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https://health.usnews.com/best-diet/best-healthy-eating-diets

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Best Diets For Healthy Eating - Expertly Reviewed for 2023 | U.S. News

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Search the complete lists of diets profiled by U.S. News. Read about nutrition, recipes, expert reviews and how to succeed on the diet best for your healthy lifestyle.

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Relationship of Social Media and Anxiety

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Relationship of Social Media and Anxiety

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1. Introduction

The human face has been a source of great interest to psychologists and other scientists in recent years because of the extraordinarily well-developed ability of humans to process, recognize and extract information from other's faces (see other papers in this volume). Our magazines and television screens are not just filled with any faces-they are filled with attractive faces, and both women and men are highly concerned with good looks in a potential partner [ 1]. Physical appearance is important to humans and certain features appear to be found attractive across individuals and cultures [ 2]. The same holds true across the animal kingdom; most non-human species rely on external traits, such as the size, shape and colour of adornments (e.g. feathers, fur and fins) to attract mates [ 3]. Research on animals has focused on individual traits that are attractive across individuals, and even species, such as symmetry [ 4].

Physical attractiveness has important social consequences. For example, beauty is associated with upward economic mobility, especially for women [ 5, 6], attractive people have more dates than less attractive people [ 7], and people who have dated more attractive individuals report being more satisfied with their dates [ 8, 9]. It has long been noted that there exists a 'What is beautiful is good' stereotype [ 10] ([ 2, 11, 12] for meta-analytical reviews of research on physical attractiveness stereotypes), whereby attractive individuals are perceived to possess a variety of positive personality attributions. In mock interviews, attractive people are more likely to be hired than less attractive individuals [ 13] and attractiveness can also influence judgements about the seriousness of committed crimes [ 14]. Outside the laboratory, attractive people also appear to lead favourable lives; attractive individuals pay lower bail [ 15] and are more likely to be hired for jobs [ 16, 17] than less attractive individuals.

Despite research on social consequences, exactly what it is that makes a face beautiful remains poorly defined. One of the major deterrents in determining the features of an attractive face lies in the widespread belief that standards of attractiveness are learned gradually through exposure to culturally presented ideals (e.g. through the media in Western society) and this has also led to a general belief that cultures vary dramatically in what they perceive to be attractive [ 18]. If this were true, it would mean that attractiveness is arbitrary and what is beautiful now could, in a different time or place, be considered unattractive. The well-known phrase 'beauty is in the eye of the beholder' is a testament to our belief that attractiveness is ephemeral. For example, the philosopher David Hume is often quoted for making the argument that beauty, 'is no quality in things themselves: it exists merely in the mind which contemplates them; and each mind contemplates a different beauty' [ 19, pp. 208-209].

Darwin [ 20] was also struck by cultural differences, such as those evident in preferences for skin colour, body hair and body fat, and those revealed in practices such as lip ornamentation and teeth filing, 'It is certainly not true that there is in the mind of man any universal standards of beauty with respect to the human body' (Darwin cited by [ 21]). Such convictions were supported by early cross-cultural work by Ford & Beach [ 22] who catalogued differences between cultures in preferences for body weight, breast size and other aspects of female physique and suggested little consensus.

While individual and cross-cultural differences exist (see later), this politically correct view of beauty is to some extent false. In fact, agreement between individuals is one of the best-documented and most robust findings in facial attractiveness research since the 1970s. Across many studies it has been found that there is a high degree of agreement from individuals within a particular culture and also high agreement between individuals from different cultures (see [ 2] for a meta-analytical review). If different people can agree on which faces are attractive and which are not attractive when judging faces of varying ethnic background (e.g. [ 23]), then this suggests that people everywhere are all using the same, or at least similar, criteria in their judgements.

Cross-cultural agreement on attractiveness is evidence against the notion that attractiveness ideals are slowly absorbed by those growing up within a particular culture and this suggests that there is something universal about attractive faces (and unattractive faces) that is recognized both across individuals and cultures. In the next section, we discuss traits that are proposed to be generally attractive by reasoning based on evolutionary theories, but we return to the notion of individual variation later. While some traits are proposed to be on average preferred across individuals, an adaptive view of preference suggests that individuals will indeed vary in what they prefer and we examine differences in more detail in §3.

