Part 5 (1/2)

In this context it is easy to imagine how pleasure might play a pivotal role in this process. As we have seen in the chapters to this point, the pleasure instinct drives the emergence of distinct receiver biases-preferences for certain forms of sensory stimulation that are critical for normal brain development and maturation. If pleasure-a.s.sociated preferences for particular forms of stimulation guide an organism toward traits that are also good fitness indicators, this combination may prove to be very useful during mate selection.

Imagine Sally is being pursued by Harry. If Sally makes her selection of a mate purely on the basis of fitness indicators without regard to whether they bring her pleasure, she might do so by simply summing up the tally and attributing an equal value to each indicator. A completely different approach would be to use the pleasure a.s.sociated with the appearance of a particular fitness indicator to gauge its current importance relative to others. In this context, pleasure is the common currency for organizing and prioritizing competing goals and interests. Using this approach, Sally would be able to a.s.sess and rank-order which fitness indicators are more important than others so she could make a more informed choice that matches her current needs. This process would be far more flexible and adaptive to changing environmental circ.u.mstances than one based on a simple summation of overall values across all indicators.

If, at this particular time, Sally has not eaten for two days and is faced with a choice between Harry, who has taken her to dinner, and Tom, who has taken her to a movie sans dinner, the choice will be very different depending on the approach. If she chooses solely on the basis of fitness indicators, either Harry or Tom will do, since they have both done things to display their fitness. Harry has shown that he can provide food, a critical resource for survival. Tom has shown that he has sufficient wealth that he can waste money on things not directly related to survival. Both have provided evidence of their fitness. If forced to choose simply on the basis of fitness indicators, Sally might have to toss a coin to decide. A different outcome would occur, however, if pleasure is used to rank-order the relative importance of fitness indicators to match her current needs. In this scenario, Harry is the clear winner, since he has delivered what she needs most at present-nutrition.

Seen in this light, pleasure provides the common reinforcement mechanism to drive and align motivated behaviors that may require very different forms of learning and that likely occur in different sensory systems. Pleasure is nature's shortcut; it enables humans to respond quickly to changing life demands by prioritizing basic needs that involve different neural systems on a single metric. The brain's motivational systems for signaling hunger for food and hunger for s.e.x are distinctly different. Both brain systems, however, interact with the brain-stem pleasure circuitry we have been discussing. The pleasure system, in this respect, is the common denominator that allows a direct comparison of the needs a.s.sociated with both competing hungers so that appropriate behavior can be chosen based on current priorities.

What are some examples of receiver biases (see chapter 8) crafted by the pleasure instinct that are also good fitness indicators? Certainly not all fitness indicators are consistent with the receiver biases created by the pleasure instinct. Likewise, most of the receiver biases we have discussed thus far are good fitness indicators, although there are, of course, exceptions. Let's look at a few cla.s.sic archetypes to guide our thinking.We will see that such biases occur in both s.e.xes in humans, since mating generally involves monogamous pair bonding.

The Hidden Persuaders It is clearly not a controversial claim to suggest that different cultures and different times tend to produce variable ideas of what is most physically attractive in a potential mate. Civilizations from the ancient Greeks onward have attempted to formulate canons for defining the ideal of physical beauty. Plato and Plotinus wrote extensively about the geometry of physical form and emphasized the inherent aesthetic appeal of things that exhibit strong symmetry, harmonious proportion, and vivid color. The emphasis on symmetry and proportion-elements that are quantifiable-began in the fifth century B.C. and has been built upon steadily by artists and philosophers ever since.

During the Renaissance, an explosion of aesthetic theories applied to human forms emerged, many of which concentrated on identifying the proper metric to measure true beauty. Leonardo da Vinci and Albrecht Durer were among the most popular artists who proposed geometric systems for measuring beauty based on symmetry and proportionality of body parts. During this period, a large number of measurement systems were proposed, each one emphasizing a particular set of metrics. For instance, Durer, inspired by the great Italian artist Jocopo di Barbari, created a formal theory of beauty based on anatomical proportions such as finding correspondence between finger length and palm width, arm length to an even-integer ratio of body length, and so forth. Although some artists from this period, such as Leon Battista Alberti, believed there was a single irreducible geometric form representing perfect beauty, da Vinci and Durer were more accepting of relative beauty in that many forms could be seen as being equally beautiful provided a few basic ratios were preserved.

