The Pleasure Instinct: Why We Crave Adventure, Chocolate, Pheromones, and Music (11 page)

The emergence of this evolutionary adaptation in our species is echoed in the development of each individual with the growth and maturity of brain pathways that connect olfactory cortical areas with midbrain and orbitofrontal regions that mediate natural reward. While the midbrain reward centers develop at a fairly early embryonic stage in humans, the pathways that connect these regions to the areas responsible for perceiving odors do not mature until rather late in gestation and are known to depend on experience. Because this is the case, many scientists believe that although these olfactory preferences are very similar across cultures, their development probably results from learning to associate these smells (in the womb) with flavorful food and the onset of an intrinsically rewarding behavior—eating.

Consummatory behaviors such as eating and sexual activity are known to increase levels of circulating neuropeptides called endorphins, those lovely chemicals that provide a feeling of relaxation and calm and that are chemically similar to morphine.The development of these pathways—which depend on exposure to odors that signal the presence of potential sources of nutrition—may occur during the last trimester, while the fetus is exposed to the coappearance of certain smells and tastes with an increase in amniotic endorphin levels that have a calming effect on mother and fetus alike. Hence, the emergence of “universal” olfactory preferences is likely to result from the same learning mechanisms that mediate olfactory labeling in all mammals.

Like it or not, we smell, and the subtle odorous messages we send and receive—often unknowingly—have a profound influence on our social identities and a wide range of behaviors, including mate selection, courtship, and the timing of ovulation.The word
pheromone
calls up a variety of images to mind: mammals communicating using a hidden language of scents; trendy socialites paying $300 per ounce for a vial of boar effluvia that promises to allure the opposite sex; and sorority sisters who menstruate in synchrony month after month.

Although it has proven rather difficult to isolate and identify a human pheromone, there is a growing body of evidence that we use them to communicate chemically much like other mammals.The first convincing evidence came from an unexpected place—an undergraduate dormitory room at Wellesley College. In 1967, an undergraduate student named Martha McClintock noticed that many of the girls in her dorm menstruated on the same days and wondered if such coordination might have survival value. She asked two simple questions in her research project: “When did you last menstruate?” and “Who are your two best friends?” The results surprised everyone.Women who spent the most time together tended to menstruate at the same time.

It wasn’t until ten years later that the mechanism that causes this synchrony (now known as the McClintock Effect) was discovered. In a simple experiment, psychologist Michael Russell and his colleagues at Sonoma State Hospital in California rubbed an extract from the underarm of a woman with a very regular twenty-eight-day cycle under the noses of sixteen other women three times a week. Within four months, all of the women were menstruating within three days of one another.The odors of a single person, it turns out, can influence the menstrual cycles of many others. It was still unclear, however, how such an effect might have survival or adaptive value.

The answer came the following year, when it was discovered that men have cycles as well, and that their regular rise and fall in core body temperature and the production of essential steroids such as testosterone can be modulated by the presence of other males. The final link came when additional experiments showed that the production of testosterone and other androgens in men often becomes synchronized to the menstrual cycles of their wives and lovers.Taken together, an impressive display of synchrony emerges between men and women in close contact with each other on a regular basis, and this patterning might facilitate the timing needed for effective sexual reproduction.

In addition to regulating menstrual and physiological cycles, pheromones have a say in other very personal affairs, such as distinguishing those we find sexually attractive from those who remind us of a sibling. But what is the physical basis for this hidden conversation of scents? Humans don’t seem very interested in smelling the urine or underarm odor of potential mates, so where do human pheromones come from and what do they smell like?

The Desana Indians of the Amazon rain forest have a cosmological worldview built around olfaction. For them, the essence of a person is revealed by their smell, which emanates directly from their bones. Mores that guide courtship and social relationships are intertwined with the relationships among different smells. The Desana believe that people of the same tribe share a common smell, and strict rules exist forbidding marriage between those who have similar scents—an olfactory-inspired incest taboo. Certain smells should never be mixed, yet some naturally go together. An answer to the puzzle of how human pheromones influence behaviors such as mate selection came out of left field and was inspired by studies of the Desana.

It has been known for ages that foreign tissue implanted into a host is often rejected by the host’s immune system. Each of our own cells bears proteins that our immune system recognizes as “self.” When a foreign cell enters the body, the immune system attempts to classify the nonself intruder by attaching a labeling protein to it and generating antibodies designed to destroy it. The immune system has a memory for intruder cells, and the next time the same foreigner is encountered, the antibodies can be launched even faster, since they do not need to be generated from scratch.

A segment of our DNA called the major histocompatability complex (MHC) codes for the immune cells that identify intruding disease organisms, essentially functioning as our immune system’s first line of defense. Unlike many genes that have only a few alternative versions (called alleles), MHC genes have upward of a hundred or so, with each providing immunity against different sets of potential disease strains.

When we think of heredity, we typically have in mind the classical pattern of single allele combinations—the dominant-recessive pairings that play a winner-take-all game with traits such as eye and hair color. If one parent has blue eyes and the other brown, one gene will dominate, meaning the gene from the other parent that controls this trait is not expressed.

MHC genes work differently in that they are codominant. Say you inherit one version of an MHC gene from your father that improves resistance to disease A and another version of the gene from your mother that happens to help fight disease B. Since MHC genes are codominant, you will be able to resist both diseases. Thus, parental combinations that have the greatest degree of MHC genetic heterozygosity
³
will produce offspring with the most robust immune functioning.These offspring would have a distinct survival advantage over offspring from parents who have considerable overlap in their MHC genes, providing resistance to a smaller spectrum of disease strains. Variation in this dimension, then, can serve as an important selection factor in the evolution of our species. The question is, what serves to attract us toward mates who have MHC genes different from our own?