2. The evolutionary basis of attraction: the functions of beauty

An evolutionary view assumes that perception and preferences serve an adaptive function: the external world provides information to guide biologically and socially functional behaviours [ 24]. If in our evolutionary past, information was present about a person's mate and/or social value (e.g. provisioning ability, genetic quality) in any way, then an advantage would accrue to those who used these signs and those individuals would leave more genes behind in the next generation. Theoretically then, preferences guide us to choose mates who will provide the best chance of our genes surviving. In many studies, this evolutionary view of attractiveness has been used to predict the specific characteristics of attractive faces (see [ 25] for review). Sexual selection is the theoretical framework for much work and a thorough discussion of this topic in general is beyond the current review. Interested readers can see Andersson [ 3] for a thorough review, including issues relating to how preferences may arise in populations.

Although we can say whether a face is attractive or unattractive, it is extremely difficult to articulate the specific features that determine this attraction. There are, however, several facial traits that have been proposed to advertise the biological quality of an individual in human faces, and hence to influence attractiveness as a mate: traits such as symmetry, averageness and secondary sexual characteristics (see also [ 26] for meta-analysis). There are many aspects of 'quality' that can be associated with certain traits but these can be broadly split into two types of benefits to the perceiver: direct benefits, whereby the perceiver directly gains for themselves or their offspring, and indirect benefits, whereby the perceiver gains genetic benefits to their offspring. The former is relevant to both same- and opposite-sex attractiveness judgements, whereas the latter has consequences for reproductive pairings. For example, avoiding a parasitized mate has obvious direct advantages whether parasite resistance is heritable or not [ 27] as there are direct benefits to choosing a parasite-free mate. Preferences for facial traits that are associated with parasite resistance may be adaptive because this can lead individuals to associate with those who are not carrying contagious parasites (which may be passed on to the individual or to the offspring) and who are able to act as good parents (providing material benefits or care). Individuals who are attracted to those having face traits associated with parasite resistance may also increase the chances of passing on heritable parasite-resistant genes to their offspring. In other words, there are several reasons why avoiding a parasitized mate is advantageous. Ultimately it may be unnecessary to consider the relative weights of indirect and direct benefits; both indirect and direct benefits are likely to be important in evolution and their contributions to attractiveness are difficult to tease apart. We note that much research has focused on women's preferences, although most traits are also relevant for men.

(a) Symmetry

Symmetry refers to the extent to which one-half of an object (image, organism, etc.) is the same as the other half. Individuals differ in their ability to maintain the stable development of their morphology under the prevailing environmental conditions under which that development is taking place [ 28, 29]. The ability of an individual to develop successfully in the face of environmental pressures is therefore one proposed indicator of genetic quality. A character demonstrates fluctuating asymmetry (FA) when symmetry reflects the normal development, and deviations from this symmetry are randomly distributed with respect to side [ 30]. FA is a particularly useful measure of developmental stability because we know that the optimal developmental outcome is symmetry. Therefore, any deviation from perfect symmetry can be considered a sub-optimal solution which will result in performance problems in the future. FA is also a useful measure as it subsumes a huge amount of individual variation in development, being the outcome of differences in genetic (e.g. inbreeding, mutation and homozygosity) and environmental (e.g. nutrient intake, parasite load) factors [ 28, 29]. Preferences for symmetry can then, potentially, provide both direct (e.g. by avoiding contagion) and indirect benefits (e.g. by providing healthy genes for offspring) to the perceiver.

Whether symmetry is actually related to quality in other animals and humans is an issue addressed by a large literature, and a complete review is not the focus of this paper. While the issue is divided, and there is some evidence that symmetry is not associated with quality (e.g. [ 31]), many studies do show links between symmetry and quality in many species [ 28, 29]. In humans, male body symmetry is positively related to sperm number per ejaculate and sperm speed [ 32] and female breast symmetry is positively correlated with fecundity [ 33, 34]. Relating to faces, one study has demonstrated that facial asymmetry is positively related to self-reported number of occurrences of respiratory disease [ 35] and some studies have observed positive correlations between symmetry and other putative indices of underlying physical condition (i.e. exaggerated sex-typical characteristics, [ 36, 37]). The relationship between symmetry and quality is not reviewed in detail here, but it should be noted that fitness-related characteristics, such as growth rate, fecundity and survivability, are positively associated with symmetry across a number of species and taxa (e.g. [ 29]; see [ 38] for a review) and ultimately, any link between symmetry and quality, no matter how weak, is sufficient to create a selection pressure on the opposite sex to choose symmetric mates in order to provide genetic quality benefits to their offspring.

