The Pleasure Instinct: Why We Crave Adventure, Chocolate, Pheromones, and Music (18 page)

This sequence is yet another example of how the brain bootstraps its own development. When workers construct a suspension bridge, they first extend a thin cable across the body of water. They then use that small cable to hoist a larger one, followed by a third, and so forth. Before long they have created a thick cable of intertwined wires that can support a Friday afternoon rush hour. The brain does something quite similar in its development. Just enough physiological maturation occurs to facilitate functional capabilities that, in turn, permit stimulation for the next phase of development. We see this modus operandi again and again in brain development—across every sensory system.

Bootstrapping as a mechanism for individual development is somewhat analogous to ratchet effects in phylogenetic development. In an earlier example, we saw how the evolution of visual motion processing paved the way for early primates’ ability to forage among the fine branches that comprise the rain-forest canopy. Living in this new ecological niche created selection pressures for being able to clearly identify pigmented fruits and predators against a background of green forest. Compelling evidence from comparative studies demonstrates that trichomacy evolved in primates as a response to such selection pressures.

Bootstrapping is a highly efficient way to drive development, given that only twenty-five thousand or so genes are available to code the staggering amount of information required to grow a human. Clearly, not every step in development is written into the genes. Bootstrapping as a general developmental mechanism requires only information about the starting conditions and a means to encourage organisms to seek out the appropriate forms of experience necessary to stimulate further growth into the next stage.

A legacy of this process is that adult humans are strangely drawn to the same forms of visual stimulation that supported their brain development as newborns, babies, and toddlers. I’m not implying that the average adult finds pleasure in spending hour upon hour watching a ceiling fan or Big Bird look for Ernie. We are, however, attracted to the same general patterns of stimuli that bootstrapping
required
for normal brain development. This, of course, is manifested in a diverse number of ways in adults, some of which are undoubtedly flavored by cultural conventions.

Humans across different cultures are attracted to bright primary colors, scenes with pronounced lateral symmetry, and high-contrast objects.These biological preferences have an important influence in shaping what we find attractive. The advertising industry has been aware of these biases in our sensory processing for decades and designs product packaging that taps into these preferences. Often we are not even consciously aware of why we are attracted to a product. Keep in mind, however, that conscious awareness is not something evolution cares about. My son Kai does not need to
know
that he is attracted to faces and objects with bright colors for these forms of stimulation to benefit his visual development.

In the parlance of evolutionary biology, such innate preferences are sometimes called
receiver biases
. The term comes from an analogy in signal theory and has been applied to studies of animal communication. Modern communication devices can be built in two very different ways. One method, for example, is to develop a general device that uses a wide range of electromagnetic frequencies—much like the AM/FM radio. A problem with this general approach, however, is that the sender and receiver both have to be on the same frequency for communication to occur. There is a certain amount of luck in this happening, since the probability of the receiver and sender naturally sharing a channel decreases with an increasing number of frequencies.

An alternative approach is to build a receiver that is pretuned to specific frequencies. In this scenario, a radio will only pick up one or two selected frequencies, but with little potential for interference from other signals. Broadcasters who want to reach listeners with these radios will, of course, have to use devices that are specifically tuned to send signals at these frequencies. Only broadcasters who can send signals at the selected frequencies and with sufficient intensity will be successful in their communication attempts.

There are abundant examples where nature has followed suit—adopting either of the two strategies in intraspecies communication. The second approach is intriguing for our present discussion because receiver biases can arise from any number of sources.There are compelling examples where a physical feature that is found to be attractive by one sex and the preference for that feature by the opposite sex did not coevolve via genetic correlations. That is, a mating preference for a feature can sometimes emerge from developmental constraints rather than adaptations related to reproductive success.

It’s important to point out, however, that most receiver biases are probably not associated with pleasure. For instance, Bolivian anuran frogs have an auditory system that is tuned to best hear vocalizations such as mating calls at 800 hertz. This particular receiver bias is the result of the physical properties of mature hair cells embedded in a frog’s cochlea. In this case, stimulation of the frog’s auditory system at 800 hertz is not a developmental requirement for normal brain growth and maturation. Indeed, what seems to work best in the anuran species during development is broad-spectrum stimulation across many different frequencies. Hence this particular receiver bias is not a bootstrapping mechanism. Rather, stimulation of the frog’s auditory system at this frequency is thought to be related more to detecting potential mates that are, in turn, tuned to vocalizing at this particular frequency.

The types of receiver biases we have been focused on thus far are those linked to the activation of key pleasure circuits in the developing brain. These pleasure-related receiver biases persist into adulthood, when they may play a critical role in driving sexual selection.