The canonization of beauty continues today in modern attempts to determine if a universal definition can be formulated that is consistent across cultures. While it has proven difficult to show that any of the historical canons match up with what modern people actually find attractive, there have been interesting findings from some of these studies.

What we generally find in modern studies is that there is no ideal physical form that all will agree on as being beautiful based on pure mathematical principles-no golden ratio of beauty. However, there do seem to be certain physical traits that people from widely diverse cultures (Western, Middle Eastern, Eastern, north and south of the equator) agree on as being beautiful or attractive in a potential mate.The questions are:What are they? Why do we like these traits so much?

Pleasure from Proportion Let's start with generic body form. The most obvious quantifiable traits that characterize body appearance are weight and height. When one looks just within a single culture, it is easy to find that certain height and weight ranges are thought to be more attractive than others. In parts of North America and Europe, there tends to be a preference for tall and thin models (of both s.e.xes), while in some South American and Polynesian cultures, those with a little more weight are considered most attractive. Given this variability for ideal height and weight by different cultures, these are clearly not universally accepted traits for beauty. A young woman raised in the Bronx might look at a possible suitor who is tall, dark, and handsome with a winsome eye, yet the same man might be seen as meek and too skinny for the likes of a highlander from Papua New Guinea. Indeed, the man might be seen as meek and too skinny for the likes of another Bronx native. Barring extremes, it turns out that body weight and height are fairly poor predictors of whether an individual is generally found to be attractive.

What seems to matter most in determining attractiveness is the overall body shape of a person. People of the same height and weight can have bodies that look remarkably different. Body shape is driven by the distribution of body fat, and as we will see, this trait is significantly correlated with a woman's s.e.x hormone profile, reproductive capability, and risk of disease. In humans, the distribution of body fat depends on both age and gender. Boys and girls have strikingly similar distributions in infancy and early childhood.At p.u.b.erty, hormonal changes lead to a s.h.i.+fting of these distributions. Increased estrogen in postp.u.b.ertal girls blocks fat buildup in the abdomen and stimulates buildup in the b.u.t.tocks and thighs. Increased testosterone in postp.u.b.ertal boys does quite the opposite, causing increased fat deposition in the abdomen and decreased buildup in the b.u.t.tocks and thighs. In general, women have greater amounts of fat in the lower parts of the body (gynoid or pear shape), and men have greater amounts in the upper portions of the body (android or apple shape).

Differences in body shape can be reliably quantified by measuring the circ.u.mference of the waist and hips, and calculating a waist-to-hip ratio (WHR). Male and female prep.u.b.ertal children have similar WHRs. After p.u.b.erty, gender-specific hormonal changes s.h.i.+ft fat distributions within each s.e.x such that a woman's WHR tends to be smaller than that of a man. Healthy premenopausal women typically have a WHR from .67 to .80, whereas healthy men usually have WHRs between .85 and .95. Hence, in women, WHR can be used as a reliable means to gauge an individual's general reproductive status (pre- versus postp.u.b.ertal).

In the early 1990s, psychologist Devendra Singh of the University of Texas began publis.h.i.+ng a series of papers demonstrating that men and women from very different cultures display remarkable similarity in what WHR they find attractive in a female. You may find it odd that Singh used both males and females in his studies of female attractiveness, but this is a critical control from an evolutionary perspective. If a trait is to be a reliable marker of attractiveness, both the signal receivers and the signal generators must be aware of its meaning. Woman, realizing that a particular trait is seen as attractive by men, might wish to accentuate or attenuate it by various means (for example, makeup, clothes, posture, and so forth) based on the desire to indicate s.e.xual availability.

