1. Introduction

Human attraction plays a fundamental role in mate choice, which may facilitate reproductive success. From an evolutionary perspective, attraction is an evolved psychological mechanism that is used to identify high-quality mates [1], such as detecting and evaluating cues indicative of fitness [2]. Indeed, attractiveness is an important attribute that both men and women find indispensable in mate preferences [3]. In the current chapter, the focus will be on sex differences in bodily attractiveness, and the perceptions that physical features convey in mate choice. It has been suggested in the literature that women may find men with physical features that convey high-quality genes and potential investment as attractive, while men may find features in women indicative of reproductive health as attractive. These sex differences in what is considered attractive may help elucidate their underlying evolutionary significance, in addition to considering how ecological constraints and individual differences are involved in those preferences.

Sexual selection is a mechanism of evolution that explains the traits important in mate choice (intersexual selection) and the traits that offer advantages in competing with members of the same sex (intrasexual selection) [4]. Darwin postulated that mating success is influenced by underlying factors that convey fitness benefits, such as physical cues that can serve as honest signals of reproductive potential and quality. According to Sexual Selection Theory, males and females should be sensitive to physical cues that advertise reproductive information, as this may potentially be beneficial for future offspring [4, 5]. For instance, in non-human species, such as the guppy, bright coloration advertises health and diet, and females show preferences for males with these color patterns and benefit from the heritable variation of such features [5, 6]. In ungulates, mate choice may be dependent on antler size, as large antlers that reflect good condition are preferred by females, and these high-quality traits are likely to be inherited by offspring [5, 7]. Antler size may also be considered an armament, signaling successful intrasexual competitive displays. In humans, there are known sex differences in mate choice, and this is influenced by what each sex considers physically attractive. It is suggested that variation in facial and bodily characteristics in humans may reflect underlying quality, such as indications of reproductive fertility, reproductive value, and an enhanced immune system [8].

Parental Investment Theory, as proposed by Robert Trivers [9] provides an understanding into the mechanisms involved in human mate choice. According to Parental Investment Theory, the sex that is more investing in parenting will be more selective in choosing a mate, while the sex with lower parental investment will compete with members of the same sex to obtain access for mating opportunities from the opposite sex. Parental investment refers to any investment in offspring that increases their reproductive success at the costs of investing in other offspring (i.e., foraging, gestation, protection) [9]. In humans, females are the more investing sex, while male investment is facultative [10]. Due to this asymmetry, the cost of selecting a wrong mate is higher for females compared to males, since females have an obligatory gestational period (i.e., pregnancy). Because male investment is minimal, it is suggested that females have evolved psychological mechanisms to be able to evaluate and detect males with cues that are associated with protection and investment, which are qualities that would enhance offspring survival. Since males are the least investing sex, and paternity is not always certain, it is in their interest seek out multiple mating opportunities and be attentive to features in females that signal reproductive capacity. This difference in sexual behaviors has fashioned evolved psychological mechanisms in males to evaluate and detect physical characteristics associated with reproductive value and fertility in mate choice.

2. Sex differences in physical attractiveness

There are known sex differences in physical attractiveness, as each sex may consider the importance that facial and bodily traits provide in reproductive success. For example, borrowing from Parental Investment Theory, it can be hypothesized that females may prefer traits in males that would be beneficial in maximizing offspring care, such as protection and increased paternal investment. Further, since males are the least investing sex, it can be hypothesized that males may focus on physical features that can maximize reproductive success, primarily by prioritizing cues associated with fecundity. Moreover, other theoretical frameworks, such as Sexual Strategies Theory [11], help elucidate the significance of prioritizing physical characteristics across temporal dimensions. Males and females throughout ancestral history have faced unique adaptive challenges and have developed a complex set of mating strategies to help solve those recuring adaptive problems associated with mating. For instance, females have faced the adaptive problem of finding and securing a mate and evaluating whether that mate would be an investing partner in her and her offspring. Physical traits that may signal an increase amount of investment may be prioritized for long-term mating, while traits that signal good genes may be prioritized for short-term mating. Males have faced the adaptive problem of determining which females are fertile, therefore, developing a mating strategy that facilitates maximizing mating opportunities is ideal. It is known that some features in women may signal fertility, and men may prioritize those features when considering women for a short-term mating relationship. In the following sections, sex differences in physical attractiveness will be considered in the framework of Parental Investment and Sexual Strategies Theory.

