FrederickHaselton-2007-PSPB-MuscularityFitnessIndicator.pdf

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Why Is Muscularity Sexy? Tests of the

Fitness Indicator Hypothesis

David A. Frederick

Martie G. Haselton

University of California, Los Angeles

Evolutionary scientists propose that exaggerated sec-

ondary sexual characteristics are cues of genes that

increase offspring viability or reproductive success. In six

studies the hypothesis that muscularity is one such cue is

tested. As predicted, women rate muscular men as sexier,

more physically dominant and volatile, and less commit-

ted to their mates than nonmuscular men. Consistent

with the inverted-U hypothesis of masculine traits, men

with moderate muscularity are rated most attractive.

Consistent with past research on fitness cues, across two

measures, women indicate that their most recent short-

term sex partners were more muscular than their other

sex partners (ds

.36, .47). Across three studies, when

controlling for other characteristics (e.g., body fat), mus-

cular men rate their bodies as sexier to women (partial

.49-.62) and report more lifetime sex partners (partial

.20-.27), short-term partners (partial

.25-.28),

and more affairs with mated women (partial

.28).

Keywords:

body image; evolutionary psychology; mate pref-

erences; muscularity; sexual selection

commitment, and less so by physical attractiveness (e.g.,

Buss & Schmitt, 1993; Pawlowski & Dunbar, 1999).

In the literature on nonhuman animals, however, there

is much research on male attractiveness and far less on

female attractiveness (e.g., Alcock, 2005; Andersson,

1994). Indeed, across species, females tend to be the sex

that invests more in offspring and therefore they are more

selective in choosing mates (Trivers, 1972). Females

appear to value male attractiveness because it is a cue of

genes that confer fitness benefits to offspring through

increased viability or reproductive success (e.g., Kokko,

Brooks, Jennions, & Morley, 2003; Moller, 1997). If spe-

cific cues index heritable quality, females should come to

value them in mates, and they should exercise this prefer-

ence in mate selection.

Might male attractiveness also play heavily in the mat-

ing decisions of human females? Recent studies suggest

that women discriminate between men on the basis of

hypothesized fitness cues, including facial masculinity

Authors’ Note:

The authors are grateful to the University of

California, Los Angeles (UCLA) Graduate Division; the FPR-UCLA

Center for Culture, Brain, and Development; and the Departments of

Psychology and Communication Studies for providing financial sup-

port to the first author. For additional papers on body image and

body type preferences, please refer to the Web site of the first author

(http://dfred.bol.ucla.edu). We would like to thank Clark Barrett,

Daniel Fessler, Gordon Gallup, Andrew Galperin, Kristina Grigorian,

Kelsey Laird, Andrea Niles, Joshua Poore, Taylor Rhoades, Letitia

Anne Peplau, Steven Platek, Leila Sadeghi-Azar, and the UCLA

Experimental Biological Anthropology Lab for their helpful com-

ments on this manuscript and project. Correspondence should be

addressed to David A. Frederick, 1285 Franz Hall, Department of

Psychology, Third Floor Mailroom, University of California, Los

Angeles, CA 90095-1563; e-mail: enderflies1@aol.com.

PSPB,

Vol. 33 No. 8, August 2007 1167-1183

DOI: 10.1177/0146167207303022

sychological research on physical attractiveness has

largely focused on the importance of female attrac-

tiveness to men and the aspects of the female body men

find most desirable in mates (e.g., Fink, Grammer, &

Thornhill, 2001; Scutt, Manning, Whitehouse, Leinster,

& Massey, 1997; Singh, 1993; Symons, 1995).

Evolutionary psychologists have generally concurred that

men possess strong preferences for female beauty because

attractive attributes are cues of fertility, and fertility

varies considerably between women and within individ-

ual women over time (for a review, see Sugiyama, 2005).

In contrast, researchers have contended that men’s desir-

ability as mates is determined by earning potential and

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PERSONALITY AND SOCIAL PSYCHOLOGY BULLETIN

associated with effort allocated to mating (McIntyre et al.,

2006) as well as with greater size and muscle mass (Bhasin,

2003). Effort allocated to developing and maintaining

these attributes can reduce budget of effort available for

maintaining other attributes (e.g., immunocompetence,

somatic upkeep) and can increase other energy demands

(e.g., increased metabolism; Buchanan, Evans, Goldsmith,

Bryant, & Rowe, 2001). This view suggests that there is a

wider array of costs beyond simply immunosuppression

that causes these traits to be honest signals of quality.

