vole, there is an extra chunk of DNA text, on average about 460 letters long, in the middle of the promoter. So Insel’s lab made a transgenic mouse with this expanded promoter and it grew up with a brain like a prairie vole, expressing vasopressin receptors in all the same places, though it did not form a pair bond.14 Steven Phelps then went out and caught 43 wild prairie voles in Indiana and sequenced their promoters: some had longer insertions than others. They varied from 350 to 550 letters in length. Are the long ones more faithful husbands than the short ones? Not yet known.15
The conclusion to which Insel’s work is leading is devastating in its simplicity. The ability of a rodent to form a long-term attachment to its sexual partner may depend on the length of a piece of DNA text in the promoter switch at the front of a certain receptor gene. That in turn decides precisely which parts of the brain will express the gene. Of course, like all good science, this discovery raises more questions than it settles. Why should feeding oxytocin receptors in that part of the brain make the mouse feel well-disposed to its partner? It is possible that the receptors induce a state a bit like addiction, and in this respect it is noticeable that they seem to link with the D2 dopamine receptors, which are closely involved in various kinds of drug addiction.16 On the other hand, without oxytocin, mice cannot form social memories, so perhaps they simply keep forgetting what their spouse looks like.
Mice are not men. You know by now that I am about to start extrapolating anthropomorphically from pair-bonding in voles to love in people, and you probably do not like my drift. It sounds reductionist and simplistic. Romantic love, you say, is a cultural phenomenon, overlaid with centuries of tradition and teaching. It was invented at the court of Eleanor of Aquitaine, or some such place, by a bunch of oversexed poets called troubadours; before that there was just sex.
Even though in 1992 William Jankowiak surveyed 168 different ethnographic cultures and found none that did not recognise romantic love, you may be right.17 I certainly cannot prove to you yet that people fall in love when their oxytocin and vasopressin receptors get tingled in the right places in their brains. Yet. And there are cautionary hints about the dangers of extrapolating from one species to another: sheep seem to need oxytocin to form maternal attachment to their young; mice apparently do not.18 Human brains are undoubtedly more complicated than mouse brains.
But I can draw your attention to some curious coincidences. A mouse shares much of its genetic code with a human being. Oxytocin and vasopressin are identical in the two species and are produced in the equivalent parts of the brain. Sex causes them to be produced in the brain in both human beings and rodents. Receptors for the two hormones are virtually identical and are expressed in equivalent parts of the brain. Like those of the prairie vole, the human receptor genes (on chromosome 3) have a – smaller – insertion in their promoter regions. Like the prairie voles of Indiana, the lengths of those promoter insertions vary from individual to individual: in the first 150 people examined, Insel found 17 different promoter lengths. And when a person who says she (or he) is in love contemplates a picture of her loved one while sitting in a brain scanner, certain parts of her brain light up that do not light up when she looks at a picture of a mere acquaintance. Those brain parts overlap with the ones stimulated by cocaine.19 All this could be a complete coincidence, and human love may be entirely different from rodent pair-bonding, but given how conservative the GOD is and how much continuity there is between human beings and other animals, you would be unwise to bet on it.20
Shakespeare was ahead of us, as usual. In A Midsummer Night’s Dream, Oberon tells Puck how Cupid’s arrow fell upon a white flower (the pansy), turning it purple, and that now the juice of this flower
…on sleeping eyelids laid
Will make or man or woman madly dote
Upon the next live creature that it sees.
Puck duly fetches a pansy and Oberon wreaks havoc with the lives of those sleeping in the forest, causing Lysander to fall in love with Helena, whom he has previously scorned, and causing Titania to fall in love with Bottom the weaver wearing the head of an ass.
Who would now wager against me that I could not soon do something like this to a modern Titania? Admittedly, a drop on the eyelids would not suffice. I would have to give her a general anaesthetic while I cannulated her medial amygdala and injected oxytocin into it. I doubt even then that I could make her love a donkey. But I might stand a fair chance of making her feel attracted to the first man she sees upon waking. Would you bet against me? (I hasten to add that ethics committees will – should – prevent anybody taking up my challenge.)
I am assuming that, unlike most mammals, human beings are basically monogamous, like prairie voles, and not promiscuous, like montane voles. I base this assumption on the testicle-size argument enunciated in chapter 1; on the ample evidence from ethnography that, though most human societies allow polygamy, most human societies are still dominated by monogamous relationships; and on the fact that human beings usually practise some paternal care – a characteristic feature of the few mammal species that live as social monogamists.21 Furthermore, as we have liberated human life from economic and cultural straitjackets, such as arranged marriage, we have found monogamy growing more dominant, not less. In 1998 the most powerful man in the world, far from treating himself to a gigantic harem, got into trouble for having an affair with one intern. The evidence for long-term, exclusive (but sometimes cheated-on) pair bonds as the commonest pattern in human relationships is all around you.
Chimpanzees are different. Long-term pair bonds are unknown, and I predict that they have fewer oxytocin receptors in the relevant parts of their brains than human beings, probably as a result of having shorter gene promoters. The oxytocin story lends at least tentative support to William James’s notion that love is an instinct, evolved by natural selection, and is part of our mammal heritage, just like four limbs and ten fingers. Blindly, automatically and untaught, we bond with whoever is standing nearest when the oxytocin receptors in the medial amygdala get tingled. One sure way to tingle them is to have sex, although presumably chaste attraction can also do the trick. Is this why breaking up is hard to do?
Having oxytocin receptors does not make it inevitable that somebody will fall in love during his life, nor predictable when it will happen, or with whom. As Niko Tinbergen, the great Dutch ethologist, demonstrated in his studies of instincts, the expression of a fixed, innate instinct must often be triggered by an external stimulus. One of Tinbergen’s favourite species was the stickleback, a tiny fish. Male sticklebacks go red on the belly in the breeding season, when they defend small territories in which they build nests, which attract females. Tinbergen made little models of fish and caused them to ‘invade’ the territory of a male fish. A model of a female elicited the courtship dance of the male, even if the model was astonishingly crude; so long as it had a ‘pregnant’ belly, it excited the male. But if the model had a red belly, it would trigger an attack. It could be just an oval blob with a crudely drawn eye but no fins or tail: still it was attacked just as vigorously as if it were a real male rival – so long as it was red. One of the legends of Leiden, where Tinbergen first worked, is that he noticed his sticklebacks would threaten the red post-office vans that drove past the window.
Tinbergen went on to demonstrate the power of these ‘innate releasing mechanisms’ to provoke an instinct in other species, notably the herring gull. Herring gulls have yellow beaks with a bright red spot near the tip. The chicks peck at this spot when begging for food. By presenting newborn chicks with a series of models, Tinbergen demonstrated that the spot was a powerful releaser for the begging
