LOVE is many things: the protective love of a mother for her child, the passion of a couple newly in love, the deep love of long-term companions and the divine love of God, to name just a few. Some cultures have 10 or more words for different forms of love, and poets and songwriters always find myriad aspects of love to celebrate. Is there anything universal behind all this diversity? As Pope Benedict recently asked in his first encyclical letter: 鈥淎re all the forms of love basically one, so that love, in its many and varied manifestations, is ultimately a single reality?鈥�
The science of love is still in its infancy. Yet scientists of many different disciplines are beginning to get their first insights into the nature and origin of love. We can now look inside brains to view their patterns of activity, measure biochemical changes that take place in different forms of love, explore diverse human experiences of love, and look for the evolutionary roots of love in other animals.
If the different forms of love have any common evolutionary beginning, where should we look? Maternal love seems a good place to start. Of all the forms of love, none seems as deep, strong, selfless or enduring as the love of a mother for her child, nor is any other bond so ubiquitous in the animal kingdom. Biologically this bond makes perfect sense. In any animal that must provide care for newborn offspring to survive, the bond is essential if that mother鈥檚 genes are to be passed on to the next generation.
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鈥淥f all forms of love, none seems as enduring as the love of a mother for her child鈥�
How is that bond created? Much of what we know about the brain chemistry of bonding comes from studies of rodents. Whether they feel 鈥渓ove鈥� we cannot say, but they will bitterly defend their young. This tendency seems directly triggered by motherhood: virgin female rats, or even pregnant ones, will avoid or attack pups, but just before giving birth their behaviour changes profoundly.
What makes newborn infants so special to their mothers? The critical link turns out to be the hormone oxytocin. Late in pregnancy, raised levels of oestrogen boost the number of receptors for oxytocin in parts of the brain. During birth, the physical stimulation of labour triggers the release of oxytocin and when the hormone hits those receptors it causes the mother to become addicted to those pups and their particular smell. 鈥淎ddicted鈥� might seem like a strong word, but the process of bonding to the newborn pups involves powerful activation of a system that carries reward information around the brain. It is this same dopamine reward circuit that can be artificially stimulated by drugs like cocaine and heroin.
The reward circuits originate near the base of the brain in the ventral tegmental area (VTA). Nerve fibres from here connect to the front of the brain, most importantly to the nucleus accumbens that lies just beneath the frontal cortex, where they release the neurotransmitter dopamine. Ultimately, it is in the cortex that reward information is coordinated with emotions and memories and where, in humans, subjective feelings are created, but it is the VTA that sends on the key information about the value of an activity and helps stamp it into memory.
When a rat is bonding to her pups, this reward system is boosted by oxytocin at the same time as the hormone facilitates sensitivity to smell, ensuring that the bonding is specific to the unique smell of the mother鈥檚 own pups. Every time the mother sniffs its pups again, it may sense that same feeling of impending reward, much as a drug addict feels cravings at the thought of the drug.
Care of the young is even more important in the higher mammals. Chimpanzees don鈥檛 leave their mothers until they are more than seven years old and human children stay around far longer. You might think that alongside the strengthening bond between mother and infant there should be a similar trend in another form of love 鈥� a monogamous bond between male and female parents, once again designed to ensure survival of the young. Think again. No such trend exists. Among mammals, love them and leave them is most often the rule: fewer than 5 per cent of mammalian species are monogamous, and there is no simple pattern that explains why it occasionally appears. Surprisingly, the need for care from two parents is unlikely to be the main driving force for its evolution, as even among species that are monogamous many have fathers that never provide for their offspring. And don鈥檛 confuse monogamy with true sexual fidelity as that is rarer still.
That leaves an interesting question: if monogamy is an evolutionary oddity rather than a trend, how does evolution occasionally redesign a species to behave so differently from other closely related ones? The answer seems to be that evolution stole the biochemistry and neural tricks that bond mother to infant and reinstalled them, so as to bind male and female together.
That is the message from two species of vole that provide a natural experiment. One species, the prairie vole, bonds extraordinarily closely to its mate. In contrast, its close relative the meadow vole is promiscuous. Partners mate and move on. The difference between them comes down to where receptors for oxytocin and a closely related hormone, vasopressin, are located in the brain. Those hormones are produced during 鈥渢he extended tactile pleasures of mating鈥� as one research paper puts it. In the promiscuous meadow vole, few receptors for the vasopressin hormone are found in the dopamine reward region, but in the monogamous prairie vole the receptors are there in abundance, turning sex into a powerful reward that bonds the male to its partner.
