Do people have a preference for symmetric mates, and if so, how does this preference impact their selections? In a landmark study, evolutionary psychologist David Buss and his colleagues interviewed more than ten thousand individuals from thirty-seven different cultures about their mating preferences. While men tended to value physical attractiveness slightly more than women in selecting a mate, both sexes from virtually every culture studied ranked appearance as one of the most important factors overall. As we have already seen, there is also remarkable agreement across cultures on aesthetic judgments of facial attractiveness, and even two-month-old infants seem capable of making the distinction. So are people with greater symmetry seen as more attractive?
Perhaps no other research area in evolutionary psychology has received more attention from the media as the study of attractiveness. Several original and replication studies have shown that in general we find potential mates with greater body and facial symmetry to be most sexually attractive. For instance, in comparison to men with high asymmetry, symmetrical men have greater facial attractiveness (as rated by both male and female observers), more sexual partners during a lifetime, and an increased frequency of sexual affairs with partners outside of their primary relationship, begin to have sex earlier in life, and produce disproportionately more copulatory female orgasms in their partners.
In a recent study, biologist Craig Roberts and colleagues from the University of Newcastle found that facial asymmetry and attractiveness correlate positively with each other and with a key marker of immune function. The investigators showed fifty women (aged eighteen to forty-nine years old) color head shots of men with a neutral facial expression. As in similar studies, all photographs were digitally masked so just the face was visible. The women were asked to rank the attractiveness of each face using a seven-point scale. The investigators extracted a composite estimate of fluctuating asymmetry by measuring symmetry at seven distinct bilateral facial landmarks. In an interesting twist, blood samples were also collected from the men and genotyped for heterozygosity at key loci in the major histocompatibility complex (MHC—see chapter 5). Recall from chapter 5 that the MHC genes code for immune cells that identify intruding disease organisms, thus functioning as our immune system’s first line of defense. Of importance in this discussion is the fact that MHC genes have upward of a hundred or so different alleles, each providing immunity against different sets of potential disease strains.The level of zygosity refers to how often a specific allele is repeated at different loci within the MHC. It is believed that greater allelic diversity in the MHC leads to a broader resistance to different pathogens. Individuals vary considerably in their degree of heterozygosity, with most people being homozygous at a few alleles. Roberts and his colleagues found that the degree of heterozygosity at key loci in the MHC correlated positively with fluctuating asymmetry in the male faces. Men with greater heterozygosity—who are presumably equipped to fight off a greater variety of invading pathogens than their more homozygous counterparts—had the most symmetrical faces. Moreover, the women rated these faces as the most attractive.
Nonfacial secondary sexual characteristics have also been found to be linked to ratings of attractiveness. As we saw earlier, waist-to-hip ratios in women of approximately .7 are seen by both men and women as most attractive. In other experiments, Devendra Singh demonstrated that breast symmetry is positively correlated with men’s judgments of attractiveness as well as their interests in both short- and long-term relationships. Like faces and other secondary sexual characteristics, when observed across a population of individuals, fluctuating asymmetry of breasts tends to be large relative to absolute size (that is, absolute breast size asymmetry divided by breast size).Whereas most body parts exhibit fluctuating asymmetry of no more than 1 percent of the overall body part size, breast asymmetry tends to be closer to 5 percent of absolute size in all cultures that have been studied.
These data indicate that people are able to detect differences in body and facial symmetry, and use this information to guide their choice of potential mates. It is interesting that when asked to define what makes a person attractive, people often say something about a particular look or a particular body part (for example, the eyes). Evidence shows that we use symmetry as an important metric in defining attractiveness and identifying preferred mates, even though we may not consciously recognize this implicit calculation. Interestingly, we use mental calculations of symmetry every day in many other contexts, such as in our appreciation of art, choices of jewelry and clothing, and what consumer products to buy. Let us now examine the broader way the pleasure of symmetry impacts our adult lives.