As we have seen, sexual selection is often used as a theoretical framework for understanding mate choice, but it goes far beyond this realm in terms of explaining behavioral phenotypes. Evolutionary biologists have never had an easy time accounting for the appearance of so many uniquely human functions such as art, music, humor, and dance from a survival of the fittest perspective.This perspective neglects the obvious fact that our ancestors had to both survive
and
reproduce for their genes to make their way through the ages. Reproduction is itself a competitive act. Individuals must identify what traits are attractive to the opposite sex and do everything possible to amplify their appearance and to hide flaws that might reveal potential weakness. The pioneering biologist Amotz Zahavi argued that organisms are naturally attracted to very specific anatomical features that are used as fitness indicators. A classic example of this is the peacock’s elaborate plume. Peahens tend to be attracted to, and prefer mating with, the most highly ornamented peacocks. The question is why. One argument, discussed earlier, is that highly ornamented peacocks are simply more conspicuous to peahens and therefore better at attracting their attention, but careful field studies fail to support this theoretical position. Experiments that remove the impact of this variable by regulating the amount of time different peacocks are exposed to the same peahen still result in a mating preference given to the peacock with the most elaborate plume.

Zahavi and other biologists have taken the position that an elaborate plume signifies the biological fitness of a peacock, since it provides evidence that the animal is strong enough to survive even though the exaggerated plume diverts precious energy resources toward its growth and maintenance. It thus serves as an energy resource handicap that must be overcome. Others have suggested that besides the energy requirement for growth, maintaining an elaborate plume is like wearing a target, since being conspicuous to potential mates also means being conspicuous to predators. In this perspective, the plume indicates fitness because it represents a survival handicap in the context of predation.

The animal kingdom abounds in examples of sexual dimorphisms such as this, a trait that becomes exaggerated in one sex—often the male—and used to attract potential mates. But why do females of the same species develop preferences for these traits in the first place? Humans are clearly not exempt from this, although the exaggerated traits occur prominently in both sexes. From breast implants to the hair weave inspired by a midlife crisis, we spend vast amounts of time and money pursuing activities designed to improve our attractiveness to the opposite sex. However, many of these “improvements” have no obvious impact on our overall health or survival. Sexual selection theory has been used as a theoretical framework for explaining this human propensity for self-adornment.Although there is notable cultural and individual variability in descriptions of physical traits that are preferred in a potential mate, there is also surprising agreement across the globe on what makes a person physically attractive. Given that there is some consensus as to what makes a person attractive, how did these biases toward specific physical features emerge?

The same forms of visual stimuli that play a role in developmental bootstrapping reemerge in the adult as pleasure-inducing receiver biases. Babies who have an innate fondness for faces and strong symmetry grow up to be adults whose eyes linger longest on potential mates with maximally symmetric faces. Small receiver biases that are present at birth can be magnified by ongoing sexual selection. Say, for example, that most females prefer tall men. Even a small bias toward taller-than-average men will have a significant effect on the evolution of male height in generations to come. Given this bias, tall men will have a higher probability of fostering offspring than shorter-than-average men. Assuming both male height and the preference for tall men in women are genetically correlated, their offspring should be taller and, most importantly, prefer taller mates. Hence, the process of sexual selection can take small receiver biases and shape them into widely accepted notions that define physical attractiveness. In the next chapter, we will see how pleasure-inducing receiver biases have become important detectors of fitness during mate selection and hence serve as perhaps the most powerful driving force of sexual selection. This fundamental mechanism has had a profound impact on many facets of our everyday lives.

—William James

—George Santayana

 

 

 

As a boy, the young Charles Darwin showed no signs of the brilliance that was to shine by his twenty-fifth birthday. He and his older brother, Erasmus, grew up playing along the banks of the Severn River in Shropshire, the idyllic countryside setting of Jane Austen’s
Pride and Prejudice
. By most accounts he was an amiable if not astute child who could just as likely be found digging for beetles as attending to the lessons foisted on him by his Latin tutors.

His father, Robert, was a physician and expected the same from both of his sons, who were shipped off to Edinburgh to study medicine in 1825. At the tender age of sixteen, the youthful Charles Darwin was quite shocked by the pace of the city, exposed to a side of life for which he had little conception. The university at the time was the center of a raging debate over Scottish nationalism that seemed an unending battle for both God and country. The greens and lecture halls were filled with rabble-rousers, each coddling their own theological baby, Jacobites, Calvinists, Loyalists, all willing to argue their case. Nor were the faculty, lecturers, and readers immune to this carnival of ideas, some of which proved very dangerous indeed.

Charles learned that his grandfather Erasmus, who died several years before his birth, had somewhat of a cult following at the university. Erasmus was considered an irreverent man by friends and family, and in his later years lived an unconventional lifestyle, championing a free-love movement of sorts. He was trained as a medical doctor and botanist, and his scientific views on nature and religion were even more scandalous than his personal life. In one of his many poems, titled “The Temple of Nature,” we find the early seeds of a theory of evolution that his grandson would harvest some sixty years later:

 

Darwin returned home following his second year at Edinburgh, and after fretting for days, gathered enough courage to tell the family he was quitting medical school. He was less at ease with medicine after attending surgical grand rounds and witnessing operations that were then conducted without the benefit of local anesthesia—sights that must have been truly horrific experiences. Robert was outraged by what he took as his son’s apathy: “You care for nothing but shooting, dogs, and rat-catching and you will be a disgrace, to yourself and all your family.” He decided that if Charles was not to be a doctor, the only respectable alternative was for him to join the clergy. So the young Darwin set off to study theology at Cambridge University, with the aim of understanding the true design of God’s nature.