We are not alone in being able to form intense and enduring social ties. Take the mother infant bond.

Whether or not the emotional connection between a ewe and her lamb, or a female macaque and her

offspring, is qualitatively similar to human motherly love, it is highly likely that these relationships s

hare evolutionarily conserved brain mechanisms. In humans, rats and sheep, the hormone oxytocin is

released during labour, delivery and nursing. In ewes, an infusion of oxytocin into the brain results i

n rapid bonding with a foreign lamb.

Long-term bonding between mates is rare in mammals. It may be regulated by the same brain mechan

isms as those involved in maternal bonding. For instance, pair bonding in the female monogamous pr

airie vole is stimulated by oxytocin released in the brain during mating. A female prairie vole rapidly

becomes attached to the nearest male if her brain is infused with oxytocin. The hormone interacts wit

h the reward and reinforcement system driven by the neurotransmitter dopamine — the same circuitr

y that drugs such as nicotine, cocaine and heroine act on in humans to produce euphoria and addictio

n. There is intriguing overlap between the brain areas involved in vole pair bonding and those associa

ted with human love. Dopamine-related reward regions of the human brain are active in mothers vie

wing images of their child. Similar activation patterns are seen in people looking at photographs of t

heir lovers.

The notion that pair bonding in humans may have evolved through a tweaking of the brain mechanis

ms underlying maternal bonding could explain certain unique characteristics of human sexuality. Fo

r example, female sexual desire may have become decoupled from fertility, and the female breast ma

y have become an erotic stimulus for males, to activate ancient maternal-bonding systems. The stimu

lation of the cervix and nipples during sexual intimacy are potent releasers of brain oxytocin, and ma

y function to strengthen the emotional tie between partners.

Pair bonding in males involves similar brain circuitry to that in females, but different neurochemical

pathways. In male prairie voles, forexample, vasopressin — a hormone related to oxytocin — stimul

ates pair bonding, aggression towards potential rivals, and paternal instincts, such as grooming offspr

ing in the nest. Variation in a regulatory region of the vasopressin receptor gene, avpr1a, predicts the

likelihood that a male vole will bond with a female.

Similarly, in humans, different forms of the AVPR1A gene are associated with variation in pair bond

ing and relationship quality. A recent study shows that men with a particular AVPR1A variant are tw

ice as likely asmen without it to remain unmarried, or when married, twice as likely to report a recen

t crisis in their marriage. Spouses of men with the variant also express more dissatisfaction in their re

lationships than do those of men lacking it.

For both voles and humans, AVPR1A genetic polymorphisms predict how much vasopressin recepto

r is expressed in the brain. The view of love as an emergent property of a cocktail of ancient neuropept

ides and neurotransmitters raises important issues for society.

For one thing, drugs that manipulate brain systems at whim to enhance or diminish our love for anothe

r may not be far away. Experiments have shown that a nasal squirt of oxytocin enhances trust and tune

s people into others’ emotions. Internet entrepreneurs are already marketing products such as Enhance

d Liquid Trust, a cologne-like mixture of oxytocin and pheromones “designed to boost the dating and

relationship area of your life”.

Although such products are unlikely to do anything other than boost users’ confidence, studies are und

er way in Australia to determine whether an oxytocin spray might aid traditional marital therapy.

We don’t yet know whether the drugs commonly used to treat disorders from depression to sexual dys

function affect people’s relationships by altering neurochemistry. But both Prozac and Viagra influenc

e the oxytocin system. The quality of patients’ relationships should be included in the list of variables

assessed in controlled psychiatric drug studies.

The possibility that genetic variation may influence the quality of our romantic relationships also has i

ntriguing implications. Perhaps genetic tests for the suitability of potential partners will one day beco

me available, the results of which could accompany, and even override, our gut instincts in selecting th

e perfect partner. Either way, recent advances in the biology of pair bonding mean it won’t be long bef

ore an unscupulous suitor could slip a pharmaceutical 'love point' in our drink. And if they did, woul

d we care? After all, love is insanity.

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Jung说：你们都还是这么偏激啊，唉……集体无意识啊……