Part 5 (2/2)

Not only do infants find faces pleasurable, they also differentiate attractive versus unattractive faces in much the same way as adults. In a series of compelling studies, psychologist Judith Langlois and her colleagues at the University of Texas at Austin found that infants as young as two months old prefer to look at attractive faces more than unattractive faces as rated independently by adults. Langlois and her colleagues began their study by taking a large collection of color slides of female faces to groups of undergraduate men and women and asking them to rank each slide based on its attractiveness from 1 (least attractive) to 5 (most attractive).The female faces were posed with a neutral emotional expression and gla.s.ses removed. Moreover, all clothing was masked so that judgments could be made based on facial features alone. There was remarkably high agreement across raters on attractiveness (0.97 coefficient alpha). Two groups were formed based on the rankings, eight slides with the highest attractiveness rating and eight slides with the lowest rating.

To determine preferences, infants were seated in their mothers' laps and shown a series of two slides positioned side by side. One slide was from the most attractive group, the other from the least attractive group. Recordings were made of the amount of time each infant spent looking at the different faces. Differential fixation time is a common metric for evaluating preferences in infants. This metric generalizes in that children and adults also look longer at self- and independently rated attractive faces than at unattractive faces. Langlois found that infants spent significantly more time gazing at faces from the most attractive group than at faces from the least attractive group. This result has since been replicated and extended to show that infants also prefer more attractive faces to lesser attractive faces within other groups, including both male and female adult Caucasians, male adult African Americans, and other infants.

Needless to say, this finding was not consistent with the prevailing theories of child development at the time. Before Langlois began her research, many researchers a.s.sumed that preferences for attractiveness were based on the gradual learning of standards within a culture through a variety of sources (for example, media and social experiences) and that these emerged much later during development. Her data are at odds with this view, since these infant subjects have presumably had very limited exposure to such forces, in that they are preverbal and only two months old. Rather, these findings suggest that preferences for attractiveness are either in place at birth or shortly thereafter.

At present, many studies have shown remarkable agreement across raters from different age, gender, and cultural groups in terms of facial attractiveness rankings. Cross-cultural studies have been done with people in the United States, throughout Europe, China, Korea, South America, and Asia using multicultural faces, with fairly consistent agreement on which faces are the most attractive. A major question that is the focus of much investigation is what features humans use to make determinations about attractiveness and why, Studies by evolutionary biologists and psychologists have found that people sometimes find the average attractive. Take a thousand faces, average their spatial characteristics, and you get a new face representing the group norm, which is generally rated to be slightly above average in attractiveness. In his cla.s.sic text on the evolution of human s.e.xuality, anthropologist Donald Symons hypothesized that natural selection drives adaptations, where at the population level, the optimum value of some trait is likely to be the mean. In this respect, a preference for the average value of a trait such as facial characteristics would be wise, since it would push the chooser toward mates with optimally adapted facial traits for things such as breathing, chewing, and any other functions linked genetically to the development of facial features. Randy Thornhill and psychologist Steve Gangestad theorized that a preference for averageness occurs because for some heritable traits, the mean value represents maximal genetic heterozygosity (allelic diversity).

Other studies have found that people tend to focus on secondary s.e.x characteristics when making judgments of attractiveness. In humans and many other species, hormonal changes occurring at p.u.b.erty can actually handicap an individual. For instance, increased testosterone production in adolescent males leads to increased musculature and energy expenditure. These changes, in turn, raise the metabolic demands of the body and draw resources away from other systems, including immune functioning. Since testosterone production creates a draw on immune function, it may be less costly for genetically fit males to have high levels of testosterone than for those more susceptible to environmental and/or genetic perturbations (for example, pathogens, maladaptive environmental conditions, genetic mutations). Likewise, increased estrogen production in p.u.b.ertal girls indicates reproductive potential and fertility. Reproductive effort places increased metabolic demands on a woman's body and draws resources away from other important biological processes, including immune function. Hence, secondary s.e.xual characteristics of the face that become exaggerated at p.u.b.erty (such as an extension of the lower jaw and broadening of the brow ridge in males) may be honest signals of phenotypic and genotypic condition.

In addition to averageness and attention to secondary s.e.xual characteristics, many studies have found a strong relations.h.i.+p (perhaps not surprisingly) between ratings of facial attractiveness and fluctuating asymmetry of the face. Indeed, as mentioned earlier, in their extensive review, Anders Moller and Randy Thornhill found that in most species, the correlation between fluctuating asymmetry and mating success was strongest for body parts that are secondary s.e.xual characteristics. The face is one such body part, since, as we have seen, it is replete with secondary s.e.xual characteristics that become differentiated at p.u.b.erty.