2.1 Women’s ideal preferences

Women’s mate preferences are driven by physical characteristics in men that convey both indirect (i.e., genetic) and direct (i.e., resources acquisition, protection) benefits [12]. These preferences are used to assess men’s overall quality and probability of investment. Research on female mate preferences has focused on the role of enhanced secondary sexual characteristics on physical attractiveness. In males, the role of androgens play a fundamental role in shaping facial features, such as pronounced eyebrows, enhanced cheekbones, elongated jawline, and in shaping bodily characteristics [13]. Since testosterone serves as an immunosuppressant, only high-quality men are able to withstand the immunosuppressant effects of testosterone while maintaining masculine characteristics, known as the immunocompetence handicap hypothesis [14]. It is suggested that women should prefer men with masculine features, as these features may be heritable and beneficial for reproductive success. Additionally, testosterone may also serve as a cue of overall effort dedicated to mating, as many perceptions of testosterone mediated features (i.e., facial and body masculinity) are perceived to be associated with male-male or intrasexual competitive displays.

2.1.1 Preferences for facial masculinity

Sexual dimorphism, or the degree in which males and females are different, is known to influence evaluations of attractiveness. The role of androgens during puberty shapes the degree of femininity and masculinity of secondary sexual characteristics, where males have much longer jawlines compared to females. Men with masculine facial features are said to be preferred because they have desirable traits that are indicative of good health and high-quality genetics. Sexual selection proposes that traits that advertise cues to health and reproductive quality in males will be selected for through female mate choice. Women who prefer men with masculine features may pass those desirable qualities to future offspring and enhance their overall viability. However, research investigating women’s preferences for facial masculinity has been mixed. Some research has shown that women do prefer men with masculine compared to feminine facial features [15, 16, 17, 18], possibly due to its association with immunocompetence [19, 20], disease resistance [21], and strength [22]. These perceptions of overall quality may influence women’s mate choice in selecting a mate that has qualities that indicate ‘good genes’ or is perceived to provide protection and resources through successful intrasexual competitive encounters.

Other studies have not found support for women’s preferences for facial masculinity. Morrison et al. [23] and Glassenberg et al. [24] showed that women did not prefer feminine or masculine faces. Further, research has shown that there are slight preferences for facial femininity in men, perhaps due to the negative perceptions that facial masculinity may signal, such as increased dominance and dishonesty [25]. Facial masculinity can also be associated with lower paternal investment, aggressiveness, and being perceived as threatening [17], which are characteristics that are not desirable in a long-term partnership. One eye-tracking study showed that women preferred and viewed longer (i.e., increased visual attention) feminine compared to masculine faces [26], suggesting that cognitive processes may be involved in driving the expression of mate preferences. Therefore, in mate choice, women may make trade-offs in choosing a mate with high-quality features (i.e., facial masculinity) over a mate who is perceived to be higher in parental investment (i.e., facial femineity).

2.1.2 Preferences for upper-body strength

Upper-body strength in males provides both indirect and direct benefits in mate choice. It is suggested that women may prefer men with formidable features because they signal good genetics [27], and the ability to offer resources and protection to their partners and offspring, which are important in parental investment. Research has shown that women consider men that display cues of strength and formidability as physically attractive, [28, 29, 30] as they may be associated with the ability to provide protection, acquire resources, compete intrasexually, and form coalitions. Individuals also perceive stronger men as more likely to be able to offer protection to children compared to men with weaker physical attributes [31]. One method in which researchers have used to assess cues of upper body strength in men is by measuring the circumference of men’s shoulders in relation to their hips, known as the shoulder to hip ratio or SHR. Men with higher shoulder to hip ratios (V-shaped bodies), indicative of broader shoulders in relation to their hip size, are considered more attractive by women [29, 30, 32, 33, 34, 35, 36], and women have reported an interest in engaging in sexual activities with men with more masculine body types [32]. Evidence from eye-tracking studies have shown that women spend a considerable amount of visual attention assessing men with formidable features [29, 33, 34, 35, 37] compared to men with more fat distribution. This may suggest that there are important cognitive mechanisms (i.e., automatic processing, attention) involved in women’s assessments of men bodies when considering them for potential mates.