In humans, Thornhill & Gangestad [ 39] found that the total number of sexual partners a man reported having was positively related to skeletal symmetry. Studies of naturally occurring human facial asymmetries also provide evidence that symmetry is found attractive, though such studies can be confounded by potential correlates. Studies measuring symmetry from unmanipulated faces have reported positive correlations with rated attractiveness [ 40- 44] and one study has even demonstrated that with pairs of monozygotic twins, the twin with more symmetric measurements is seen as more attractive [ 45].

While some studies directly manipulating human facial images have found that asymmetry is preferred to symmetry [ 46], manipulations used in these studies tend to be crude, using 'chimeric' face images manufactured by aligning one vertically bisected half-face with its mirror reflection. Studies using more sophisticated symmetry manipulations have demonstrated that symmetry can have a positive influence on attractiveness [ 47, 48] and have established that the chimeric manipulations used in the early studies introduced unnatural proportions into the symmetric faces (see [ 48]). Examples of manipulated images can be seen in . Thus, the methodologically superior computer graphic studies [ 47, 48] parallel the findings of investigations into naturally occurring facial asymmetries [ 40, 41, 43- 45]. The computer graphic studies demonstrate that increasing symmetry alone is sufficient to increase attractiveness. Subsequently, other studies have replicated preferences for symmetry using manipulated stimuli in different Western samples (e.g. [ 49, 50]), but evidence for symmetry preferences using these methods is not limited to western populations or even to humans. Preferences for symmetry using manipulated faces have been found in African hunter-gatherers [ 51], and macaque monkeys gaze longer at symmetrical than at asymmetrical face images of conspecifics [ 52].

Importantly, recent studies have implicated perceptions of health in attraction to symmetric faces [ 44, 53] and have suggested that the mechanisms underpinning preferences for symmetric faces are different from those that might drive preferences for symmetry in mate-choice-irrelevant stimuli (e.g. [ 49, 50]). Such findings suggest that preferences for symmetric faces reflect, at least in part, adaptations for mate choice.

(b) Averageness

Averageness refers to how closely a face resembles the majority of other faces within a population; non-average faces have more extreme characteristics than the average of a population. Average faces may be attractive because an alignment of features that is close to a population average is linked to genetic diversity [ 54, 55]. Thornhill & Gangestad [ 54] have argued that average faces may be preferred to less-average faces because owners of average faces possess a more diverse set of genes, which may result in less common proteins to which pathogens are poorly adapted. Parasites are generally best adapted to proteins that are common in the host population; hence, parasites are adapted to the genes that code for the production of these proteins. A second evolutionary theory for the attractiveness of averageness in faces is that extreme (non-average) genotypes are more likely to be homozygous for deleterious alleles, that is, to be more likely to possess genes that are detrimental to an individual than those with more average genotypes [ 54]. Both of these theories propose evolutionary benefits to mating with individuals possessing average faces.

Recent studies have supported the link between averageness, heterozygosity (i.e. genetic diversity) and attractiveness. Heterozygosity in the major histocompatibility complex (MHC) genes that code for proteins involved in immune response, is positively associated with facial attractiveness [ 56] and facial averageness [ 57]. More directly, another study has shown that facial averageness is positively related to medical health as measured from actual medical records in both men and women [ 58]. Facial averageness can then be potentially associated with both direct benefits in terms of associating with healthy, parasite- and/or disease-free partners and indirect benefits of heterozygous genes that can be passed onto offspring.

There is good evidence that average faces are indeed found attractive. Galton [ 59] first noted that multiple faces blended together were more attractive than the constituent faces. Recent studies have improved upon these techniques using computers to create digitally blended composite faces; generally, the more images in a composite, the more attractive it is found [ 60- 62]. Aside from composite images, Light et al. [ 63] found that, in unmanipulated male faces, more attractive faces were rated as less distinctive, and Rhodes & Tremewan [ 64] found that higher averageness was associated with higher attractiveness when manipulating averageness via digital caricaturing.

