In RL, our neurons and our hormones, our brains and our blood pressure, our stomachs and hearts, are in a state of upheaval. And if we check out the behaviour of our close primate relatives, we can detect links between human bonding of the RL type, and evolutionary survival pressures. In fact, there's no area of physiology, or behavioural and evolutionary biology, that doesn't boast an explanatory claim to RL.
The feeling that RL is akin to being "besotted" lies deep in western folk memory. From the ancient Greek myth of the centaur Nessus and his dangerous love potions, to E M Forster's lovesick Maurice's complaint that he is "drugged", our forebears have characterised RL as a potent substance. But is it generated within, or outside, the body? Many of our ancestors cited external influences: drugged arrows, spells, planetary forces, charms, potions.
A medieval recipe for a "true-love powder" states: "Take elecampane, the seeds and flowers of vervain, and the berries of mistletoe. Beat them, after being well dried in an oven, into a powder, and give it to the party you design upon in a glass of wine and it will work wonderful effect to your advantage."
But Galen, the 2nd-century Greek "prince of physicians", insisted that the affliction was purely a matter of internal chemistry. It's what happens, he asserted, when the crucial four bodily fluids, or humours — yellow bile, black bile, phlegm and blood — get into a muddle. For Galen's followers, right down to Robert Burton, author of The Anatomy of Melancholy, the theory of humoralism held good. Galen was eventually displaced by 19th-century theories of cell biology, but modern physiologists nevertheless share his broad conviction that RL is induced by powerful natural bodily chemicals.
In our own day, the favoured chemical explanation focuses on a molecule called PEA: phenylethylamine, a kind of natural amphetamine that revs up the brain and the central nervous system. PEA causes the experience of euphoria, hyperventilation, increased heart rate, dilated pupils, and secretions of odours that can seduce an unsuspecting love object. The eye of the chemical storm is in the brain.
The brain in RL resembles a huge geological and meteorological event: earthquakes, cyclones, tsunamis. It's as if the ecosystem of the lover's brain, the pulsing grey-blue-green planet in the skull, suffers a drastic depletion of the protective cortical ozone layer, triggering neuronal global warming with consequent atmospheric storms.
The notion that reason goes to pot in RL fits with a popular mind-brain theory first proposed in the 1970s. The outer brain, or cortex, which evolved late in evolution, is associated with rational thought and intelligence. The midbrain, known as the limbic system, regulates the emotions. But there's a deep inner core, located at the final bulb where the spinal cord enters the brain, dubbed by the physiologist Paul MacLean "the reptilian brain", where lurk our darker, primeval, instinctive behaviours of territoriality, mating and reward-seeking.
When two separate researchers, the British brain mapper Semir Zeki and the American anthropologist-psychologist Helen Fisher, used functional magnetic resonance imaging to explore the neural basis of RL, the brains of their lovelorn volunteers lit up precisely in that deep region known as the caudate nucleus: the site of the reptilian brain, thought to be 65m years old in evolutionary terms. The more passionate the subjects, the more active the caudate nucleus. The reptilian brain connects directly with the limbic system, where, according to Helen Fisher, "the chemical storms, leading to infatuation, almost certainly have their physical origin". In RL the music of cortical sweet reason is drowned out by the primitive drumbeats of our limbic and reptilian brains, stimulating cascades of PEA in the central nervous system. At the same time, adrenaline levels are boosted, prompting the release of a chemical called dopamine. Dopamine is associated with highly targeted attention, stamina, energy, all focused on reward.
As these powerful chemicals run riot down the neuronal pathways, they dilute and cancel out the nerve chemical called serotonin. Serotonin controls impulses, unruly passions, obsessive behaviour: it aids the sense of power over action, the feeling of "being in control". A severe depletion of serotonin can induce panic, anxiety, queasiness, manic behaviour, depression, obsession: "I can't get her/him out of my mindÉ I'm thinking about her/him all the time."
Patients with compulsive disorders — such as nonstop hand-washing and bulimia — are often prescribed Prozac (fluoxetine hydrochloride), which increases the activation of lowered serotonin in the synapses. In the language of pharmacology, Prozac is an SSRI, a selective serotonin-reuptake inhibitor. SSRIs prevent serotonin being swallowed up too quickly in the synapses. There are serious suggestions that the lovesick should be given a good dose of an SSRI.
So what, chemically speaking, is happening to the lovesick — the jilted, the jealous? The lover needs the constant fix of encounters with the love object to satisfy and dampen the excitation of those cataracts of PEA; any thwarting ("I can't get no-o... satis-faction!") can only lead to further drenching of PEA, resulting in even more drastic loss of serotonin. This explains the highs and lows of the lovesick, the out-of-control symptoms of possessiveness, goose pimples, butterflies in the stomach, restlessness, inability to concentrate, sleeplessness: that generalised delicious agony called infatuation.
But should RL be reciprocated, there follows the second stage: sexual fulfilment, in which the hormone testosterone becomes rampant in men, and also in women, especially at ovulation and even beyond the menopause. When actual contact with the love object occurs — stroking, love play, kissing, leading eventually to coitus — a heady hormone identified as oxytocin explodes like a firework display in the brain, releasing showers of natural opiates known as endorphins, a kind of natural crack: a mega-reward! At orgasm a man's oxytocin levels can increase by a factor of five. In women the oxytocin levels can be even higher during intercourse. Oxytocin, moreover, combines with the hormone vasopressin, which is associated with vivid emotional memories, visual, tactile, aural and nasal, consolidating the image and associated deep feelings for the love object. That piece of music, that particular scent, the purr of their voice, the shape of that nose exciting so much passion. The oxytocin highs, with their consequent endorphin hits, do much to explain the withdrawal symptoms when the love object goes cold and, worst of all, is seen in the arms of another. Small wonder psychiatrists have likened disappointed love to acute depression.