Numerous studies have demonstrated that both facial and body symmetry involving secondary s.e.xual characteristics are related to reproductive success and health in general. In a diverse range of species tested, increased fluctuating asymmetry has been shown to be related to decreased fecundity, growth rate, survival, and metabolic efficiency. For instance, increased fluctuating asymmetry in men has been shown to be related to a number of fertility measures. Population biologist John Manning and his colleagues from the University of Liverpool studied males referred to a reproductive medicine clinic at a local hospital for routine s.e.m.e.n a.n.a.lysis.They found that men with greater body asymmetry had fewer numbers of sperm per e.j.a.c.u.l.a.t.e, lower sperm speed, and reduced sperm migration relative to their more symmetric counterparts. In women, breast asymmetry has been found to correlate negatively with fecundity and to the probability of marriage.

Other studies have shown that body symmetry is related to how efficiently our bodies use energy. If one has a normally low metabolic cost a.s.sociated with the maintenance of body processes, this should theoretically free energy for use in other ways, such as maximizing developmental homeostasis. Consistent with this ideal, Manning's group has also found that people with greater body symmetry exhibit a lower resting metabolic rate, measured as oxygen consumption at rest.

The relations.h.i.+p between overall physiological health and body symmetry has also been examined by several investigators. Gangestad and Thornhill gave a series of health questionnaires to 203 romantically involved couples where both the man and the woman rated self and partner. The questions asked about the individual's current condition in eight specific physical health domains, including muscularity, energy, stamina, vigorousness, robustness, lethargy, physical tightness, and cardiovascular fitness. The researchers found a significant negative correlation between a composite physicality score (summed from the individual domain scores) and body asymmetry. As asymmetry increased, perceived general physical health decreased.

Other investigators have found similar results when focusing just on facial asymmetry. In a study of 101 college students, evolutionary psychologists Todd Shackleford and Randy La.r.s.en examined the relations.h.i.+p between facial asymmetry and a broad of range of physical, emotional, and psychological health indicators. Head shots of students were taken and used to extract estimates of facial asymmetry using bilateral measurements at the outer eye, inner eye, nostril width, cheekbone width, and jaw width. Each subject was also given an extensive battery of health questionnaires concerning mood and emotional state, personality, life orientation, sociability, impulsivity, and general symptomatology. Additionally, subjects were requested to complete reports on their daily activities, moods, and physical symptoms twice each day. These included standard physical symptoms such as headaches, trouble concentrating, runny nose, sore throats and coughs, gastrointestinal problems, and so forth. Finally, each partic.i.p.ant had his or her overall aerobic fitness a.s.sessed by measuring cardiac recovery time following a standard exercise protocol to raise their heart rates to at least thirty beats per minute beyond resting state.

A separate set of college-age observers were asked to rate each photograph of the subjects along a number of dimensions including attractiveness, happiness, reliability, agreeableness, intelligence, emotional stability, activeness, and others. The overall results of the study painted a complex picture revealing how variation in facial asymmetry impacts several key aspects of daily life in the bearer and how others view them.

In terms of physical health, subjects with greater facial asymmetry were more likely than their symmetric counterparts to report negative physical symptoms, such as backache, muscle soreness, reduced vigor, and trouble concentrating. They were also more likely to complain of depression, perform more impulsive acts, and view their lives as being outside of their personal control. Men with greater facial asymmetry, in particular, tended to score higher on measures of mania and schizophrenia components of the Minnesota Multiphasic Personality Inventory (MMPI), a cla.s.sic clinical a.s.sessment, than those who were more symmetric. Consistent with this observation, several studies have now shown that schizophrenia in adults and hyperactivity disorder in boys are both a.s.sociated with increased body and facial fluctuating asymmetry.

On the flip side, individuals with relatively more facial symmetry when compared to their counterparts were more likely to be optimistic, view themselves as superior, and score higher on measures of narcissism. Interestingly, extroversion seems to correlate positively with the degree of facial asymmetry in women and negatively in men. Women with more facial asymmetry tend to be more extroverted, while men tend to be more introverted.

Independent judgments by external observers were also systematically related to the degree of facial asymmetry in the subjects. Individuals with greater facial symmetry were viewed by others as being more conscientious, intelligent, active, agreeable, and genuine than those with less symmetry.This study confirms other reports that facial asymmetry correlates with a number of important markers of physiological, emotional, and psychological health.

Symmetry Signals and Pleasure Thus far we have found that increased body and facial asymmetry are a.s.sociated with decreases in certain indicators of fertility, as well as self-reported physical, emotional, and psychological well-being. But does greater facial symmetry translate into improved mating success? In other words, do people actually have a preference for symmetric mates?

