Love is the quintessential romantic, irrational, human experience. It is in the first place the domain of its clients, although readily owned by the poets. As for others. The philosophers are permitted access, though they often regress into poets too; describing love as sacred, profound, and utterly existential. In the penultimate place in the line for love, in front only of the lawyers whose focus is appallingly materialistic, are the scientists. But the scientists, too, are accused of materialism, of soulless reductionism, mistaking passion for a chemical equation.
I am here as a scientist, though you would do well to remember that behind the label stands another human being, and the scars I bear you may recognize too. I am here to answer that trite question, “What can science ever know of love?” That question is too broad for my mind, and I write with a more particular view in mind. Thus, ladies and gentlemen, lovers and scorned alike, I present to you, a great and amazing production, written using the quills of Cupid’s arrows, and inspired by the musings of the great bard himself. I give you:
The Molecular Neurobiology of Pair Bonding
The scene is familiar. You were there. And so was I. Cocooned in sunlight. Starry Heavens. Watching. Holding. Kissing. Wanting. Having. Do you remember the shape of the moon as I held you by the side of the silent road. I remember the rhythm of your breath. You were asleep when I told you this is perfect. I write this now, tired, sick, alone, remembering. And you?
Simply put, love is the phenomenological correlate of a behavioural tendency which favours pair bonding, and that is an evolutionary selected strategy benefiting offspring survival in particular species. In other words, love induces an obsession with a potential mate, eliciting a preference for monogamy, and this effect naturally overlaps with a period of time sufficient for the conception and protection of maturing progeny.
I might as well note here that, scientifically speaking, it is unclear whether humans are biologically monogamous (cf. culturally), nor what is the length of time of human monogamy (cf. culturally).
So step right up, five pence to the bearded woman if you don’t mind, and I promise entertainment, education, and if you’re good, a baring of my very heart and soul.
I shall explain the following. Most prominently, I describe the molecular components known to be involved in pair bonding (you say, love, I say potata). There shall be talk of the avenues of the brain in which these molecules are to be found. And all is topped by a conclusion in the form of a philosophical coda, a musing on the music of the cells.
The original love story
It seems likely that the pair bonding instinct evolved from other behaviours, like the care of a mother for her offspring. Not only are the two behaviours similar, as well as both being called “love”, they also share molecular mechanisms. Oxytocin. It’s a molecule. Important for forming a bond between a child and their mother, enhanced by behaviours like grooming and breastfeeding, it is also important for birthing, when a giant surge can precipitate labour.
Oxytocin is famous as the “trust molecule”, as dosing enhances our tendency to trust those around us. A less well-known fact is that it can enhance our mistrust of strangers; yin-yang. Thus it can be seen that boyfriend/girlfriend love is a capacity that evolved out of the more primordial maternal love, which itself is a subset of an even more general tendency to stick together with those you know and away from those you don’t.
Alongside oxytocin operates its less famous cousin, vasopressin, which in itself is better known for regulating fluids (sexy!) It synergizes oxytocin’s effects and its presence is at least necessary for strong pair-bonding.
You make me happy
Dopamine. Obviously not just anywhere in the brain. But basically, the reason you’re stupidly happily in love is because of some errant dopamine flinging itself about in a rather unbecoming manner. Dopamine is the neurotransmitter the media loves to talk about. It makes for a good, simple story – “the reward molecule”, a fact only in part.
Yes, dopamine is involved in such horrors as addiction and experiences of euphoria. It’s also linked to the formation of relationships, as well as forni-fucking (for want of a word). So far, it makes perfect, simple, sense; love is predicated on a reward mechanism. But that’s too simplistic, and as I hope you’ll find clear before long, dopamine is involved also in inhibiting pair-bonding, and molecules in the body don’t have jobs like “happiness”, and “love”.
But then what are these molecules doing?
So if there’s no love molecule, what’s the significance of all these molecules? Without getting into the nitty-gritty of brains and neurons and neurotransmitters and receptors and action potentials and long-term potentiation and Bayesian statistics, I can say the following:
The brain is a jumble of tendencies. These tendencies can cause you to commit a behaviour, but they can also be affected by what you do or experience. For instance, if you were to engage in nurturing behaviour then parts of your brain will respond by sending oxytocin into the parts of your brain that trust your friends and encourage that tendency. That happens by making that part of the brain more receptive to future oxytocin, as well as more likely to send signals to other parts of the brain.