2.1.3 Preferences for height

Height is another sexually dimorphic trait that plays a fundamental role in mate choice. Taller men are often perceived as physically stronger, aggressive, dominant, formidable, and higher in social status [38], which are features important for successful contest competitions among members of the same sex [39]. It is reported that taller men have higher levels of self-esteem and self-confidence, and these psychological traits are critical in being successful in interpersonal relationships [38]. Research has shown that taller men are more successful in acquiring long-term partners [40] and sexual partners [41] compared to relatively shorter men. Women have reported that height is an important physical trait in rating men’s attractiveness and in selecting them as a mate. Although both sexes prefer opposite sex mates that are taller than the average height, women’s preferences for taller men are greater than men’s preferences [42].

2.1.4 Preferences for facial and body hair

Facial and body hair are sexually dimorphic traits that are known to be associated with men’s attractiveness. Beards are androgen-dependent traits and may serve as an honest cue to men’s heritable fitness, as proposed by the immunocompetence handicap hypothesis [14]. As the most conspicuous secondary sexual trait in men, men with beards are perceived to be more masculine, dominant, aggressive, and threatening, compared to clean-shaven men. This may suggest that beards may function as an intrasexual competitive display, perhaps by enhancing masculine features such as the jaw line. In reference to physical attractiveness, research on beards enhancing men’s attractiveness has been mixed. Research has shown that women rate men with full beards as attractive [43, 44, 45], while others have shown that men with a heavy stubble [46], light stubble [47], and clean shaven [48] are rated higher in attractiveness. Interestingly, men with beards are often rated higher in parental ability [49] and are rated higher in attractiveness for a long-term partnership [46, 47]. This may be due to the perceptions beardedness has on age, maturity, and ambition, which are attributes that are desirable in a long-term partner. Relative to facial hair, research on women’s preferences for men’s body hair has also been mixed. Some studies have shown that women rate men with considerable body hair as more attractive [50, 51], while other studies have shown that hairless men are rated higher in attractiveness [51, 52, 53, 54, 55, 56, 57]. The ectoparasite avoidance hypothesis proposes one reason why hairless men may be considered more attractive. Hairless men may be less susceptible to ectoparasites, which may serve as a cue that they are healthy and not transmitting unwanted parasites.

2.1.5 Women’s mating strategies and attractiveness

Women obtain benefits from utilizing short-and long-term mating strategies. For a short-term mating strategy, women can obtain immediate resources, assess a partner for a potential long-term relationship, and obtain good genetics. For a long-term mating strategy, women can obtain investment, commitment, status transmission, and physical protection [11]. It is suggested that in order to obtain high-quality genes from a partner, women may prioritize physical attractiveness in mate choice for a short-term mating context, such as showing preferences for facial masculinity, body muscularity, and formidability. Short-term mating-oriented women, as measured by the sociosexuality orientation inventory (SOI), are more likely to engage in sexual activity with minimal commitment, and this may be calibrated to obtain genetic benefits from an attractive male. According to Strategic Pluralism Theory, some women may prioritize physical traits due to their genetic inheritance if they outweigh the cost of short-term mating (i.e., less paternal investment, partner desertion) [58]. Indeed, research has shown that short-term mating-oriented women find masculine faces [18, 59, 60, 61, 62] and muscular body types [61, 63, 64] more attractive. This may translate to pursuing a mate that has ideal traits in order to maximize the genetic benefits that may be obtained from successful courtship. Women’s preferences for men’s physical traits as a function of mating strategy are also expressed at the cognitive and behavioral level. Behavioral evidence in the form of eye-tracking studies have shown that women pursuing short-term mating strategies view men with variable physical traits associated with muscularity differently. Short-term mating-oriented women are more like to view men’s upper body regions longer compared to other regions of interest (i.e., waist, legs) [34], supporting the claim that cues to upper body strength are important in assessments of attractiveness. For long-term mating, women may prefer men that are able to signal investment and commitment. Muscular men are perceived to be physically dominant, more likely to pursue multiple mating opportunities, and less committed in a relationship [65, 66], which might influence women’s preferences for less muscularity in mates for a long-term relationship. Taken together, women’s mate preferences may be based on mating strategies and possible trade-offs in choosing a partner for obtaining indirect or direct benefits.

