Can science finger a philanderer? Not like this!

Two left hands make a heart. Source: Leon Brocard/Flickr
Two left hands make a heart. Source: Leon Brocard/Flickr

Are people naturally monogamous, polygamous or promiscuous? It’s one of those questions that most people feel quite confident in answering. Ask a few people and you’re likely to come up with a variety of contradictory answers, each delivered with considerable confidence. But the question is far more slippery than it first appears.

I will return later to the question of humanity’s “natural” mating system, but lately I’ve been far more interested in why people hold such strong opinions on the subject. And I think it’s mostly out of keenness to understand ourselves and those we love, to navigate the perilous tension between monogamy and non-monogamy that runs through our own lives. And, often, to validate our own proclivities.

With the Hallmark Holiday of St Valentine’s Day just a few days away, a recent study that touches on the monogamy-promiscuity tension deserves close examination. Particularly because various media outlets made it sound like a litmus test of whether someone is a likely ‘strayer’ or a certain ‘stayer’.

Sociosexuality

In ‘Stay or Stray: evidence for alternative mating strategy phenotypes in both men and women’, Rafael Wodarski, John Manning and Robin Dunbar probe the statistical distributions of two traits related to sexual behaviour. They ask whether sociosexuality and the relative lengths of the second and forth fingers (2D:4D ratio) conform to distributions with one peak or two. A bimodal distribution, with two peaks, suggests there may be two different groups of individuals within a given sample.

An individual’s sociosexuality reflects how restricted their attitudes toward sex and their sexual behaviours are. Wlodarski’s team used answers from the following six questions in the 9-item Revised Sociosexual Orientation Inventory (SOI):

  1. With how many different partners have you had sex within the past 12 months?
  2. With how many different partners have you had sexual intercourse on one and only one occasion?
  3. With how many different partners have you had sexual intercourse without having an interest in a long-term committed relationship with this person?
  4. Sex without love is OK. (this and the next two questions answered on a scale from strongly disagree to strongly agree)
  5. I can imagine myself being comfortable and enjoying “casual” sex with different partners.
  6. I do not want to have sex with a person until I am sure that we will have a long-term, serious relationship.

High scores (plenty of agreeing, and lots of casual sex) characterise an unrestricted sociosexuality, whereas people with restricted sociosexuality tend to get low scores.

The statistical tests showed that within fairly large samples of British and American subjects, there was evidence that both women’s and men’s SOI scores are distributed bimodally. But there’s plenty of overlap between the peaks (modes). The authors infer that within each sex exists a more restricted, monogamous group of individuals and another group of unrestricted promiscuous people.

Here, and in almost every study using SOI, men tend to have more unrestricted sociosexuality than women, on average. The difference in means could be pinned on small differences in the percentages of men and women in the restricted and unrestricted SOI groups.

Digit Ratios

The group also analysed a sample of hand measurements of 1314 British subjects. They looked at the ratio of the index finger (second digit, or 2D) to the ring finger (4D). Adults who were exposed to higher levels of testosterone when they were in the womb, tend to have relatively short index fingers (small 2D:4D ratio).

Hand with index finger shorter than the ring finger, resulting in a small 2D:4D ratio, pointing to a high exposure to testosterone in the uterus. source: Wikimedia Commons
Hand with index finger shorter than the ring finger, resulting in a small 2D:4D ratio, pointing to a high exposure to testosterone in the uterus. source: Wikimedia Commons

Now that you’ve stopped looking at your fingers, can we move along?

Prenatal testosterone exposure is also thought to bias individuals toward more promiscuous sexuality when they reach adulthood. The pattern also works across species: monkey and ape species with long-term pair bonds and a knack for monogamy tend to have high 2D:4D ratios.

Interestingly, when Wlodarski’s team applied their statistical tests to the distribution of 2D:4D ratios, they again found evidence of bimodality. A similar pattern in two very different traits associated with promiscuity-monogamy suggested to them a provocative conclusion:

Perhaps we are dealing here with two different types of people.

What if some folks are good at monogamy whereas others are rather better at … the other stuff? Perhaps. Funny thing about sex research is that any conclusion you arrive at will leave some people feeling validated and an almost equal number something more like violated.

We go on foot from here

But the idea is worth exploring. The valuable thing about this study is that it challenges our too-common tendency to see every trait as a continuum, with a few individuals at either end and the majority somewhere in the middle.