Average faces are generally more symmetric and symmetry is typically attractive in faces (discussed in more detail above). Several studies have controlled for this confound in the original studies. When averageness and symmetry were independently manipulated, one study found that both manipulations positively and independently influenced attractiveness judgements [ 65]. Other studies have used perfectly symmetric images manipulated in averageness and still have demonstrated preferences for averageness [ 66, 67]. Indeed, by comparing preferences for averageness when the effects of symmetry were controlled for and were not controlled for, Jones et al. [ 66] demonstrated that the contribution of symmetry to the attractiveness of average faces was minimal.

It has also been noted that, in the original composite studies, the more images that are blended together the smoother the skin texture becomes, as imperfections such as lines or blemishes are averaged [ 68]. Skin colour/texture has been controlled in studies that normalize the texture/colour of all the faces seen and all these studies demonstrate that average is attractive [ 62, 64, 66, 67]. Examples of composite images and the effects of shape and colour averaging can be seen in .

While the majority of the work described above has been carried out in North America, Britain and Australia, averageness has also been found to be attractive across different cultures. For example, facial averageness is also found attractive in Japanese participants [ 69] and in African hunter-gatherers [ 67].

(c) Secondary sexual characteristics in faces

Male and female faces differ in their shape. Mature features in adult human faces reflect the masculinization or feminization of secondary sexual characteristics that occurs at puberty. These face shape differences, in part, arise because of the action of hormones such as testosterone. Larger jawbones, more prominent cheekbones and thinner cheeks are all features of male faces that differentiate them from female faces (e.g. [ 70]).

From an evolutionary view, extremes of secondary sexual characteristics (more feminine for women, more masculine for men) are proposed to be attractive because they advertise the quality of an individual in terms of heritable benefits; they indicate that the owners of such characteristics possess good genes. In other words, such traits advertise the possession of genes that are beneficial to offspring inheriting them in terms of survival or reproduction. One explanation of the importance of these facial traits is that they represent a handicap to an organism [ 71] and the costs of growing the trait means that only healthy individuals can afford to produce them. In this way, these 'honest' handicaps are proposed to indicate the fitness of the owner. For example, secondary sexual characteristics are proposed to be linked to parasite resistance because the sex hormones that influence their growth, particularly testosterone, lower immunocompetence. Testosterone has been linked to the suppression of immune function in many species [ 72], including humans [ 73, 74]. Larger secondary sexual characteristics should be related to a healthier immune system because only healthy organisms can afford the high sex-hormone handicap on the immune system that is necessary to produce these characteristics [ 75].

In many non-human animal studies, there is a positive association between secondary sexual trait expression and immunocompetence (e.g. [ 76]). The relationship between masculinity/femininity and good genes in humans is less clear. A study by Rhodes et al. [ 77], however, has shown that perceived masculinity correlated positively with actual measures of health in male adolescents. No relationship was found between femininity and actual health in female faces, though [ 77]. Another study has demonstrated that men's facial masculinity and women's facial femininity are negatively related to self reports of respiratory disease [ 35]. If health is heritable, then female preferences for masculinity and male preferences for femininity may indeed also reflect the choice of mates with good genes. There is also a link between hormonal profile and face shape. Women with higher circulating oestrogen have more feminine faces [ 78], while men with high testosterone have more masculine faces ([ 79], but see also [ 80]). If women with high oestrogen and men with high testosterone are valued as mates, preferences for cues of hormonal profile could drive preferences for sexually dimorphic face shape. shows faces manipulated in facial masculinity and femininity.

There is considerable evidence that feminine female faces are considered attractive. Studies measuring facial features from photographs of women [ 40, 81, 82] and studies manipulating facial composites [ 83] all indicate that feminine features increase the attractiveness of female faces across different cultures. If oestrogenized female faces provide cues to fertility and health, then male preferences for such features are potentially adaptive. This reasoning does not require oestrogen to be immunosuppressive or part of a handicap.