2.2 Men’s ideal preferences

According to Parental Investment Theory, the least investing sex will compete with members of the same sex to access fertile females [2, 9, 67]. In order to determine reproductively viable females, males must rely on key features of the female body to determine reproductive value (i.e., expected future fertility) and fertility (i.e., probability of present reproduction). Youthfulness is a feature that men find attractive in women and shows cross culture generality, where men are more likely to marry younger women, and consider attractiveness and youthfulness in a mate as indispensable traits [3]. Other bodily traits, such as women’s breast morphology and waist to hip ratio, have received considerable attention in their perceptions of youthfulness and fertility, which contribute to men’s ratings of attractiveness. It is also known that men pursue both short- and long-term mating strategies. Because of the asymmetry of parental investment, where men’s investment is lower than women’s, men will be more likely to pursue multiple mating opportunities and desire more interest for short-term mating [3]. Although an interest in short-term mating can increase a man’s odds of maximizing reproductive success, there are benefits obtained from pursuing a long-term mating strategy. Human males are one of the small percentages of mammals (3–5%) [10] that contribute a considerable amount of paternal investment to their offspring. By pursuing a long-term mating strategy, not only do men monopolize their partner’s reproductive resources, but increase the survivability of their offspring by providing investment from both parents.

2.2.1 Preferences for facial femininity

It has been proposed that facial femininity reflects underlying cues to women’s reproductive health. Facial femininity may signal overall reproductive condition, as variations in fat distribution and ecological constraints (i.e., food availability) may affect levels of estradiol and progesterone [68]. Since estradiol is associated with facial femininity, femininity may serve as a cue to hormonal systems that are involved in reproductive effort and fertility, as hormones fluctuate across the fertile periods of the menstrual cycle. It could be proposed that men would prioritize facial femininity in mate choice to maximize their probability of reproductive success. Facial femininity has been associated with higher levels of circulating estrogen [69] which is an important biomarker in reproduction and may explain why some men consider women with more feminine faces more attractive. Research has shown that men consider facial femininity over averageness as physically attractive [25, 70]. Further, women with higher levels of late-follicular estradiol report more interest in ideal number of children, and women with facial features indicative of maternal features are rated as more feminine [71]. It is important to note that preferences for facial femininity in men declines with age, and it is a notably stronger preference for younger men. This may be because there are more costs associated for older men competing with younger men in order to access women with ideal traits [72].

2.2.2 Preferences for ideal breasts

In human females, breasts are conspicuous secondary sexual traits that males find sexually attractive. Unlike other mammals, where breasts only enlarge around the time of lactation, female breasts stay permanently enlarged after puberty [73]. Breasts are considered sexually arousing and are thought to be driven by sexual selection, as variation in breast morphology alters perceptions of attractiveness, fertility, reproductive success, health, and age. According to the nubility hypothesis, female breasts are an indicator of age and reproductive value, as breast size varies considerably in their conspicuousness in pre-pubescent compared to young mature women [74]. Research examining preferences for women’s breast sizes has been mixed and show considerable variation across cultures. Men have shown preferences for large breasts [75, 76, 77], medium-sized breasts [78, 79, 80], and small breasts [81, 82], in addition to other morphological features, such as pendulous breasts [83]. In line with the nubility hypothesis, breasts that are non-ptotic (i.e., firm) are associated with reproductive relevant information, such as fertility, health, reproductive success, and overall age. Compared to breasts that are high-ptotic (i.e., sagginess), men and women rate non-ptotic (i.e., firm) breasts higher in fertility, health, youthfulness, and attractiveness, while rating high-ptotic breasts higher in reproductive success (i.e., number of children) and older in age [76, 84]. Women with high-ptotic breasts are often rated higher in reproductive success since breast ptosis increases with each pregnancy [85]. Overall, there is much more consistency in ratings of women’s breasts when it directly relates to features associated with reproductive relevance, such as breast ptosis compared to overall size. Men’s preferences for women’s breasts have also been elucidated by studies incorporating eye-tracking methods to highlight the important role of cognitive mechanisms. Eye-tracking studies have shown that men focus a considerable amount of visual attention to the chest region of women compared to other regions of interest [29, 30], such as waist to hip ratios, which is another reproductive relevant cue men find attractive in women.