In his first series of studies, Devendra Singh created a set of line drawings depicting women with three body weight categories (underweight, average, and overweight). Within each weight category, he used line drawings to represent four WHRs. Two were typical gynoid shape with WHRs of .7 and .8, and the other two had a typical android shape with WHRs of .9 and 1.0. He and his colleagues showed the drawings to men and women of different ages (eighteen to eighty-five years old), professions, educations, and ethnicities, and asked them to rate each figure based on its attractiveness. The results were very interesting. Men and women rated drawings with a WHR of .7 as the most attractive within each weight category. The drawing seen as most attractive was of a female figure with a WHR of .7 and average weight.The drawings seen as least attractive were of female figures with .9 and 1.0 WHRs from the overweight category.

A possible critique of these findings might be that they have focused entirely on Western cultures, since Singh's studies only included Americans of European, Mexican, and African descent.To show that a preference for a particular trait is shaped by a selection process, the first step is to demonstrate that it is at work in humans of diverse cultures and ethnic groups. To see if WHR is indeed a universal marker of attractiveness, Singh next did a series of studies using men and women from nineteen different cultures. In these studies, he included people from America, Europe, Australia, Africa (Kenya, Uganda, Guinea-Bissau), the Azore Islands, the s.h.i.+wiar tribe of East Ecuador, Indonesia, China, India (Sugali and Yanadi tribes) Chile, and Jamaica. He showed the same line drawings as used in the earlier studies and asked subjects to rate their attractiveness, healthiness, youthfulness, and desirability as a marriage partner.

Despite coming from incredibly diverse cultural backgrounds, there was a clear preference for a WHR of .7 in each weight cla.s.s. Moreover, there was a strong correlation among the variables in the findings. Perceived attractiveness, youthfulness, and healthiness were ranked in an almost identical fas.h.i.+on across the different cultures. For these variables, positive rankings decreased systematically with increased WHR.

One might argue that there could still be the influence of Western media on these findings, since it tends to a.s.sociate a particular body shape with beauty. Granted, but this is unlikely to have impacted these results, since both the Azoreans and subjects from Guinea-Bissau had virtually no exposure to Western media, yet they ranked the drawings similarly to most other cultures, including those from the United States. Hence it seems that WHR may be a universal marker for attractiveness in a potential mate. In females, the maximal ranking seems to be about .7. Of course, as mentioned earlier, many different body types and images of beauty can have a WHR of .7. The cla.s.sic beauties Marilyn Monroe at 36-24-34 and Audrey Hepburn at 31.5-22-31 had very different hourgla.s.s figures, but shared the same WHR of .7.

The next logical question to ask, given the seemingly universal appeal of a .7 WHR in females, is why such a trait is important. Why do we find looking at a female with a .7 WHR more pleasurable than one with a WHR of, say, 1.0? The answer is not to be found in some ancient canon of beauty, prescribed by the divine or mathematical. Rather, it can be found in the way WHR reveals basic information about the bearer's general health and fecundity. Reproductive success for a man in ancient environments must surely have depended on selecting a mate with good health, genetic fitness, and excellent reproductive capacity. Of course, these characteristics are not directly observable; hence s.e.xual selection has shaped certain mental mechanisms for measuring genetic fitness indirectly. There is now convincing evidence demonstrating that WHR is a fairly good predictor of long-term health risk, mortality, and reproductive endocrinological status.

Women with a WHR lower than .8 have a significantly reduced risk relative to women with a WHR above .8 for key conditions that are known to hinder reproductive success and fertility, including hyperandrogynism, menstrual irregularity, suboptimal s.e.x hormone profiles (optimal is high estrogen and low testosterone), and abnormal endocervical mucus pH. Women with WHRs below .8 are also significantly more likely than their age-matched counterparts with WHR above .8 to have a successful pregnancy outcome after artificial insemination or in vitro fertilized embryonic transfer. Thus WHR is a reliable marker for estimating a woman's reproductive health.

It is important to note that these are statistical observations. Certainly, there are many women with WHR well above .8 who have perfect reproductive and general health. Likewise, there are undoubtedly loads of women with WHR of .8 and below who do not share this health. s.e.xual selection shapes traits that are expressed to various degrees within a population. It is nature's way of playing the odds. If a healthy man mates with a female with a WHR of .7, there is no guarantee of offspring. s.e.xual selection has crafted a psychological mechanism-a preference for females with a particular WHR range-that, all other things being equal, increases the odds of producing offspring who will preserve his genes, since the female has better odds of increased genetic fitness, disease resistance, and fecundity.