The link between masculinity and attractiveness in male faces is less clear. Cunningham et al. [ 84] and Grammer & Thornhill [ 40] used facial measurements and found that women preferred large jaws in men. 'Masculine' features, such as a large jaw and a prominent brow ridge are reliably associated with ratings of dominance in photographic, identikit and composite stimuli [ 83, 85- 88]. Despite some findings showing a preference for more masculine and dominant faces, several studies have shown that feminine characteristics and faces of low dominance are of increased attractiveness [ 62, 83, 84, 89- 91]. Many studies have made use of computer graphic techniques to manipulate masculinity. Sexual dimorphism in face shape can be manipulated by taking the geometrical differences between average male and female face shapes and applying this difference to new faces, making more or less masculine or feminine versions [ 83]. This process simultaneously changes all dimorphic shape characteristics in the face. For example, 'masculinizing' a male face shape by increasing facial proportions relative to the differences between a male and female average increases the size of the jaw and reduces lip thickness because male jaws are larger than female jaws and the lips of men are thinner than those of women. Perrett et al. [ 83] presented both Japanese and Caucasian faces in their country of origin. For the male face stimuli, the shape selected by Caucasians as most attractive was significantly feminized for both the Caucasian male face and the Japanese male face continua. Similarly, Japanese participants also selected significantly feminized versions of the male stimuli for both the Japanese and Caucasian male face continua. Thus, in both cultures it was found that participants showed a preference for feminized male faces. Since then, several studies have also documented preferences for femininity [ 62, 90, 92, 93], but some similar computer graphic studies have also reported preferences for masculinity [ 94, 95]. Although some of this variation may be attributed to other characteristics of the faces that varied between sets of stimuli [ 96], this does not explain the variability in preferences. We discuss the sources of individual differences in preferences for sexually dimorphic shape cues in the latter sections of our article.

(d) Skin health and colour

The face traits discussed so far have often been measured and manipulated but also studied in terms of perception and related to attractiveness. The reasoning for why traits like symmetry are preferred is often related to underlying health. Thus, it is important to examine perceptions of facial health directly. Perceived health is difficult to relate to any one metric, but people will readily rate faces for perceived health and show very high agreement on such ratings (e.g. [ 44, 97]). See for examples of healthy and unhealthy appearing traits. In evolutionary terms, there is a large and obvious selective advantage in detecting healthy partners both for social exchange and mate choice. Indeed, while the role of health in mate preferences is clear (see below), recent work has demonstrated that participants are more willing to reciprocate trust from healthy-looking social partners than from social partners who are relatively unhealthy-looking [ 98]. Such findings demonstrate the importance of health perceptions for social interaction generally. Again, as for previous traits, there may be both direct and indirect benefits to partnering with individuals who are perceived to be healthy.

There have been several studies that have addressed how facial appearance relates to the healthiness of an individual in humans. The three traits discussed above are often manipulated by changing only face shape, but health perception appears to be related to facial colour and texture also. Fewer studies have examined how colour and texture of faces influence attractiveness judgements. One study has examined how well ratings of health from small patches of skin of faces are related to overall rated attractiveness when the whole face image is available. Jones et al. [ 43] found that apparent health of facial skin is positively correlated with ratings of male facial attractiveness. In other research, homogeneity of skin colour was positively related to attractiveness [ 99]. Findings have also suggested that more heterozygous men also have healthier appearing skin [ 56]. Skin health may be a particularly useful marker of current health condition as it is more changeable than aspects such as symmetry or averageness.

Coloration is directly related to the appearance of skin. Coloration also appears to be an important component of sexual selection in many species. Red coloration is associated with dominance in fish [ 100], birds [ 101] and non-human primates [ 102, 103] and, consequently, is linked to attracting the opposite sex. Recent evidence has suggested that primate trichromatic vision is an adaptation to distinguish colour modulations in skin based on blood flow, allowing assessment of the state and/or mood of conspecifics [ 104]. It has been noted that primates with trichromatic vision are generally bare-faced [ 104] and that, at least in humans, facial flushing is associated with anger and confrontation [ 105].

In research on non-human primates, there has been much interest in colour. For example, experimental manipulation of colour shows that female rhesus macaques prefer images of redder male faces [ 103], while males prefer images of redder female hindquarters [ 106]. In mandrills, red facial colour is related to rank in males [ 102], and females sexually present more frequently to brighter males and also groom them more frequently [ 107]. Red coloration also has consequences for behaviour in other species. For example, in bird species, the addition of red to stimuli can increase social dominance [ 108].