Symmetry is a basic property that is preferred and sought after by newborns and toddlers alike. They recognize more quickly and exhibit greater pleasure in viewing symmetric objects than those that are asymmetric. I have shown in earlier chapters that an anatomical and developmental imperative exists for symmetry-seeking almost immediately after birth, since it represents an optimal form of stimulation for experience-expectant maturation of the primary visual cortex and additional downstream visual areas (for example, V2, V3, and inferotemporal cortex).

The preference for symmetry right after birth would be expected to continue through childhood, since synaptic pruning of these visual cortical areas continues for decades.This early preference thus forms the basis for a fondness of symmetry in adults. Moreover, since body and facial symmetry are related to general health (and are, hence, potential fitness indicators), an inborn preference for symmetry that mediates normal brain growth and development would be a perfect trait to be co-opted through s.e.xual selection mechanisms. If this scenario is true, one would predict all six of the following conditions to be true as well.

Condition 1 The preference for symmetry is expressed at or very near birth. The preference for symmetry is expressed at or very near birth. Cell proliferation in the primary visual cortex is maximal at about the time that infants begin to take pleasure from highly symmetric objects. Stimulation of V1 and related downstream visual cortical areas is required for normal brain growth and development, including synaptic pruning (see chapter 8). Laterally symmetric objects represent an ideal form of stimulation during this developmental period, since they have redundant spatial information. Such redundancy may facilitate the increased recognition speed that infants exhibit in processing symmetric relative to asymmetric objects. Cell proliferation in the primary visual cortex is maximal at about the time that infants begin to take pleasure from highly symmetric objects. Stimulation of V1 and related downstream visual cortical areas is required for normal brain growth and development, including synaptic pruning (see chapter 8). Laterally symmetric objects represent an ideal form of stimulation during this developmental period, since they have redundant spatial information. Such redundancy may facilitate the increased recognition speed that infants exhibit in processing symmetric relative to asymmetric objects.

Condition 2 The preference for symmetry generalizes across many object forms. The preference for symmetry generalizes across many object forms. This condition is critical to the hypothesis that a preference for symmetry originated as a mechanism for ensuring normal brain development and maturation independent of its co-option as a fitness indicator through s.e.xual selection. If a fondness for symmetry in newborns is only fulfilling a role as a potential fitness indicator, we would expect to see the preference emerge strictly in relation to bodies and faces-those objects to which fitness most applies. On the other, if the preference for symmetry in newborns is first and foremost a mechanism to facilitate brain growth, one would expect that all types of symmetric objects are preferred to their asymmetric counterparts. Indeed, the data show the latter to be true, since newborns merely a few months old prefer a wide range of symmetric objects to their asymmetric versions. Such objects include abstract drawings, solid geometric forms, landscape features, faces, and many others. This condition is critical to the hypothesis that a preference for symmetry originated as a mechanism for ensuring normal brain development and maturation independent of its co-option as a fitness indicator through s.e.xual selection. If a fondness for symmetry in newborns is only fulfilling a role as a potential fitness indicator, we would expect to see the preference emerge strictly in relation to bodies and faces-those objects to which fitness most applies. On the other, if the preference for symmetry in newborns is first and foremost a mechanism to facilitate brain growth, one would expect that all types of symmetric objects are preferred to their asymmetric counterparts. Indeed, the data show the latter to be true, since newborns merely a few months old prefer a wide range of symmetric objects to their asymmetric versions. Such objects include abstract drawings, solid geometric forms, landscape features, faces, and many others.

