One important marker of phenotypic condition that has caught the attention of researchers in the past decade is bilateral symmetry of the body. Imagine a line drawn from top to bottom down the middle of your body. If I were to take calipers and carefully measure the relative size of many body parts (such as the width of your feet, ankles, and ears, or the length of your fingers from both sides), I would typically find slight variations in relative size from left to right. The variations are usually small, on the order of about 1 percent of the overall size of the body part being measured. The distribution of these bilateral differences is referred to as fluctuating asymmetry, since departures from perfect symmetry vary randomly along the body axis. Fluctuating asymmetries are very different from directed asymmetry, such as handedness, since the former average out to zero in the general population.
Fluctuating asymmetry can be caused by a number of factors during development. Since the corresponding sides of the same body part are coded by the same genes, fluctuating asymmetries typically emerge from either environmental stressors or genetic perturbations within the genome that reduce developmental stability. Such stressors include things such as parasites, pathogens, pollutants, and other environmental challenges such as extreme temperatures or marginal habitats. Fluctuating asymmetries also increase with genetic perturbations caused by things such as inbreeding, the presence of certain recessive genes, chromosomal abnormalities, and homozygosity. Considering this, it is thought that fluctuating asymmetry is a measure of the extent to which an individual has been able to maintain a normal developmental trajectory by resisting such challenges. A person with a large number of harmful genetic mutations or who is less able to resist pathogens should on average exhibit greater fluctuating asymmetry.
There is now a substantial body of literature showing that fluctuating asymmetry is a reliable indicator of overall phenotypic quality. Increases are associated with decrements in biological fitness in a number of key domains, including reproductive success, growth rate, an ability to resist disease, metabolic efficiency, immunocompetence, and overall survival rate. Insofar as fluctuating asymmetry has been found to be partly heritable and is a reliable marker of phenotypic quality and biological fitness, several authors have suggested that ultimately it is a marker of genetic quality. As such, theoretical models of sexual selection by mate choice and competition would predict that fluctuating asymmetry should exhibit a strong relationship with mating success.That is, individuals with increased symmetry should, in general, enjoy more successful mating than their more asymmetric counterparts.
Indeed, in the majority of species tested, males with the highest degree of bilateral symmetry tend to have the greatest mating success. In a large-scale review of sixty-five studies involving forty-two species across four major taxa, biologists Anders Moller and Randy Thornhill found a number of interesting results that were predicted by theoretical models years earlier.
First, in the vast majority of species tested—from fruit flies to humans—males showed the strongest association between fluctuating symmetry and measures of mating success. Females also exhibited a statistically significant relationship between fluctuating asymmetry and mating success in many species (especially humans), but the relationship was strongest in males across most species, which is exactly what would be predicted from evolutionary models of sexual selection based on female choice. Choosier females would lead to greater sexual selection in males, who would, in turn, develop more pronounced traits (for example, ornamentation) to be assessed by females as part of their selection criteria. Accordingly, a sexually selected trait such as fluctuating asymmetry should bear a stronger relationship to mating success in males who have to compete with each other for female attention in species where female choice dominates.
In contrast, there should be a more pronounced relationship between fluctuating asymmetry and mating success for females in species where male choice dominates. In species where mate choice is more equitable among the sexes, the relationship between fluctuating asymmetry and mating success should exist in both sexes to roughly equal amounts.
A second important finding was that in most species, including humans, the relationship between fluctuating asymmetry and mating success was stronger for body parts involving secondary sexual characteristics (traits that distinguish the two sexes of a species but that are not directly part of the reproductive system) than for other parts. For instance, in the dozen or so studies of humans, investigators have examined symmetry at several stops along the primary axis of the body, including the feet, ankles, hands, fingers, arms, chest, shoulders, ears, face, breasts, and the overall figure. In terms of gauging mating success, researchers have measured things such as the rated attractiveness of a potential mate, the likelihood of accepting a date with that person, the likelihood of engaging in sex with them, and others. In general, the most persistent and robust relationship between fluctuating asymmetry and mating success in humans was found to involve parts of the body that are most meaningful during intimate encounters, such as the face, shoulders, chest, and breasts. Why should this be the case? One might argue, perhaps, that it is exactly features such as the face, chest, and breasts that naturally draw our attention because we find them pleasurable, so we are inclined to tune in to them when deciding about a possible mate. But this is a circular argument in this context. True, we focus on these features because we find viewing them pleasurable—certainly more so than focusing on a potential mate’s ear length or ankle width. But the primary question is:Why is it so much fun to look at these features in the first place? A second, related question is: Why do these features carry more relevant information (in terms of fluctuating asymmetry) than, say, our feet, knees, or elbows when gauging the suitability of a sexual partner?