2.2.3 Preferences for waist to hip ratios

Waist to hip ratio (WHR), or the circumference of a women’s waist compared to her hips, is a sexually dimorphic feature that is associated with women’s attractiveness. Waist to hip ratio has been considered a cue to reproductive capacity, and it has been suggested that reproductive hormones, such as estradiol, are higher in women with lower WHRs [86]. Given its importance as a reproductive cue, men should find women with ideal WHRs as physically attractive. Across research, men rate women with low WHRs as the most attractive [87, 88, 89]. This may be due to their perceptions of reproductive relevance, such as better health, fertility, reproductive value, and parity (i.e., number of previous pregnancies) [90]. Women with higher WHRs are often rated lower in attractiveness, and this may be due the association between higher WHRs and obesity [91]. Further, higher WHRs are associated with irregular menstrual [92] and fewer ovulatory cycles [93] compared to women with lower WHRs, which may play a role in men’s ratings of women’s attractiveness. Evidence from neuroscientific studies further highlights the importance of women’s WHRs on men’s cognitive processes in mate choice. Parts of the brain associated with reward processing and decision making are activated when viewing images of women with ideal WHRs [94]. This suggests that neuroanatomical regions are involved in identifying and detecting ideal features in women’s bodies when making judgments of attractiveness.

2.2.4 Men’s mating strategies and attractiveness

Short-term mating strategies in men are characterized by low commitment and a desire for multiple sexual partners. In order to maximize reproductive success, men pursuing short-term mating would have benefitted by being able to detect and identify women with physical features that may connote reproductive health. Although both sexes consider physical attractiveness important in mate choice, men prioritize physical attractiveness when utilizing a short-term mating strategy over a long-term mating strategy [95]. Short-term oriented men rate feminine faces [96], low WHRs and BMI’s as more attractive [97], which are features that are associated with fecundity. In a study examining sociosexuality and preferences to women’s breasts, men with a short-term mating orientation rated women with larger breast sizes, most notably C and D cup sizes, as more attractive [77]. Large breast sizes are associated with higher levels of estradiol [86], fertility and reproductive success [98], which may provide cues to reproductive potential in short-term mating.

3. Ecological conditions and mate choice

Human mate choice can also vary across ecological conditions, such as living in an environment with limited food supply or increased mortality hazards. The ecology provides individuals with cues of available resources, and this may influence which mating strategy is most optimal. In harsh environments, characterized by unpredictability of obtaining resources, there may be more advantages in adopting faster reproductive strategies by prioritizing good genetics [99], since the risk of mortality is high and finding a partner is not always certain. Some research has shown that women prefer men with masculine characteristics in environments of unpredictability (i.e., violence, resource scarcity, pathogen prevalence [100]. Although men with masculine characteristics are perceived to be less investing in a long-term relationship, they are preferred because they can provide protection and immediate provisioning to their partner and offspring in harsh environments [101]. One study showed that men with stronger body types are preferred over weaker body types when primed with resource scarcity [102]. Lyons et al. [101] showed that women primed with a resource scarce prompt viewed masculine faces longer, however, this was dependent upon their relationship status, where the effect was stronger in partnered women. Nonetheless, adopting a strategy that favors high investing men in harsh environments can also beneficial. Biparental care may be an optimal strategy for offspring if they are to receive resources from both parents instead of one [103]. Lee and Zietsch [104] showed that women preferred men with “good dad” traits when primed with an ecologically harsh prime. Another study showed that women who reported higher levels of fear of crime preferred formidable men as long-term partners, possibly due to these men providing increased protection [105]. Interestingly, safe compared to harsher environments may also influence women’s preferences for masculine men. Little et al. [100] showed that women preferred masculine men in a safe environment for a short-term mating context, and Marcinkowska et al. [106] found that short-term mating-oriented women were more likely to prefer masculine men in ecologically safe countries. These studies suggest that in a safe environment there are fewer benefits obtained from an investing partner, therefore, women’s mate preferences are centered around obtaining high-quality traits.