Interestingly, the two measures, SOI and 2D:4D ratio were from different samples. At no point did the researchers provide any evidence that the two groups separated on digit ratio were the same individuals as those separated on SOI. They couldn’t be: they were different samples.

The links between SOI and 2D:4D ratios are, at best, equivocal. Some studies find that low 2D:4D is associated with higher SOI. Other studies fail to find such effects. And a study of women and men from Brazil and from the Czech Republic, found that in both sexes a more feminine (higher) 2D:4D ratio is associated with less restricted sociosexuality.

But you wouldn’t know it from the media coverage. The Daily Mirror over-promised, trumpeting that “Boffins” have learned “How to work out if your partner is cheating on you? Check their fingers.” Well, checking if they’ve been hiding their wedding ring might tell you something, but checking out their 2D:4D ratio won’t help at all. Nonetheless, Valentine’s Day dinners are going to involve a lot of quizzical staring at fingers this year. (And not out of daydreaming that he might just put a ring on it.)

The Telegraph took a more introspective line under the headline “Are you promiscuous or faithful? Measure your index finger to find out.” Actually, a better way to figure out if you are promiscuous or faithful, or if you are likely to be in the future, is to ask yourself the questions in the Sociosexual Inventory. It’s pretty straightforward: if you’ve had plenty of one-off sex and lots of partners in the last year, then odds are that you bend toward promiscuity. At least at this point in your life. But I can imagine folks on both side of the 2D:4D distribution reassuring themselves that they are doing the right thing.

Sometimes I wonder why scientists even bother talking to the media. The public love to learn the latest things that “boffins have figured out”, but they deserve journalism that makes at least a token effort to grapple with the research or speak to said boffins.

What are we?

The distribution of SOI and 2D:4D cannot tell us all that much about humanity as a whole, other than that both women and men vary in their openness to casual sex and their proportional finger length.

But this variation is part of what makes human sexual behaviour so fascinating. Some people do seem at ease with life-long monogamy whereas others are shockingly bad at it.

How that variation arises presents a very interesting bevy of questions. Cue the usual intellectually bereft wrangling over nature and nurture as though the two were alternatives.

I’m sure there is more than one reason, but an obvious candidate for variation in sociosexuality is religion. Perhaps those who buy in to religious practices are more likely to be on the “restricted” end of the sociosexuality distribution, whereas those who have rejected or never embraced religion are more likely to be in the “unrestricted” peak?

For now, my preferred answer to the question “Are people naturally monogamous, polygamous or promiscuous?” is “YES”.

We have evolved adaptations that make some of us rather good at monogamy, some of the time. Until we’re not. We also have an evolved capacity to leave one partner for another, or to partner up with more than one person at a time, depending on our circumstances.

If you’re looking for natural history to vindicate your own particular preferred way of life over the alternatives, then you’re always going to be disappointed.

Monogamy can be complicated too. The Police knew this. Check out “Wrapped Around Your Finger

The Conversation

This article was originally published on The Conversation.
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A complex cocktail: alcohol, sex and cute monogamous mammals

Influencing the drinking patterns of others. Apart from being in a short, overly fussy glass and sporting a slice of lemon rather than a lime, the good thing about these gins and tonics is they were made by somebody else. And so I’m not complaining. Source: cyclonebill/Flickr
Influencing the drinking patterns of others. Apart from being in a short, overly fussy glass and sporting a slice of lemon rather than a lime, the good thing about these gins and tonics is they were made by somebody else. And so I’m not complaining.
Source: cyclonebill/Flickr

How does alcohol consumption affect romantic life? Let me count the ways.

If popular advertising is to be believed, the consumption of high-end spirits almost guarantees a steady variety of glamorous amour. I was always surprised that James Bond – before Daniel Craig – opted to take his vodka martinis shaken rather than stirred. Bond was never short of anyone to stir his martinis.

From Dutch courage to a shared glass of champers to drunken would-rather-never-remember sex, alcohol’s tendency to reduce our inhibitions has changed the way drinkers meet and mate. But drinking is also a cause and a consequence of relationship breakdowns and considerable associated misery.