In both the immunocompetence and the more gen-

eral cost models, however, the prediction is the same:

Traits produced by high levels of testosterone are cues

of heritable fitness or good condition because they indi-

cate that the male can afford to generate these costly

traits. Selection should have shaped a female preference

for these traits because, all else equal, males displaying

them sire more viable offspring.

(e.g., Johnston, Hagel, Franklin, Fink, & Grammer, 2001;

Penton-Voak & Perrett, 2000; Penton-Voak et al., 1999)

and body scents associated with symmetry (Gangestad &

Thornhill, 1998). We examined whether women find

muscular male bodies sexy and, if so, whether this prefer-

ence may also be a product of sexual selection.

SEXUAL SELECTION AND BODY MORPHOLOGY

Some traits are fitness cues because they demonstrate

that a male is in good condition. Life history theorists

think of organisms as entities that capture energy from

the environment and then convert it to survival and

reproduction-enhancing activities, including by develop-

ing metabolically expensive physical features that are

attractive to the opposite sex (for a review, see Kaplan &

Gangestad, 2005). Because of differences in genetic

makeup, combined with the challenges faced during

development, individuals differ in their ability to allocate

energy to generating costly traits that are attractive to the

other sex. Zahavi (1975) proposed that males who dis-

play traits that are costly to maintain (e.g., the peacock’s

tail) are attractive to females precisely because they are

costly and thus demonstrate that the male is in good

enough condition to produce them. Females who mate

with these males would pass on the attractive traits to

their offspring, increasing their viability or reproductive

success, or both.

THE COSTLY SIGNALING HYPOTHESIS AND

PREFERENCES FOR MUSCULARITY

We propose that the metabolic expense and levels of

testosterone necessary to build and sustain muscle mass

make muscularity a fitness cue. Numerous studies indi-

cate that increased muscle strength is associated with

naturally occurring levels of testosterone, as well as

with testosterone treatments in normal, hypogonadal,

adolescent, and older male patients (e.g., Storer et al.,

2003; Wang et al., 2000; for a review, see Bhasin,

2003). Thus, men who are more muscular are exposed

to greater levels of testosterone than other men.

For women to gain some genetic benefit by mating

with muscular men, however, muscularity must be her-

itable. Estimates of the heritability of traits associated

with muscularity indicate that extremity circumferences

(e.g., bicep circumference), static strength (e.g., how

much weight a person can hold in place), and explosive

strength (e.g., vertical jump) range from 20% to 80%

depending on the given trait (Loos et al., 1997; Thomis,

Beunen, Maes, et al., 1998; Thomis, Beunen, Van

Leemputte, et al., 1998; Thomis et al., 1997). One twin

study assessing gains in strength across a 10-week train-

ing period also found that the ability to add muscle

mass beyond one’s baseline degree of muscularity is her-

itable (Thomis, Beunen, Maes, et al., 1998). The finding

that there are underlying genetic differences related to

muscle mass indicates that muscularity, along with the

suite of traits correlated with it, can be passed on to off-

spring. This provides offspring with the advantage of

developing traits that are attractive to females, further

enhancing the women’s reproductive success in later

generations.

SEXUAL SELECTION AND

TESTOSTERONE-LINKED TRAITS

As an extension of Zahavi’s (1975) hypothesis,

Folstad and Karter (1992) introduced the immunocom-

petence signaling hypothesis. This hypothesis suggests

that secondary sexual characteristics are reliable indica-

tors of mate quality because the reproductive hormones

required for their development, including testosterone,

suppress the immune system (e.g., Peters, 2000;

Rantala, Vainikka, & Kortet, 2003). The expression of

testosterone-linked traits reveals that men are in good

enough condition to withstand the deleterious effects of

immunosuppression, and women who selected these

men as mates would have transmitted features associ-

ated with good condition to their offspring.

An alternative perspective suggests that testosterone-

linked traits are costly signals for reasons other than

immunocompetence (see Kaplan & Gangestad, 2005;

Kokko et al., 2003). In this view, fit males benefit more

than other males from devoting a greater share of their

energy budget to mating effort (competing for mates, dis-

playing attributes desired by mates). Higher testosterone is

Frederick, Haselton / MUSCULARITY AS A FITNESS INDICATOR

STRATEGIC PLURALISM AND WOMEN’S

PREFERENCE FOR FITNESS INDICATORS

Effects of Mating Context

The perspective described previously predicts that

women should find muscular men sexually desirable.

