Lust: “The Interolerable Neural Itch”

W. H. Auden called the sex drive “an intolerable neural itch.” Scientists have long regarded that itch as a distinct emotion system that is innate and common to all birds and mammals—lodged in the avian and mammalian brain. Moreover, they have long understood at least the basic neuroanatomy and physiology of the libido, agreeing that it is predominantly associated with the androgens in both men and women. The estrogens also play substantial roles in the sex drive in many mammals, but only a secondary role in humans.

The biological relationship between the sex drive and the attraction system has not been well defined in most mammals; but in the small rodents called prairie voles, studies have shown that the two systems regularly interact. When a female prairie vole receives a drop of male urine on her upper lip, the neurotransmitter norepinephrine is released in specific areas of the olfactory bulb in her brain. This helps to stimulate the release of estrogen and contributes to triggering sexual behavior. In the prairie vole, attraction is a brief, spontaneous, chemically induced, excitatory reaction that initiates sexual desire, sexual physiology, and sexual behavior.

Lust and attraction do not always go hand in hand in people. When middle-aged men and women are injected with testosterone, their sex drive increases, but they do not fall in love. Moreover, men and women can express sexual desire toward those for whom they feel no obsessive attraction or deep attachment.

“Lust is the oldest lion of them all,” says an Italian proverb. The factors that trigger the libido vary from one individual and one species to the next, but the sensation itself, which is associated with a specific constellation of neural correlates, evolved to initiate the mating process. This emotion system, however, probably also contributes to many cases of date rape and other forms of inappropriate human sexual conduct.

Attraction: The “Delirium of Eros”

Robert Lowell called love “this whirlwind, this delirium of Eros.” Romantic love, obsessive love, passionate love, infatuation: Call it what you will, almost all men and women around the world have known its ecstasy and anguish.

In 1991, anthropologists surveyed accounts of 166 societies and found evidence of romantic love in 147 of them. (In the other 19, researchers had simply failed to examine this aspect of daily living). Everywhere they looked, they found evidence of this passion. People sang love songs or composed romantic verse. They performed love magic, carried love charms, or brewed love potions. Some eloped. Some committed suicide or homicide because of unrequited love. In many societies, myths and fables portrayed romantic entanglements. Thus, anthropologists believe that romantic attraction is a universal or near-universal human experience. I will go even further: I think romantic love, attraction, is common to all mammals and birds.

Naturalists have implicitly acknowledged the existence of this emotion system for over a century. In 1871, Darwin wrote of a female mallard duck who became attracted to a pintail duck, a bird of a different species. Citing the report of a colleague, Darwin wrote, “It was evidently a case of love at first sight, for she swam around the newcomer caressingly… From that hour she forgot her old partner.” The animal literature is filled with such descriptions. Dogs, horses, gorillas, canaries: Males and females of many species assiduously avoid mating with some individuals and resolutely focus their attention on others.

Darwin further discussed attraction when he wrote about the evolution of the “secondary sexual characteristics,” all of the gaudy, cumbersome accoutrements that creatures flaunt, such as the peacock’s unwieldy tail feathers. He reasoned that birds and mammals evolved these bodily decorations for one of two reasons: to impress or fight members of the same sex to win breeding opportunities or to attract members of the opposite sex. Yet he failed to note that these physical traits must trigger some type of physiological attraction response in the viewer.

Today many scientists call this attraction “favoritism,” “selective proceptivity,” “sexual preference,” “sexual choice,” or “mate choice.” As yet, however, they have not examined the biological process by which the viewer comes to prefer and choose a mate. I theorize that birds and mammals have evolved a specific “attraction circuit” in the brain that becomes active when an individual sees, hears, smells, or touches an appropriate mating partner—a neural circuit that creates a condition humans have come to call romantic love.

My hypothesis is that feelings of romantic attraction are associated with high levels of the neurotransmitters dopamine and norepinephrine and with low levels of serotonin. I arrived at this thesis after culling 13 psycho-physiological characteristics of romantic love from the past 25 years of psychological literature, then matching these traits, where possible, with known properties of dopamine, norepinephrine, and serotonin. Several of the 13 traits can be so matched with what is now known about brain chemistry. These traits include the experience of novelty, intrusive thoughts, focused attention, increased energy, and powerful feelings of elation.

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