular or luteal menstrual phase on the day of testing. A second analysis examined only those participants who had been tested during the time of highest
Discussion
The morph movie used in the current experiment offers a sensitive tool for examining facial preferences by providing participants with a choice among male and female facial images that have been masculinized and feminized to varying degrees. Unlike caricaturing techniques that masculinize or feminize faces on the basis of questionable mathematical assumptions, the morph movie systematically changes the major facial features and proportions that are perceived to characterize human maleness and
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