When considered from the “good genes” perspective, it would seem that symmetry of secondary sexual characteristics such as the face and breasts varies most strongly with mating success because these features are hard to fake (at least in ancestral times) and are honest indicators of true fitness. For instance, the biological complexity and metabolic cost of building a face makes it particularly susceptible to genetic or environmental perturbations during development that would leave a visual record of such events in the form of increased asymmetry. As discussed earlier, preferred features that are genuinely related to fitness should increase in prevalence, assuming that the preference and appearance of the trait are genetically correlated. This increase in the expression of the trait is tempered by metabolic costs associated with its development. Without such constraints, a simple Fisherian model would predict a runaway process of the trait becoming ever exaggerated and everyone having maximal symmetry throughout the population.This obviously does not occur. Body parts that are the most costly to build are the best candidates for being honest fitness indicators, since they have developed despite metabolic costs and the possibility of environmental or genetic perturbations.
Another important reason why symmetry of secondary sexual characteristics might be more important indicators than symmetry of other body parts is that these features change dramatically at puberty, announcing sexual maturity. Take the face, for example. It is often difficult to determine the sex of a baby by just looking at its face if no supporting clues are available such as gender-typical clothing. Toddlers can also have very similar facial appearances across the sexes, but marked differences in facial appearance generally occur by puberty. During adolescence hormonal changes sculpt these differences. Boys’ faces become larger and more angular, especially the lower jaw and brow ridge. Girls’ faces retain a smoother forehead and smaller lower jaw, giving a rounder impression.A smaller nasal bridge relative to boys gives the impression of larger and wider-spaced eyes in girls. Clearly, the developmental growth of a face involves much more than simply scaling up the size of the prepubertal face.
Many body parts undergo a qualitative change at puberty where fat redistributes in a sex-specific manner. By comparison, however, the face undergoes extreme changes with many more opportunities for environmental and/or genetic challenges to the developmental stability needed to achieve perfect symmetry. This sensitivity to challenges that occur during development is what makes facial symmetry a potential selection mechanism for identifying mates, assuming it correlates with actual fitness. So let’s see what is so important about faces that make them the center of attention in the mating game.
A Fit Face
As we saw in earlier chapters, there is something special about faces that makes them a naturally pleasurable stimulus. Even babies that are a mere ten minutes old gaze longest (a proxy for measuring preference in infants) at illustrations with anatomically correct compositions of a face when compared to control illustrations that have all the same components but are ordered in a random manner.They will also visually track a line drawing of a face at this age. Right out of the womb, babies have a preference for faces. Within another day, newborns develop a preference for their mother’s face as opposed to that of other similarly aged women who have recently given birth. By day three, infants can mimic certain facial expressions, such as sticking out a tongue in response to a similar gesture from Mom or Dad. Add a few months and infants develop an ability to discriminate one unfamiliar face from another and detect different emotional expressions, of which they prefer joyful over angry faces.
There might be many different reasons why faces seem naturally interesting and attention-grabbing to humans. The prevailing theory is that an infant’s fascination with faces emerges as an adaptive mechanism to promote parent-child attachment. Being able to recognize and engage the primary caregiver increases the likelihood that an infant will become emotionally bonded with that individual and receive proper nurturance. The need to recognize, engage, and extract information from faces continues, of course, through childhood and into adulthood. Being able to read the minds of others in a social group is also important for survival and reproductive success. Humans can’t read minds, but the next best thing is being able to understand the emotional mind-set of your peers. No other body part even comes close to yielding such rich emotional information about the bearer as is the case with the face.
The “face as a kin recognition device” theory is well supported in the literature and is consistent with a large number of primate studies, including humans.There may, however, be additional reasons why infants (and adults) find faces so pleasurable. Faces are composite objects made up of smaller, complex stimuli—eyes, lips, nose, jaw, brows, skin, and so forth. Each of these elemental objects is itself a potentially rich source of stimulation for growing brains and indeed has physical characteristics that are known to be naturally preferred at or near birth by newborns. For instance, newborns have a preference for stimuli with strong lateral symmetry and can recognize vertically symmetric objects (symmetric around a vertical axis like the letters “A” and “V”) more quickly than asymmetric objects.They also prefer objects that are smooth rather than rough, complex rather than simple, and have high-contrast contours, curves, and concentricity. Faces generally have smooth skin punctuated by high-contrast elemental objects. The elemental objects all exhibit strong lateral symmetry, high-contrast curves, and a high degree of concentricity (as does the whole face).
In this respect, the face might be regarded as a veritable treasure trove of pleasure-inducing stimuli. For the newborn, even an unfamiliar face is pleasure-inducing because it stimulates multiple core features across multiple sensory domains (for example, touch, audition, and vision) that its experience-expectant brain requires for normal development. Clearly, the pleasure experienced by a newborn looking into a caregiver’s face only increases over time as it comes to recognize this person and bond with her or him. Before this occurs, however, there must be a neural mechanism that increases the likelihood that the newborn spends more time looking at faces rather than, say, knees. The face provides a hedonic wonderland for the newborn, since a single experience can stimulate developing visual, somatic (touch), and auditory cortical regions in an integrated manner. As we saw in chapter 7, newborns also prefer certain forms of auditory stimulation, such as sounds that have slowly increasing and decreasing pitch contours—the singsong melody of motherese. Motherese, of course, emanates from mouths embedded in faces. Such pleasurable sounds serve to draw the attention of the newborn to the face, where they can be grouped together with other pleasurable features.
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 glasses 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.