Condition 3 Objects that have multiple markers of symmetry, such as faces, are especially pleasurable. Objects that have multiple markers of symmetry, such as faces, are especially pleasurable. If a general preference for symmetrical features exists independent of the form in which they appear, objects that have multiple salient points that exaggerate this condition should be especially pleasurable to view. Newborns tend to look longer (a proxy for preference) at symmetrically complex objects than at simpler symmetric objects. Newborns also prefer line drawings of faces with the proper symmetry of features preserved more than drawings where the features are s.h.i.+fted and symmetry is broken. Taken together, such evidence indicates that objects with multiple salient features that emphasize symmetry should be even more pleasurable and preferred to simpler, symmetric forms with fewer features. If a general preference for symmetrical features exists independent of the form in which they appear, objects that have multiple salient points that exaggerate this condition should be especially pleasurable to view. Newborns tend to look longer (a proxy for preference) at symmetrically complex objects than at simpler symmetric objects. Newborns also prefer line drawings of faces with the proper symmetry of features preserved more than drawings where the features are s.h.i.+fted and symmetry is broken. Taken together, such evidence indicates that objects with multiple salient features that emphasize symmetry should be even more pleasurable and preferred to simpler, symmetric forms with fewer features.Condition 4 Symmetry is a reliable marker of phenotypic quality. Symmetry is a reliable marker of phenotypic quality. As we have just reviewed, body and facial symmetry are related to improved markers of fertility in both men and women. Both body and facial symmetry have also been shown to be related to better self-reported physical, emotional, and psychological health and well-being. Moreover, independent observers rank people with highly symmetric faces as more conscientious, agreeable, and intelligent, among other positive traits, compared to faces bearing less symmetry. Hence there is evidence that individuals with greater symmetry enjoy better health and fitness and that they are perceived by others as being more likely to possess certain positive personality traits in comparison to their less symmetric counterparts.These and other studies suggest that body and facial symmetry are reliable markers of phenotypic condition, and thus might be used as fitness indicators during mate selection. As we have just reviewed, body and facial symmetry are related to improved markers of fertility in both men and women. Both body and facial symmetry have also been shown to be related to better self-reported physical, emotional, and psychological health and well-being. Moreover, independent observers rank people with highly symmetric faces as more conscientious, agreeable, and intelligent, among other positive traits, compared to faces bearing less symmetry. Hence there is evidence that individuals with greater symmetry enjoy better health and fitness and that they are perceived by others as being more likely to possess certain positive personality traits in comparison to their less symmetric counterparts.These and other studies suggest that body and facial symmetry are reliable markers of phenotypic condition, and thus might be used as fitness indicators during mate selection.

Condition 5 Adults prefer symmetrical bodies and faces. Adults prefer symmetrical bodies and faces. If body and facial asymmetry are fitness indicators that are used during mate selection, individuals should be able to detect variation in symmetry and prefer it in a potential suitor. Consequently, everything else being equal, individuals with greater body and facial symmetry should be seen as more attractive than those who are less symmetrical. By extension, individuals with the greatest body and facial symmetry (again, all else being equal) should have greater mating success than their asymmetric counterparts. If body and facial asymmetry are fitness indicators that are used during mate selection, individuals should be able to detect variation in symmetry and prefer it in a potential suitor. Consequently, everything else being equal, individuals with greater body and facial symmetry should be seen as more attractive than those who are less symmetrical. By extension, individuals with the greatest body and facial symmetry (again, all else being equal) should have greater mating success than their asymmetric counterparts.

Condition 6 Adults prefer symmetrical to asymmetrical versions of most objects in general, even those unrelated to faces and bodies. Adults prefer symmetrical to asymmetrical versions of most objects in general, even those unrelated to faces and bodies. If a preference for symmetry emerges in newborns to guide proper brain development through experience-expectant mechanisms, such stimulation need not be in one specific form or another. Faces are excellent forms of stimulation in this scenario since, as we have seen, they possess many cardinal features that activate the pleasure instinct and facilitate neural development. But all symmetric objects should facilitate this process to some degree. Hence, if the preference for symmetry in newborns and children is carried over to adulthood, we would expect it to generalize to other object forms beyond faces and bodies. Let us now turn to a discussion of the evidence for the final two conditions. If a preference for symmetry emerges in newborns to guide proper brain development through experience-expectant mechanisms, such stimulation need not be in one specific form or another. Faces are excellent forms of stimulation in this scenario since, as we have seen, they possess many cardinal features that activate the pleasure instinct and facilitate neural development. But all symmetric objects should facilitate this process to some degree. Hence, if the preference for symmetry in newborns and children is carried over to adulthood, we would expect it to generalize to other object forms beyond faces and bodies. Let us now turn to a discussion of the evidence for the final two conditions.

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 s.e.xes 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 s.e.xually 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 s.e.xual partners during a lifetime, and an increased frequency of s.e.xual affairs with partners outside of their primary relations.h.i.+p, begin to have s.e.x earlier in life, and produce disproportionately more copulatory female o.r.g.a.s.ms 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 h.o.m.ozygous 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 h.o.m.ozygous counterparts-had the most symmetrical faces. Moreover, the women rated these faces as the most attractive.

Nonfacial secondary s.e.xual 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 relations.h.i.+ps. Like faces and other secondary s.e.xual characteristics, when observed across a population of individuals, fluctuating asymmetry of b.r.e.a.s.t.s 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 cla.s.sic 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 a.s.sociation 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 s.e.xual selection processes. In the next chapter we will discuss a temporal example and examine the complicated manner in which our preference for repet.i.tion and rhythm is expressed in our everyday lives.

Chapter 10.

Pleasure from Repet.i.tion and Rhythm It appears probable that the

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