The Pleasure Instinct: Why We Crave Adventure, Chocolate, Pheromones, and Music (20 page)

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 sex 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 associated 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 classic archetypes to guide our thinking.We will see that such biases occur in both sexes in humans, since mating generally involves monogamous pair bonding.

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 Dürer 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, Dürer, 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 Dürer 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?

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 sexes), 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 sex 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 puberty, hormonal changes lead to a shifting of these distributions. Increased estrogen in postpubertal girls blocks fat buildup in the abdomen and stimulates buildup in the buttocks and thighs. Increased testosterone in postpubertal boys does quite the opposite, causing increased fat deposition in the abdomen and decreased buildup in the buttocks 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 circumference of the waist and hips, and calculating a waist-to-hip ratio (WHR). Male and female prepubertal children have similar WHRs. After puberty, gender-specific hormonal changes shift fat distributions within each sex 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 postpubertal).

In the early 1990s, psychologist Devendra Singh of the University of Texas began publishing 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 sexual 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 Shiwiar 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 class. Moreover, there was a strong correlation among the variables in the findings. Perceived attractiveness, youthfulness, and healthiness were ranked in an almost identical fashion 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 associate 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 classic beauties Marilyn Monroe at 36-24-34 and Audrey Hepburn at 31.5-22-31 had very different hourglass 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 sexual 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 sex 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. Sexual 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. Sexual 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.

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 Sex
, 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 sexual 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 sexual 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 sexually 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.