In humans, it has been shown that wearing red in a variety of physically competitive sports is associated with an increased chance of winning over opponents [ 109]. This has been interpreted as natural associations of red with dominance being extended to artificially displayed red in the same way that artificial stimuli can exploit innate responses to natural stimuli [ 108, 110]. One study pitting red versus blue shapes found that red shapes were seen as more aggressive, dominant and more likely to win in physical competitions [ 111]. Red does generally seem to have aversive effects on human behaviour. For example, when taking exams, individuals move their body away from tests with red covers more than they do from those with green or grey covers [ 112]. While these studies suggest the colour red may be seen as a threatening stimulus in humans, red also appears to enhance attraction in some instances. For example, women are seen as more attractive by men when presented with red backgrounds or with red clothing, relative to other colours [ 113]. This effect appears to be specific to attractiveness judgements; red colour does not influence judgements of other traits such as kindness or intelligence and does not influence women's attractiveness judgements of other women [ 113]. Further research has examined red coloration in faces and demonstrated a positive association with perceived health [ 114]. The authors suggest that perception of healthy, oxygenated blood may drive associations between red and healthiness. Alongside redness, people also appear to think that skin yellowness is associated with healthy appearance in faces [ 114]. Yellowness may advertise health via an association with diet, as carotenoids are associated with skin yellowness and are absorbed via the intake of fruit and vegetables [ 114]. Taken together, these studies suggest that information on attractiveness and health is available from surface skin and that facial attractiveness is not dependent only on traits that display limited variation in adult life: skin texture and skin colour can vary over weeks or even days.

(e) Facial cues associated with personality attribution

In a classic social psychology study, Dion et al. [ 10] found that strangers rated attractive people as possessing 'socially desirable' traits to a greater extent than unattractive people, and that attractive people were also expected to lead better lives than unattractive people. For example, attractive individuals were thought to be able to achieve more prestigious occupations, be more competent spouses with happier marriages and have better prospects for personal fulfilment. There has been a wealth of studies examining this attractiveness stereotype, demonstrating that attractive people are seen in a positive light for a wide range of attributes compared with unattractive people. On the basis of such studies, it has been suggested that there exists a stereotype associated with physical attractiveness, famously-'What is beautiful is good' [ 2] (see [ 11, 12] for meta-analytical reviews of research on physical attractiveness stereotypes).

Studies on attractiveness stereotypes have generally not addressed the particular characteristics of faces that make individuals either attractive or unattractive, or the features that elicit personality attributions, although different faces reliably elicit the same personality attributions [ 115]. Expression certainly has large effects, with, for example, faces shown with smiles rated as more attractive and as having more positive personality traits than neutral faces (e.g. [ 116]). Such facial expressions are transient, however, and will differ rapidly within individuals over time and across photographs. Both baby-like and mature/dominant facial qualities are related and are more stable aspects of appearance that reliably elicit personality attributions cross-culturally (e.g. [ 88, 117]), but their effect on attractiveness judgements, at least of men, is still in dispute, as noted earlier. Despite some findings showing a preference for more masculine and dominant male faces (e.g. [ 40]), several studies have shown that feminine characteristics and faces of low dominance are of increased attractiveness [ 84, 89].

Personality traits are reported cross-culturally to be among the most important factors in partner choice by both sexes [ 1, 118]. If desired personality is so important, it would appear likely that personality attributions elicited by a face would affect its attractiveness. For example, women who value cooperation and good parenting may avoid masculine-faced men. Thus, instead of feminine faces being attractive and this attractiveness driving positive personality attributions, it may be that the personality attributions are driving the attractiveness judgements.

Individuals may use personality stereotypes in mate selection to select partners with a personality that they desire. Some perceptual attributions to facial photographs are somewhat accurate (e.g. [ 119]), and so choosing a partner based on perceived personality may result in acquiring a partner who actually possesses desired personality traits. Attraction to faces based on personality stereotypes may happen regardless of whether attributions are accurate or not, especially as many individuals do believe that face provides an important guide to character [ 115, 120]. In fact, it is possible that visually appearing to possess certain traits may be more important in initial selection processes than actually possessing desired traits because the visual stereotypes are more easily available than information about stable behaviour. One study has indeed demonstrated that a desire for some personality traits influences judgements of facial attractiveness [ 121]. Individuals valuing particular personality traits find faces appearing to display these traits attractive. Conversely, those not valuing particular traits find faces attractive that are perceived to possess that trait less. Thus, desired personality influences perceptions of facial attractiveness in opposite sex faces, changing the result to: 'what is good is beautiful' [ 121]. In terms of benefits to perceivers, it is easy to see why traits such as appearing trustworthy would make a face appear more attractive. For individual-specific traits, the logic is more complicated, but such preferences could be related to behavioural compatibility within couples, as people do tend to desire partners with personalities similar to their own [ 122].