Which is why I’m fascinated to see how the world’s media covers a paper out today in the Proceedings of the National Academy of Science of the US (PNAS). It has an irresistible combination of clickbait-ready elements: a cute small mammal, booze and serious questions about monogamy.

The first element, the cute mammal, is the prairie vole, poster-child for wishful thinking anthropomorphising about monogamy and the power of love. Male-female pairs form long-lasting bonds, sleeping together, grooming one another and raising pups together.

The prairie vole looks even more virtuous alongside its shadier close relative, the montane vole, which tends to mate promiscuously and form no such pair bonds. Which means comparisons of the two species, from ecology to the molecular biology of receptors on the brain, can help resolve the mechanisms involved in prairie vole monogamy.

Early work on these species implicated the hormones vasopressin and oxytocin. Block their release in prairie voles, and they come over all promiscuous – just like montane voles. And it turns out the reward centres of prairie vole brains bustle with receptors for these hormones, but montane vole brains don’t. Decades of work on these closely related vole species have resolved, in considerable detail, the brain regions, receptors and molecules involved in regulating prairie voles’ much-admired mostly monogamous ways. Last June even saw the 21st century’s dark magic – epigenetics – implicated in understanding the neuroscience of how prairie voles “fall in love”.

But like so many mythologised monogamous relationships, a dark secret stalks the prairie vole love story. Because the vole has a bit of a drinking problem.

Not only do voles “self-administer” in much the same way that I do when I mix a gin and tonic, they can also “influence the drinking patterns of a social partner” in exactly the same way I do when I make one for a friend or beloved. But instead of the taste sensation of Hendricks, tonic and a quarter lime, the little fuzzies opt for a 1:10 mix of ethanol and water. They prefer it to plain water.

With an eye for a compelling study, Allison M.J. Anacker and colleagues saw an opportunity to study how self-administered alcohol consumption affects social bonding and the neural mechanisms by which prairie voles form monogamous couples. The effects of alcohol, it turns out, differed between female and male prairie voles.

Huddling up together is a reliable predictor that a couple of voles are likely to mate. Male voles that drank alcohol during a period of cohabitation were as likely to huddle up with a strange female as with a sexually receptive partner. Males who drank only water strongly preferred the partner animal over a strange female. Alcohol, it seems, disrupted the males’ tendency to behave like monogamous paragons of pair-bonded virtue.

Female voles that had been drinking alcohol, on the other hand, grew slightly more likely to huddle up to their partner and not a strange male. It seems that alcohol might enhance female pair-bonding at the same time as diminishing the male commitment to pair-bonding.

Detailed study of the voles’ behaviour as well as, eventually, their brains, revealed that alcohol directly affected the brain structures responsible for pair bonding rather than exerting indirect effects by causing drowsiness or altering levels of aggression. Alcohol, drunk during cohabitation with a potential mate, seems to affect male and female brains and behaviour differently.

Of course humans aren’t voles. Deciding how much to infer about human behaviour from a study on some other organism always presents a tricky challenge. It would be far too much to infer that drinking influences human pair-bonding in exactly the same way as it does for voles. But it would be ignorant to imagine that voles can teach us nothing of value here.

Insights from voles have already led to intriguing findings about the roles of vasopressin, oxytocin and their receptors in human commitment to their romantic partners and families. Men with a particular form of the vasopressin receptor gene, for example, exhibit a suite of traits that suggest they don’t form the same deep pair bonds that other men form: they make less committed partners, suffer from more marriage problems, are more likely to be unfaithful mates and, as a result, their relationships don’t last as long.

Alcohol inhabits a prominent yet complex place in the social and sexual lives of many adults. It can enhance bonding, ease courtship and foster closeness. And it can both moderate and cause stress, dominance/control issues and violence. The issues surrounding alcohol are so important, and so pervasive, that they should not be oversimplified or narrowly construed.

For me the most interesting thing about the vole study is the sex-specific way in which alcohol influenced pair bonding and behaviour. The study of sex differences in brain anatomy and function is an area replete with controversy and contesting ideological claims. Here is an area in which the study of sex differences can transcend simplistic ideological bickering about sexism and gender-free wishful thinking. Surely understanding the ways in which environments, genes and brains interact – over questions as socially pressing and consequential as alcohol and family function – could potentially be harnessed to great effect.

Rob Brooks does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.

This article was originally published on The Conversation.
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