The initial clues emerged from animal experiments. If a female mouse is offered two suitors, she inevitably chooses the mate whose MHC genes have the least overlap with her own, and it is now known that they do this through smell. Scientists have found that each version of the MHC gene codes protein by-products that are excreted from the body, and they have a unique odor. Mice that have damage to their olfactory nerve or olfactory epithelia cells perform this test at chance levels—deciding on the heterozygous mate only about half of the time.

So if you’re a smart rodent, a big part of your mate selection process is in deciding if a suitor has the right smell. Mice that are most attracted to the smells of potential mates with dissimilar versions of MHC genes will be less likely to inbreed and will maximize the genetic fitness of their offspring. Can such a process be important for human mate selection? The fact that perfume and cologne sales account for approximately 12 to 15 percent of annual consumer luxury item spending suggests that we believe smell is a key factor in shaping our own attractiveness.

Intrigued by the idea that humans may use a very similar process to select mates, evolutionary psychologist Chris Wedekind and his colleagues conducted an experiment in which they asked more than one hundred men and women to score the odor of T-shirts worn for two consecutive days by male and female subjects. Each person tested was brought into a room with six odorous T-shirts stored in separate plastic containers and asked to rank them in terms of their “sexiness” and “pleasantness.” The results were surprising.

Scores of the pleasantness and sexiness were indeed found to relate to the degree of MHC similarity between the smeller and the T-shirt wearer. For most subjects, the most pleasant and sexy smells were associated with members of the opposite sex whose MHC genes had the least overlap with their own. When asked why they liked a specific smell, many subjects offered that it reminded them of their present lover or an ex-mate. Interestingly, lower-ranking odors were said to remind the smeller of a sibling or other relative. This is one case where opposites definitely attract, and for a good reason—the observed mating preferences stemming from these choices would naturally increase immune system heterozygosity of the offspring.

In this mechanism we find that there is no single attractive smell that works for everyone—one person’s
sentir bon
may repulse another. To take advantage of this adaptation, lonely singles in search of a mate would have to get a genetic fingerprint of his or her intended before a scent could be custom-designed. But all hope is not lost. In 2001, Wedekind’s group showed that we unwittingly use our own MHC geneotype information in choosing perfume and cologne for personal use. In this study 137 male and female students who had been typed for MHC were asked to rate 36 different scents for personal use—“Would you like to smell like that yourself ?” The researchers found a significant correlation between MHC genotype and scent rankings, indicating that people with similar MHC alleles preferred to wear similar-smelling perfumes. These results suggest “that perfumes are selected ‘for self ’ to amplify in some way body odors that reveal a person’s immunogenetics.” While it is commonly assumed that perfumes are worn primarily to mask a person’s natural odors, Wedekind has argued that we actually prefer to wear scents that accentuate these olfactory cues, announcing our MHC genotype through a form of olfactory advertising.

 

 

In this chapter we’ve seen that there are a few treasured scents that are universally appealing. Floral and fruity smells top the list in most countries, probably because they signify the presence of nutritious food sources. Newborns and children alike are attracted to these scents, independent of the culture in which they were raised.

Newborns and infants are also universally attracted to the smell of their own amniotic fluid and those that are breast-fed come to associate maternal odors with the arrival of food. Attraction to maternal odors has obvious survival benefits by keeping the offspring in close proximity to its mother.

We have been wired by natural selection factors to find pleasure in these and similar smells because they have survival value. Pheromones, on the other hand, have a different kind of universal appeal. While no single odor is pleasurable to everyone, the rule is very simple (and universal): sexy/pleasurable smells signify the presence of potential mates that can lead to viable offspring.The pleasure we find in these “hidden” scents is driven not by natural selection factors, but rather through sexual selection because these adaptations have clear reproductive value.

In the next chapter we turn our search for pleasure toward the epicurean in us all. We will discover that our lust for certain tastes fosters normal immune system and brain development, but at a growing cost to public health in Western societies.

—
Comedian Alan King

—
G. K. Chesterton

 

 

 

If you ever make it to the Amazon, ask your guide to show you what is likely the most revered tree throughout all of South and Central America. The
theobromo cacao
, or “food of the gods,” was named by Linnaeus, the great eighteenth-century cataloger of nature. The designation makes clear his admiration for the almost indescribably savory taste of its fruit and seeds as well as the role of the tree in world history. Before taking a bite, few people stop to think—and who can blame them?—about the influence of chocolate on the social, political, and economic evolution of those cultures that came in contact with cacao beans as they spread from equatorial America to Europe, and then Asia.

After a short trek into the jungle, your guide will stop in front of an odd-looking tree, probably no taller than ten meters or so. If it is a mature tree—older than three years—it will have large patches of pink or blue cauliflorous growth on its bark, but your eyes will skip right past this feature and focus on the strange football-size pods that dangle expectantly from its trunk. The outer covering of the tree’s fruit is a tough hide of corrugated green and yellow, which when broken reveals a soft, whitish pulp. The taste of the pulp will catch you off guard. Most people anticipate the tangy sweet flavor of fruit with their first bite, only to be surprised by the subtle, bittersweet taste of chocolate. Enveloped inside the pulp are dark, purple-colored seeds—about thirty to forty per pod—that after being dried and processed can be recognized by epicureans around the world as “chocolate beans.”