Small changes in a single gene determine whether there will be many vasopressin receptors in the reward region or not. Larry Young and his colleagues at Emory University in Atlanta, Georgia, were able to make promiscuous meadow voles monogamous simply by injecting them with a virus that carried the prairie vole gene variant into their brain cells. Just as predicted, if prairie voles are given drugs that block their vasopressin receptors, they become as promiscuous as meadow voles.
Of course, voles aren鈥檛 humans and their pair bonding can鈥檛 really be called love. But it is worth noting that among humans, there is considerable individual variation in the gene that controls the distribution of the receptors, although no one knows if it correlates with fidelity. Oxytocin and vasopressin certainly seem important in human love. When Andreas Bartels and Semir Zeki at University College London scanned the brains of couples who had been together for several years while they looked at a picture of their partner, they found that activity rose in just those parts of the brain that are rich in receptors for these two hormones.
鈥淥xytocin levels rise during orgasm in women and arousal in men鈥�
Oxytocin levels rise during orgasm in women and sexual arousal in men, as they do from touching and massage. Oxytocin also boosts trust, which is an important step in developing a loving relationship. In a laboratory investment game devised by neuro-economist Ernst Fehr at the University of Zurich in Switzerland, almost half the people playing the role of 鈥渋nvestors鈥� would hand over all their money to an anonymous trustee, with no guarantee of its return, if they sniffed an oxytocin spray beforehand.
Inspired by this study, Andreas Meyer-Lindenberg鈥檚 team at the National Institute of Mental Health went on to see what was happening in the brains of volunteers who sniffed oxytocin. They found the hormone reduced activity in the amygdala, a part of the brain that signals fear, and its action seems to help overcome 鈥渟ocial fear鈥�, making it easier to bond to another person.
Of course, before that bonding can take place, male and female have to get together, and for many people that means passing through the ups and downs of falling in love. What happens during this rollercoaster of intense feelings? At its height, romantic love certainly does seem to set the brain on fire. Helen Fisher鈥檚 team at Rutgers University scanned the brains of couples who were newly in love while they gazed at photos of their sweethearts. Activity soars in the brain鈥檚 reward system. That results, Fisher says, in 鈥渇ierce energy, concentrated motivation to attain a reward, and feelings of elation, even mania 鈥� the core feelings of romantic love.鈥�
Other areas linked with negative emotions and assessing other people鈥檚 intentions switch off. And they switch off too when mothers look at pictures of their babies. No wonder that love is blind and to love someone is, as Fran莽ois Mauriac wrote, 鈥渢o be the only one to see a miracle invisible to others鈥� (see a comic strip explaining what really goes on inside the brains of lovers, 200k file).
Not everything is the same in romantic and maternal love. Romantic love also includes activation of the hypothalamus, where the sex hormone testosterone is produced. Lust, the sexual part of love, is, unsurprisingly, switched on in romantic love but not in maternal love.
Overall, then, science tends to confirm what human experience teaches. The various forms of love 鈥� maternal, pair-bonded and romantic 鈥� are biologically related and have neurochemical circuitry in common.
But what about an even wider form of love 鈥� the religious love for God and humankind? Love that extends to strangers, outcasts and even to enemies, is central to the Christian message. Other religions stress love and compassion for fellow creatures too and Buddhism in particular has developed meditative practices intended to develop these feelings.
In searching for correlates between this wider religious love and changes within the brain, it is thus not surprising that researchers have turned to Tibetan Buddhist monks who have practised meditation on loving compassion for many years. Such people do appear to be exceptionally good at reading facial expressions and empathising with others. First results also show that Tibetan Buddhist monks have unusual brain activity when they meditate on loving compassion: Richard Davidson at the University of Wisconsin-Madison found exceptionally high levels of integrated electrical activity during mediation, especially in the right prefrontal cortex. Curiously enough, separate experiments have shown that areas of the prefrontal cortex also light up when a mother gazes at a picture of her child.
These are just the first small steps in looking for the roots of religious love. But they suggest the Pope may be on the right track when he says in his letter that 鈥渓ove is a single reality, but with different dimensions鈥�.
