The sex life of voles - and the law

Voles are rodents who have a sex life which may be of considerable interest in understanding some expects of human sexuality and in reflecting on some aspects of the law relating to it.

In brief, there are two varieties of vole – the prairie and the montane voles. They differ from each other significantly with respect to their taste for monogamous and polygamous relationships. They have been studied by Larry Young and Thomas Insel in the United States. By and large, prairie voles lead a largely monogamous existence, with apparently the occasional fling. By contrast, montane voles are promiscuous and polygamous. What is it that differentiates these two varieties? Is it morality, or what may pass for morality in the world of vole ideas? (We would, or at least some would, perhaps refer to polygamous humans as immoral). Is it their up-bringing, or is it their biological constitution?

As you might have guessed, the latter seems to be the causative factor and has been traced to two neuro-hormones called oxytocin and vasopressin. These neuro-hormones have many effects but most relevant here is that they are involved in bonding between individuals and are effective in learning and memory in a social context. They are released in the reward centres of the brain when voles (and humans) have sex, which becomes therefore a rewarding experience with the chosen partner. If release of the two hormones is blocked in prairie voles, they too become promiscuous. If, on the other hand, they are injected with these hormones but prevented from having sex, they continue to be faithful to their partners, that is to have a monogamous but chaste relationship. On the other hand, injecting montane voles with these neurohormones does not make of them monogamous creatures, for the simple reason that they do not have sufficient receptors in their brains for them.

There is no evidence that these two neurohormones act in exactly the same way in humans. It would indeed be surprising if they did, given the infinitely more complex structure of the human brain and of human behaviour. But there is good reason to suppose that what applies in these animals also applies, at least in some form, in humans. And humans too can be categorized as being strictly monogamous (or serially monogamous) at one end and promiscuous to varying extents at the other. It would be highly interesting to learn whether (mainly) monogamous humans have higher concentrations of oxytocin and vasopressin, and a richer concentration of receptors for them in the reward centres of the brain, compared to more promiscuous humans. It may be that humans can be divided into several categories – ranging from the strictly monogamous to the extremely promiscuous - depending upon these concentrations. We might even find that there is a straightforward, linear, relationship between the concentration of these neuro-hormones and the incidence of promiscuity.

Which brings me to the law. No doubt the law of divorce regarding promiscuity and adultery has been much liberalised in Western countries. I do not know how the law apportions blame or arranges settlements in these instances. But the demonstration raises, it seems to me, broader issues that are relevant to the law and to how it may be modified through legislation in the future, in light of scientific findings like the one described briefly above, and others like them. For if individuals can demonstrate scientifically that their conduct, however much disapproved of by society and prohibited by law, is the consequence of their biological constitution, then the law would probably ultimately have to take account of that, as indeed it already does in certain instances. It will have to address the thorny question of the balance between biological imperatives and social prohibition. I therefore see a broadening of interest among legal legislators and the judiciary in neurobiological studies that relate to complex human conduct which comes within the province of the law. It is for this reason that, when Editor, I initiated a special issue of the Philosophical Transactions of the Royal Society on the theme of Law and the Brain. It was perhaps but a small step in a debate which, I am certain, will become increasingly important in the future.