However, if muscular men are sexually desirable but

less likely to commit to their partners, women’s attrac-

tion to muscularity should differ depending on mating

context. According to strategic pluralism theory

(Gangestad & Simpson, 2000), men have evolved to

pursue reproductive strategies that are contingent on their

value on the mating market. More attractive men accrue

reproductive benefits from spending more time seeking

multiple mating partners and relatively less time invest-

ing in offspring. In contrast, the reproductive effort of

less attractive men, who do not have the same mating

opportunities, is better allocated to investing heavily in

their mates and offspring and spending relatively less

time seeking additional mates.

From a woman’s perspective, the ideal is to attract a

partner who confers both long-term investment benefits

and genetic benefits. Not all women, however, will be

able to attract long-term investing mates who also dis-

play heritable fitness cues. Consequently, women face

trade-offs in choosing mates because they may be forced

to choose between males displaying fitness indicators or

those who will assist in offspring care and be good long-

term mates (Gangestad & Simpson, 2000). The most

straightforward prediction that follows is that women

seeking short-term mates, when the man’s only contri-

bution to offspring is genetic, should prefer muscularity

more than women seeking long-term mates.

Preferences in Extrapair Mates

As a partial solution to the problem of trade-offs,

women may have evolved to pursue a dual-mating

strategy by securing investment from a long-term mate

and obtaining genetic benefits from extrapair mates

(Haselton & Gangestad, 2006). Several lines of evi-

dence support this proposal. Although estimates vary,

the human extrapair paternity rate is approximately 2%

to 4% (for a review, see Anderson, 2006). Thus, a sub-

stantial portion of men raise offspring who are not

genetically their own. Men also appear to possess anti-

cuckoldry mechanisms that lead them to detect the

degree of resemblance between babies’ faces and their

own and adjust their investment accordingly (Platek

et al., 2003). Last, women are most attracted to men

other than their primary mate when fertility is high

within the ovulatory cycle (and thus the benefits of

extrapair mating for genetic benefits are highest;

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Gangestad, Thornhill, & Garver-Apgar, 2005). This is

especially true for women whose primary mates lack

sexual attractiveness—the women who, in theory, have the

most to gain from extrapair mating with men who display

costly fitness indicators (Gangestad et al., 2005; Haselton

& Gangestad, 2006; Pillsworth & Haselton, 2006).

One prediction that follows from the dual-mating logic

is that men who display cues of fitness should be chosen

most often as affair partners. Symmetry is a purported

index of fitness (see Moller, 1997); therefore, Thornhill

and Gangestad (1994) examined partner number in men

varying in symmetry. As predicted, more symmetrical men

reported having a greater overall number of sex partners,

more sexual affairs, and a greater number of sex partners

who were themselves mated to other men at the time of

the affair. Hughes and Gallup (2003) found a similar pat-

tern in men with higher shoulder-to-hip ratios, a trait that

may be linked with testosterone. In sum, both theory and

existing evidence suggest that women attend to cues of fit-

ness when selecting sex partners, particularly short-term

mates and affair partners.

THE INVERTED-U HYPOTHESIS OF

MASCULINE TRAITS

When individuals consider others as mates, is more of

a valued trait always better? Recent work by Kenrick and

colleagues provided compelling evidence that the answer

is no. For example, there comes a point where possessing

additional income does not make one significantly more

desirable as a mate (Kenrick, Sundie, Nicastle, & Stone,

2001). Having more money, however, does not decrease

one’s attractiveness on the mating market.

In contrast to financial resources, there is reason to

believe that high levels of masculine physical features,

including extreme muscularity and facial masculinity, can

decrease a man’s desirability as a mate (Dixson, Halliwell,

East, Wignarajah, & Anderson, 2003; Johnston et al.,

2001). In a study by Johnston et al. (2001), women rated

the behaviors and dispositions of men varying in facial

masculinity. Facial masculinity was positively correlated

with sexual desirability, but the relationship was nonlin-

ear, with the most masculine faces perceived as being

somewhat less sexually desirable, trustworthy, and sensi-

tive than less masculine faces. Highly masculine faces

were also rated as being more dominant, volatile, selfish,

and impulsive than somewhat less masculine faces.

Johnston et al. concluded that “the aesthetic preference

of human females could be viewed as an adaptive com-

promise between the positive attributes associated with

higher-than-average testosterone (health cues) and the

negative attributes associated with more extreme mas-

culinization” (p. 262).

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PERSONALITY AND SOCIAL PSYCHOLOGY BULLETIN

shape, with muscular men being more attractive and

desirable than nonmuscular and very muscular men.