One reason for variability in preferences for male facial masculinity may lie in the personality traits that masculine- and feminine-faced men are assumed to possess. Increasing the masculinity of face shape increased perceptions of dominance, masculinity and age but decreased perceptions of warmth, emotionality, honesty, cooperativeness and quality as a parent [ 83]. Cunningham et al. [ 84] have suggested that, because both masculine and feminine faces are only rated as moderately attractive, a resolution to this conflict could be that very attractive male faces possess a combination of factors and so reflect 'multiple motives' in female mate choice (i.e. the desire for a dominant and a cooperative partner, as advertised by a combination of masculine and feminine features). It appears then that 'socially valued' traits such as honesty, warmth, cooperation and skill as a parent are associated with feminized versions of male faces, while traits such as dominance are associated with masculinized face shapes. Indeed, recent work has shown that masculine facial characteristics are associated with indices of physical dominance, such as physical strength [ 123], and the perception of such traits [ 124], and that feminine men show weaker preferences for short-term relationships and stronger preferences for committed, long-term relationships than their masculine peers do [ 125]. Feminization of male face shape may increase attractiveness because it 'softens' particular features that are perceived to be associated with negative personality traits. Women's face preferences may thus represent a trade-off between the desire for good genes and the desire for a cooperative partner. This trade-off means that masculinity may be more or less attractive under certain contexts and to certain individuals; we discuss this in §3.

(f) Other traits and interactions

Of course, the five types of trait listed above are not a complete list of factors involved in the judgement of facial attractiveness. Other face traits include factors such as age [ 126], weight/adiposity [ 127], hair and eye colour [ 128], facial hair in men [ 129] and make-up use in women [ 130]. Similarity to self also appears to be an influential trait and we review this factor in more detail in §3 b as self-similar preferences vary according to social context [ 131, 132].

While individual traits impact on attractiveness, there is also scope for interaction between them. We review studies on interactions between emotion and attention in §3 b as preferences for these traits also change according to social context. Certain face traits also appear to interact in generating preferences, however. For example, preferences for masculinity vary as a function of the healthiness of the face [ 96] and women's preferences for facial self-similarity are higher when men are more facially masculine [ 133]. Such interactions highlight that facial attractiveness judgements are not simple: many factors contribute to facial attractiveness and these factors can interact with one another.

3. Adaptive individual differences

In humans, while individuals may share certain basic criteria for finding faces attractive, many factors may influence the specific types of face they find attractive. In this section, we review three broad areas leading to individual differences in preferences: internal factors (e.g. hormonal state), context (e.g. mate-choice versus same-sex preferences) and exposure (e.g. visual experience).

(a) Internal factors

Research suggests that internal factors predict individual differences in several aspects of face perception, including attractiveness judgements. Importantly, the nature of these individual differences suggests adaptive design in face perception and face preferences. In the following section, we discuss two broad types of internal factors: (i) those related to hormone levels and fertility and (ii) those related to own condition, attractiveness and personality.

(b) Context

While factors such as hormones and own attractiveness can explain differences in face preferences between individuals, the context under which judgements are made can also contribute to variation in standards of beauty. In the following section, we discuss how context affects face preferences in three types of contexts: (i) social contexts, such as when judging potential mates versus potential cooperative partners; (ii) temporal contexts, such as long- versus short-term relationships; and (iii) environmental contexts, such as environments with high versus low pathogen load.

(c) Visual experience

Individuals are confronted with a myriad of faces and social interactions every day. Research has shown that such experience leads to changes in preferences for faces. In the following section, we discuss two aspects of visual experience examining: (i) how exposure can impact on preferences and (ii) how observing the choices of others may affect our preferences.

# data[].attributes.attachment_link.description
The human face has been a source of great interest to psychologists and other scientists in recent years because of the extraordinarily well-developed ability of humans to process, recognize and extract information from other's faces (see other papers in this volume).