Seduction by Symmetry Another excellent example of a receiver bias crafted by the pleasure instinct that does double duty as a fitness indicator is our love of symmetry. In his book The Descent of Man and Selection in Relation to s.e.x The Descent of Man and Selection in Relation to s.e.x, Charles Darwin wrote, ”The eye prefers symmetry or figures with some regularity. Patterns of this kind are employed by even the lowest savages as ornaments; and they have been developed through s.e.xual selection for the adornment of some animals.” As we saw in the previous chapter, at about the time the primary visual cortex (V1) reaches maximal cell proliferation, babies begin to become keenly attracted to objects exhibiting strong lateral symmetry. As if on cue, this preference emerges just at the right time to promote experience-expectant synaptic pruning of V1 and downstream visual cortical areas, hence promoting normal maturational development. The pleasure obtained in self-stimulating V1 and downstream visual areas with highly symmetric objects as an infant forms the basis for a host of preferences as adults, including an attraction to things bearing strong lateral, rotational, and radial symmetry. The most obvious example of this is how this preference impacts our choice of mates.

As we have been discussing, the fitness indicator theory of s.e.xual selection suggests that individuals develop preferences for potential mates who possess traits that are reliably linked with good genetic quality and increased likelihood of bearing viable and vigorous offspring. Indeed, many s.e.xually selected traits (such as WHR) seem to have comparable genetic variability to that commonly observed for fitness traits (for example, fecundity), which indicates they may have evolved as signals of overall phenotypic condition.

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 h.o.m.ozygosity. 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 a.s.sociated 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 s.e.xual selection by mate choice and compet.i.tion would predict that fluctuating asymmetry should exhibit a strong relations.h.i.+p 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 a.s.sociation between fluctuating symmetry and measures of mating success. Females also exhibited a statistically significant relations.h.i.+p between fluctuating asymmetry and mating success in many species (especially humans), but the relations.h.i.+p was strongest in males across most species, which is exactly what would be predicted from evolutionary models of s.e.xual selection based on female choice. Choosier females would lead to greater s.e.xual selection in males, who would, in turn, develop more p.r.o.nounced traits (for example, ornamentation) to be a.s.sessed by females as part of their selection criteria. Accordingly, a s.e.xually selected trait such as fluctuating asymmetry should bear a stronger relations.h.i.+p 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 p.r.o.nounced relations.h.i.+p between fluctuating asymmetry and mating success for females in species where male choice dominates. In species where mate choice is more equitable among the s.e.xes, the relations.h.i.+p between fluctuating asymmetry and mating success should exist in both s.e.xes to roughly equal amounts.

A second important finding was that in most species, including humans, the relations.h.i.+p between fluctuating asymmetry and mating success was stronger for body parts involving secondary s.e.xual characteristics (traits that distinguish the two s.e.xes 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, b.r.e.a.s.t.s, 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 s.e.x with them, and others. In general, the most persistent and robust relations.h.i.+p 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 b.r.e.a.s.t.s. Why should this be the case? One might argue, perhaps, that it is exactly features such as the face, chest, and b.r.e.a.s.t.s 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 s.e.xual partner?

When considered from the ”good genes” perspective, it would seem that symmetry of secondary s.e.xual characteristics such as the face and b.r.e.a.s.t.s 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, a.s.suming that the preference and appearance of the trait are genetically correlated. This increase in the expression of the trait is tempered by metabolic costs a.s.sociated 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 s.e.xual characteristics might be more important indicators than symmetry of other body parts is that these features change dramatically at p.u.b.erty, announcing s.e.xual maturity. Take the face, for example. It is often difficult to determine the s.e.x 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 s.e.xes, but marked differences in facial appearance generally occur by p.u.b.erty. 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-s.p.a.ced eyes in girls. Clearly, the developmental growth of a face involves much more than simply scaling up the size of the prep.u.b.ertal face.

Many body parts undergo a qualitative change at p.u.b.erty where fat redistributes in a s.e.x-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, a.s.suming 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 ill.u.s.trations with anatomically correct compositions of a face when compared to control ill.u.s.trations 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.