Symmetry and Aesthetics: An Example of a General Process
Adults are not only drawn to symmetric mates. As would be predicted by the theoretical perspective being discussed throughout this book, the pleasure we take from seeing highly symmetric objects extends beyond bodies and faces. For instance, similar to newborns, adults are able to recognize and process vertically symmetric objects more quickly than objects with similar features that are not symmetric. Moreover, symmetric objects and patterns are preferred by adults over asymmetric versions even if they do not serve any apparent biological function (for example, such as mate selection). Indeed, there is widespread use of symmetric designs for decorative art among cultures diverse in region, ethnicity, and time.
In a series of interesting studies, psychologist Lauren Harris adopted classic abstract designs seen in different cultures (for example, Aonikenk, Navajo, and Yoruba) and manipulated them in terms of their symmetry. In one condition, the geometric shape was manipulated such that two versions of the same form were presented to adult subjects—one perfectly symmetrical, the other asymmetrical. The subjects were asked to “choose the design that is more attractive in each pair of designs.” In a second condition, both object shape and coloration were varied—symmetrical in one object and asymmetrical in the comparison. Finally, in the third condition, objects varied in the orientation of their symmetry—vertical versus at forty-five degrees.This third condition was included to replicate previous work that has shown that both newborns and adults recognize objects with vertical symmetry more quickly than other orientations.
Harris and her colleagues found that in all comparisons the symmetric version was seen as being more attractive than the asymmetric counterpart. This suggests that symmetry is preferred in nonbiological signals outside the context of fitness, and that the preference is robust since it occurred with respect to the primary features of shape, color, and orientation. Condition two, in which both color and shape symmetry were varied, was found to have the largest effect size (differences in the mean number of designs chosen as most attractive in each condition) of all the conditions. This finding is exactly what would be predicted from the theory being considered, namely that there are several core preferences (across all sensory domains) that emerge during development that facilitate normal brain growth and maturation. The pleasure instinct prods us to seek these basic stimulus forms to fine-tune each sensory system to the environment in which the individual resides.These core features are additive in the sense that objects with multiple pleasure-inducing stimulus forms should be preferred to those objects with fewer forms. Hence it is more pleasurable to look at a real face (in both newborns and adults) with smooth skin, high contrast and concentricity, and symmetry than a simple line drawing of a face with just symmetry. In this case we see that color symmetry acts additively with shape symmetry to produce an even greater preference than either would have alone.
Interestingly, Harris and her colleagues also examined the impact of symmetric and asymmetric facial painting on overall facial attractiveness, noting that such practices are common across geographically diverse tribal societies (for example, the Selk’nam of South America, the Huli of Papua New Guinea, the Kikuyu of Africa, and Blackfoot Indians of North America). Again, they had three conditions. In the first condition, unpainted symmetric versions of a face were compared against unpainted asymmetric versions of the same face. In the second condition, symmetric faces with laterally symmetrical paint designs were compared against asymmetric faces with laterally asymmetric designs. In the final condition, symmetrical faces with asymmetrical paint were compared to asymmetric faces with symmetric paint. Subjects were asked to “choose the face that is physically more attractive in each pair of faces.”
The researchers found that symmetric faces with symmetric paint were viewed as the most attractive in terms of absolute preference across all of the conditions. They also found, as expected, that unpainted symmetric faces were preferred to unpainted asymmetric faces. Interestingly, the application of an asymmetric design to a symmetric face decreased its attractiveness, while the application of a symmetric design increased the attractiveness of asymmetric faces. This pattern of results indicates that additional symmetry features that have no association with overall fitness or phenotypic quality have an additive effect on preference ratings of biologically relevant features that do indicate fitness.