Regarding men’s mate choice across ecological conditions, some research has focused on the role of ecological harshness and men’s perceptions of women’s breasts. Aside from functioning as a signal of reproductive health, women’s breasts may also serve as a cue to fat reserves. Men in harsh ecological environments may pursue faster sexual strategies that optimize reproductive output over parental investment. This may influence preferences that men show for women with differences in breast morphology. Some studies have found evidence that men find larger breasts more attractive in ecologically harsh environments [107, 108]. Larger breasts may signal cues of fat storage in environments of unpredictability, which is important in pregnancy and lactation. These findings may point to the role harsh ecologies have on driving mate preferences for physical traits signaling reproductive health.

The availability of resources has been thought to be a key component in men’s preferences for women’s waist to hip ratios. In societies where resources are not readily available and work is energetically expensive, men might prefer women with higher WHRs, as noted by research on the Hazda, a hunter-gather population from Tanzania. One study comparing U.S. and Hazda preferences for women’s WHR showed that Hazda men preferred women with much higher WHRs, rated them healthier, and more desirable as a wife compared to U.S. men [109]. One possible explanation is that in environments of uncertainty, thinness may be associated with disease prevalence, malnutrition, and poor reproductive health. Indeed, thinness is associated with later menarche, irregular ovulatory cycles, and lower capacity to support pregnancy [110, 111]. In Peru, males from the Yomybato village considered high WHR women higher in attractiveness and desirable as a spouse, while males from Shipetiari, a more westernized population, considered low WHR women more attractive and desirable as a spouse [112]. Perhaps, as the food supply became more readily available and predictable, higher WHRs became associated with obesity, leading to an overall preference for women with low WHRs.

4. Mate value and preferences for ideal traits

Individuals also vary in what they prefer and find attractive in potential partners. Mate value, or one’s own assessment of physical attractiveness and mating market value, can calibrate preferences to high-quality mates. Individuals high on mate value may be better able to obtain the desired features and mate with potential partners of similar levels of attractiveness [113]. Research on mate value and physical attractiveness has mostly focused on women’s assessment of their physical attractiveness and preferences for ideal traits in men, such as indicators of masculinity [114]. Some studies have shown that women who rate themselves higher on physical attractiveness prefer men with masculine faces [112, 115], as masculine faces may be characteristic of high-quality men. One study demonstrated that women high on mate value rated men with low waist to chest ratios (broad shoulders relative to the waist) higher on attractiveness, and they were more likely to view them longer using an eye tracking device. Women high on mate value may also be better at matching their ideal preferences in a mate with their actual relationship choices [116], as well as being better at controlling their partner’s behavior to prevent mate desertion [117]. Buss and Shackelford [113] showed that attractive women desire men who have indicators of good genes and parenting traits, suggesting that attractive women are able to raise their standards on these fundamental traits compared to women lower on attractiveness.

Men also calibrate their ideal mate preferences according to their own mate value and self-perceived attractiveness. Men who rate themselves higher on self-perceptions as a long-term partner are more discriminative in their mate preferences [118], such that men who rate themselves higher on the trait of physical attractiveness also considere physical attractiveness important in their mate preferences. Arnocky [119] found that men’s facial attractiveness was associated with their preferences for good health in a partner. Further, men who rated themselves higher on mate value reported stronger preferences for a partner who desired children, had good social status, and had good looks. More research is needed in this domain of men’s mating preferences, as most research has focused primarily on women’s mate value and preferences for mates with similar features.

5. Conclusion

Attraction is an evolved psychological mechanism that aids in identifying and detecting traits in the opposite sex that indicate high-quality genes and reproductive health. Facial and bodily traits are used in this assessment, and there are sex differences in how they are prioritized in mating. In women, it has been shown that facial and body masculinity may provide indirect benefits, but at the expense of lower parental investment. In men, physical traits in women may signal reproductive health, which may explain why men value and find desirable women’s breasts morphology and waist to hip ratios physically attractive. Mating strategies may also facilitate these preferences, as men and women may gain reproductive advantages from pursuing short- or long-term mating strategies. Moreover, it is important to consider that mate preferences vary across ecologies, as the availability of resources has been shown to drive what men and women prioritize in mating. Lastly, individuals differ in what they consider attractive, and they may choose mates who match their levels of attractiveness, which is in line with assortative mating.

Conflict of interest

The author declares no conflict of interest.

Sung-Jin Choi

Chapters authored

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