It gets tricky when we start thinking about the molecules which are actually affected. Oxytocin and dopamine can react with different receptor molecules, which leads to some counter intuitive effects. For instance, although the “D2” dopamine receptor is vital for post-coital pair-bonding, the D1 dopamine receptor inhibits pair-bonding. This is the case because the particulars of D1’s location is such so as to prevent one from falling in love with everyone you meet after sex (well, only the nice people are an issue).
Lots of words but what’s happening
This is an incomplete story, but it’s a nice one, and will hopefully give some impression of some of what happens.
Step one, Alex and Alice flirt and maybe a back rub is involved, but most certainly they mix their private parts. They later spend a lot of time together, and fall in love.
Here’s what happened to poor Alice. The flirting and grooming behaviours engaged her oxytocin pumps, sending them into two important systems: one is involved in the sense of reward (nucleus accumbens, part of the mesolimbic pathway), and the other is more generally social (ventral pallidum, where it meets vasopressin). Thus Alice already feels trust for Alex. But it gets worse. The pre-existing oxytocin combines with dopamine and even more oxytocin during and after sex. This has a couple of effects. One is a longer term bias towards monogamy with Alex, and away from others. The other establishes even greater potential for affection for Alex. The dopamine plus oxytocin combo enhances the ongoing production of oxytocin’s receptor (in the nucleus accumbens, this is achieved via histone deacteylation so as to enhance transcription of the receptor’s gene, and consequent, parallel translation). The effect of higher amounts of receptors means that Alice is more vulnerable to the effects of future oxytocin, so that Alex’s advances are all but certain to succeed. This is all, of course, a viscous cycle, engaging a positive feedback loop that is enhanced by sex and happiness. Ugh.
Ok, cool, I think I get it
Well, I don’t, but never mind. Let’s change track and focus on how complicated it can get. After acknowledging that the above was an incomplete picture, let’s zoom into one detail. The oxytocin’s receptor’s gene is called OXTR.
The OXTR gene can have various “point mutations”, this is when some of the atoms in the gene are different from the average. Genes are made up of molecules that can easily be thought of as letters. The OXTR gene has over 19,000 letters. Most point mutations have not been studied, and of those that have been recorded, most have no effect, and of course, some have a lethal effect. In between those extremes are single letter changes that are associated with a range of behavioural abnormalities. These include not only autistic tendencies, but also differences to threat-associated-prosocial behaviours, sensitivity of parenting (including the capacity for hearing infants’ cries), the stress-protective-effects-of-socializing, and much more.
Some of these changes must be due to developmental effects, that is, relating to the role of oxytocin in the formation of the brain, a topic well outside the current perusal. But some of these changes are due to a more interesting subtle effect. Perhaps a slight change in the way oxytocin interacts with the receptor, or in the way the receptor interacts with other receptors, or in the locations in the cell that the receptor preferentially positions itself, or in its capacity to dock associated molecules in the cell, or in its tendency to be made (i.e. transcribed or translated) or trafficked (e.g. through the endoplasmic reticulum and Golgi), or in its effects on DNA topology, or in any of a huge number of other possibilities, or, almost certainly, in some combination thereof. Love is all so very complicated.
So love is just a chemical reaction and we’re all robots doomed to our predestined lives
I see it like this. Without questioning whether our lives are predestined and whether we have free-will (pro-tip: yes and yes), we can ask what it means to value a particular set of experiences when we know that their entire process is a series of molecular mechanisms interacting on a neurological scale. Or does it mean nothing?
My answer: Just like staring into the abyss, it is inevitable that the study of one’s own mechanical nature will disabuse one of particular notions. But it can also be empowering. Instead of striding blindly forward into society’s absurdly condoned strange madness, aka love, one can grasp it with both hands and choose to manipulate oneself accordingly.
Do you want to fall in love with her. Then do this. And if you don’t, well, you’d better stop what you’re doing. Biology as a key to the Socratic imperative.
Which is all a very long way of saying: I choose to love you.