Our innate preferences for symmetry and proportion that impact everyday behaviors are but two simple spatial examples of what I believe represent a general process. As mentioned in earlier chapters, the pleasure instinct creates strong preferences for distinct stimulus features in every sensory domain. The developing brain is thirsty for particular experiences that optimize both synaptogenesis and synaptic pruning. Some feature preferences crafted by the pleasure instinct are likely carried into adulthood unaltered and continue to steer our everyday behaviors and choices in subtle and not so subtle ways (for example, our love of sugars and fats). But my sense is that many of the preferences that facilitate brain development (arguably adaptations driven by natural selection) have also been amplified at some point in our phylogenetic history by sexual selection processes. In the next chapter we will discuss a temporal example and examine the complicated manner in which our preference for repetition and rhythm is expressed in our everyday lives.
Chapter 10
Pleasure from Repetition and Rhythm
It appears probable that the progenitors of man, either males or females or both sexes, before acquiring the power of expressing their mutual love in articulate language, endeavored to charm each other with musical notes and rhythm.
—Charles Darwin,
The Descent of Man
Existence equals pulsation.
—Ellen Dissanayake,
Homoaestheticus
As we saw earlier, our innate preferences for proportion and symmetry have been crafted by the pleasure instinct as a means to encourage newborns, toddlers, and older children to seek out optimal forms of spatial stimulation to fine-tune the developing visual system during synaptogenesis and synaptic pruning (see chapter 8). In the previous chapter, we discussed how this preference has been co-opted during our species’ phylogenetic history through sexual selection, since such visual features can also be used as fitness indicators for mate identification and mate choice. In this chapter we will consider a temporal example and examine how our innate preference for repetition and rhythm impact many of our everyday behaviors, some of which may also be magnified through sexual selection.
The development of the primary auditory cortex and associated brain structures of all mammals depends critically on the precisely timed expression of key genes triggered when the organism experiences environmentally relevant stimuli to fine-tune the system. As was remarked earlier, the details of development are not in the genes, but rather in the patterns of gene expression. Between the twenty-fifth and thirtieth weeks of gestation, fetuses become sensitive to sounds and will move in relation to Mother’s voice especially. But beyond Mother’s voice, they also hear the steady rhythm of her heartbeat and respiration.
After birth, infants continue to seek out certain forms of auditory stimulation, particularly those that are repetitious and rhythmic. For instance, newborns and infants are extremely sensitive to the prosodic elements of speech, those that are rich in emotional meaning. Of course, prosody is the backbeat of the well-known singsong style of motherese that dominates parent-infant dialogue during the first year of life, with its emphasis on simple pitch contours, broad pitch range, and syllable repetition. Experiencing acoustic rhythm during the latter period of gestation and then as a newborn and toddler seems to be a key requirement for normal growth and maturation of the neural systems that process auditory information. Recall from chapter 7 the experiments by neurobiologists Michael Merzenich and Edward Chang of the University of California at San Francisco showing that newborn rats failed to develop a normal auditory cortex when reared in an environment that consisted of continuous white noise. After only a few months, the scientists found significant physiological and anatomical abnormalities in the auditory cortex of the noise-reared rats when compared to rats raised in a normal acoustic environment. Moreover, these abnormalities persisted long after the experiment ended. Most interesting of all, when the noise-reared rats were later exposed to repetitious and highly structured sounds, their auditory cortex rewired and they regained most of the structural and physiological markers that were observed in normal rats.
Rhythm also seems to have a calming effect on newborns and children through several sensory modes. For instance, it is commonly known that older infants, children, and even adults employ various “comfort actions” of rocking back and forth, or doing some other form of rhythmic, repetitive action to relieve tension or stress. Various religious groups practice meditation and prayer using a combination of rhythmic chanting, rocking, and repetition of key phrases. Babies as young as four months old are calmed by repetitive sounds containing consonant rather than dissonant intervals. Auditory rhythms built using intervals such as the “perfect fifth,” with a pitch difference of seven semitones, or the “perfect fourth,” with a pitch difference of five semitones, are calming to babies and adults from all cultures. Hence the experience of repetition and rhythm is critically important during brain development and can have a marked impact on the behavior of babies, children, and adults.