Nature via Nurture: Genes, experience and what makes us human. Matt Ridley
beings, meanwhile, masturbation increases oxytocin levels in both sexes. All in all, oxytocin and vasopressin in the brain seem to be connected to mating behaviour.
Now all of this sounds rather unromantic: urine, masturbation, breastfeeding – hardly the essence of love. Be patient. In the late 1980s, Tom Insel was working on the effect of oxytocin on maternal behaviour in rats. Brain oxytocin seemed to help the mother rat form a bond with its young and Insel identified the parts of the rat brain that were sensitive to the hormone. He switched his attention to the pair bond, wondering if there were parallels between a female’s bond to her young and to her mate. At this point he met Sue Carter, who had begun to study prairie voles in the laboratory. She told him how the prairie vole is a rarity among mice for its faithful marriages. Prairie voles live in couples and both father and mother care for the young for many weeks. Montane voles, on the other hand, are more typical of mammals: the female mates with a passing polygamist, separates quickly from him, bears young alone and abandons them after a few weeks to fend for themselves. Even in the laboratory, this difference is clear: mated prairie voles stare into each other’s eyes and bathe the babies; mated montane voles treat their spouses like strangers.
Insel examined the brains of the two species. He found no difference in the expression of the two hormones themselves, but a big difference in the distribution of molecular receptors for them – the molecules that fire up neurons in response to the hormones. The monogamous prairie voles had far more oxytocin receptors in several parts of the brain than the polygamous montane voles. Moreover, by injecting oxytocin or vasopressin into the brains of prairie voles, Insel and his colleagues could elicit all the characteristic symptoms of monogamy, such as a strong preference for one partner and aggression towards other voles. The same injections had little effect on montane voles, and the injection of chemicals that block the oxytocin receptors prevented the monogamous behaviour. The conclusion was clear: prairie voles are monogamous because they respond more to oxytocin and vasopressin.11
In a virtuoso display of scientific ingenuity, Insel’s team has gone on to dissect this effect in convincing detail. They knock the oxytocin gene out of a mouse before birth. This leads to social amnesia: the mice can remember things, but they have no memory of mice they have already met and will not recognise them. Lacking oxytocin in its brain, a mouse cannot recognise a mouse it has just met ten minutes before – unless that mouse was ‘badged’ with a non-social cue such as a distinctive lemon- or almond-scented smell (Insel compares this to an absent-minded professor at a conference who recognises friends by their name tags, not their faces).12 Then by injecting the hormone into just one part of the animal’s brain in later life – the medial amygdala – the scientists can restore social memory to the mouse completely.
In another experiment, using a specially adapted virus, they turn up the expression of the vasopressin receptor gene in the ventral pallidum, a part of a vole’s brain important for reward. Pause here to roll that idea around your mind a few times to appreciate just what science can do these days: they use viruses to turn up the volumes of genes in one part of the brain of a rodent. Even ten years ago such an experiment was unimaginable. The result of turning up the gene’s expression is to ‘facilitate partner preference formation’, which is geekspeak for ‘make them fall in love’. They conclude that for a male vole to pair-bond, it must have both vasopressin and vasopressin receptors in its ventral pallidum. Since mating causes a release of oxytocin and vasopressin, the prairie vole will pair-bond with whatever animal it has just mated; the oxytocin helps in memory, the vasopressin in reward. The montane vole, by contrast, will not react in the same way, because it lacks receptors in that area. Female montane voles express these receptors only after giving birth, so they can be nice to their babies, briefly.
So far I have talked of oxytocin and vasopressin as if they were the same thing, and they are so similar that they probably stimulate each other’s receptors somewhat. But it appears that to the extent that they do differ, oxytocin makes female voles choose a partner; vasopressin makes males choose a partner. The male prairie vole becomes aggressive towards all voles except its mate when vasopressin is injected into his brain. Attacking other voles is a (rather male) way of expressing his love.13
All this is astonishing enough, but perhaps the most exciting result to emerge from Insel’s lab concerns the genes for the receptors. Remember that the difference between the prairie vole and the montane vole lies not in the expression of the hormone, but in the pattern of expression of the hormone’s receptors. These receptors are themselves the products of genes. The receptor genes are essentially identical in the two species, but the promoter regions, upstream of the genes, are very different. Now recall the lesson of chapter 1: that the difference between closely related species lies not in the text of genes themselves, but in their promoters. In the prairie 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