Furthermore, we predicted that women would infer that

very muscular men would be more likely to be physi-

cally dominant and volatile compared with less muscu-

lar men. Last, consistent with the mating trade-off

hypothesis (Gangestad & Simpson, 2000), we predicted

that women would rate muscular men as less committed

to their partners. These predictions were tested in

Studies 1 and 2 by examining women’s ratings of the

attractiveness of computer-generated images and sil-

houettes of men varying in level of muscularity.

Women’s Preference for Muscular

Short-Term Partners

If muscularity is a cue of fitness, it should be more

important to women selecting a short-term mate, when

the man’s only contribution to offspring might be genetic

(Gangestad & Simpson, 2000). Although women may

also desire muscularity in long-term mates, strategic plu-

ralism theory predicts that not all women will be able to

secure attractive mates as long-term partners. We pre-

dicted that women would report preferring a more mus-

cular short-term partner than long-term partner. This

prediction was tested in Study 2. Furthermore, we pre-

dicted that women would report that their recent short-

term sex partners were more muscular than their other

sex partners. This prediction was tested in Study 3.

Muscularity Associated With Male Partner Number

and Self-Perceived Desirability

If women prefer muscularity in short-term mates,

muscular men should be able to capitalize on this prefer-

ence and successfully attract multiple sex partners. Thus,

muscular men should report more lifetime sex partners,

brief sexual affairs, and affairs with mated women than

less muscular men. Male muscularity should also be pos-

itively associated with self-rated attractiveness to women.

These predictions were tested in Studies 4, 5, and 6.

We term this proposal the

inverted-U hypothesis of

masculine traits:

Women will not prefer mates with

extremely high and extremely low levels of masculinity

(e.g., muscularity, facial masculinity, shoulder-to-hip ratio,

and chest-to-waist ratio). Very high levels will be viewed as

unattractive because these men are viewed as volatile and

threatening, perhaps presenting a direct danger to the

woman. Low levels will be viewed as unattractive because

these men are viewed as weak and submissive. Men with

moderate to high levels should be preferred most as mates.

HYPOTHESIS AND PREDICTIONS

We hypothesized that women possess context-sensi-

tive preferences for muscularity owing in part to an

underlying evolved psychology shaped by sexual selec-

tion. To investigate this hypothesis, we tested the fol-

lowing predictions.

Attraction to Muscularity

If muscularity is a cue of fitness, women should be

more attracted to muscular men than to nonmuscular

men. Past research generally supports this prediction. In

questionnaire-based studies, women in Western societies

indicated that men with muscularity or high waist-to-

chest ratios were attractive (e.g., Dixson et al., 2003;

Franzoi & Herzog, 1987; Li & Kenrick, 2006; Maisey,

Vale, Cornelissen, & Tovee, 1999; Swami & Tovee,

2005) but not if they were highly muscular (Dixson et al.,

2003). Although there has been little cross-cultural

research on women’s preferences for muscularity, some

evidence in non-Western societies suggests that women

prefer men with powerful body builds (Cassidy, 1991;

Dixson et al., 2003; for an exception, see Swami &

Tovee, 2005). In parallel, men in societies spanning four

continents believe that women are attracted to men who

are more muscular than average (Taiwan: Yang, Gray, &

Pope, 2005; Samoa: Lipinski & Pope, 2002; Austria and

France: Pope et al., 2000; Kenya: Campbell, Pope, &

Filliault, 2005; Ghana and the Ukraine: Frederick et al.,

in press; and the United States: Frederick et al., in press;

Olivardia, Pope, Borowiecki, & Cohane, 2004). The

majority of these studies, however, presented women

with crude hand-drawn silhouettes of men. In Study 1,

we tested whether women find muscular men more sexu-

ally desirable than nonmuscular men when evaluating

relatively realistic computer-generated stimuli of men.

The Inverted-U Hypothesis of Masculine Traits

We predicted that the extent to which women find

muscularity attractive would follow an inverted-U

STUDY 1: SOCIAL PROFILES OF SIX COMUPTER-

GENERATED IMAGES OF MEN

This study investigated whether women believe mus-

cular men are more sexually desirable, more physically

dominant, more volatile, and less committed to their

romantic partners than less muscular men. We predicted

that women’s ratings of the physical attractiveness and

sexual desirability of muscular men would show an

inverted-U pattern, with nonmuscular and very muscu-

lar men being rated as less attractive than moderately

Frederick, Haselton / MUSCULARITY AS A FITNESS INDICATOR

muscular men. These predictions were tested by exam-

ining women’s ratings of six computer-generated

images of men.

Method

Participants.

A total of 141 undergraduate women

with a mean age of 20.44 (SD

3.59) from the

University of California, Los Angeles (UCLA), partici-

pated in exchange for extra credit as part of their

psychology or communication studies course.

Stimuli.

The stimuli were images of shirtless men cre-

ated using MyVirtualModel.com, a program that allows

manipulation of physical features (see Appendix A).

Aside from muscularity and body weight, all features of

the models were held constant, and height was set at 6 ft

0 in. The first dimension varied was defined as muscular

versus nonmuscular by the program. The second dimen-

sion was defined as total body weight by the program:

large (230 lb), medium (190 lb), or small (150 lb). The

program also offers a limited ability to control shoulder

and waist proportions, and we attempted to standardize

shoulder-to-hip ratio across images.

There were six images in total: brawny (large, mus-

cular), built (medium, muscular), toned (small, muscu-

lar), slender (small, nonmuscular), typical (medium,

nonmuscular), and chubby (large, nonmuscular).

Participants were not exposed to these labels, only the

images. These levels were chosen because of their face

validity; they appeared to differ systematically in body

fat and muscularity. The validity of this manipulation

was tested by presenting the images to 21 judges who

rated how muscular and how fat each of the images

appeared using a 0-100 scale (0

not at all,

a little,

somewhat,

very,

100

extremely).

Planned

comparisons conducted within the context of one-way

ANOVAs revealed the following patterns of results for

muscularity (brawny

built

toned

slender

typical

chubby) and for body fat (brawny

built

toned

slender

typical

chubby) using

.001 as the signif-

icance criterion. Thus, to participants, there appeared to

be four levels of muscularity at the same level of body

fat: slender (small, nonmuscular), toned (small, muscu-

lar), built (medium, muscular), and brawny (large, mus-

cular). As a preliminary test of the inverted-U

hypothesis, we were interested in whether women found

men with moderate muscularity (toned, built) to be

more sexually desirable than men with very low or very

high levels of muscularity (slender, brawny).

To test whether the images differed in shoulder-to-

hip ratio, a graphics designer unaffiliated with the

project measured the length of the shoulders and hips

using a graphics program. The ratios (shoulder length

1171

divided by hip length) were similar among built (1.26),

toned (1.24), and slender (1.24) individuals; slightly

larger for the brawny individual (1.32); and slightly

smaller among typical (1.15) and chubby (1.13) indi-

viduals. Finally, in an attempt to remove ethnic and

racial cues, faces were covered with a small black oval

and the images were printed on a laser printer in black

and white to yield images with ambiguous skin color.

Procedure.

Participants were asked 10 questions about

each image and made all ratings using a 9-point Likert

scale (1

not at all,

a little,

somewhat,

very,

extremely).

The question stem for each item was

“How likely is it that this man . . .” The participants

rated his physical dominance (“is physically intimidating

to other males”), his commitment to his partner (“would

remain sexually faithful to you” and “would be sensitive

to your emotional needs”), his volatility (“has a bad tem-

per” and “would be abusive”), and his sexual desirabil-

ity (“would be sexually exciting,” “would be a good

sexual partner,” and “would be able to satisfy your

sexual desires”). The sexual desirability category also

included the item “How physically attractive is this

man?” All Cronbach’s alphas for each category with

more than one item, for each of the six images, were

greater than .70. We therefore computed the category

means for each image.

Results and Discussion

To examine differences in how women rated each

body for each dimension (sexual desirability, domi-

nance, commitment, and volatility), we conducted a

one-way ANOVA with body type (brawny, built, toned,

slender, typical, chubby) as the independent variable

followed by planned comparisons of each cell. The

means are summarized in Figure 1 and Table 1. Because

our predictions pertained to differences between partic-

ular cell means, we report the results of planned com-

parisons among women’s ratings of each image rather

than the results of the omnibus tests. Because we con-

ducted multiple pairwise comparisons, we used a con-

servative alpha level of .001. All differences were

significant at this level unless otherwise noted.

In support of the inverted-U hypothesis, brawny and

slender men were rated as less sexually desirable than

built men. They were also rated as less desirable than

toned men, although the difference between brawny

and toned was marginally significant (p

.004). In

support of the predictions, each of the muscular men

(toned, built, and brawny) was rated as more dominant

than each of the nonmuscular men (slender, typical, and

chubby). Among the muscular men, the brawny man

was rated the most dominant and the toned man was

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