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

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Economic dependence promotes prudishness

Stay home, bake whitebread, don't sleep around.
Stay home, bake whitebread, don’t sleep around.

Marriage, according to those who habitually preface the word with “traditional”, is a collaboration. With complementary roles, filled as predictably by one woman and one man as peanut butter fills the gap between two slices of white bread.

If you encounter somebody clinging to this view of marriage in which women happily traipse down the church aisle into economic dependence on their menfolk, then I’m sure you can predict their views on sex and the thousand other issues that inhere to sex:

Sex education? Abstain until marriage, ‘cos true love waits.

The pill? Okay if you’re using it to control your acne.

Abortion? Causes all those calamities the greenies like to pin on climate change.

Okay, my clumsy stereotype grows unkind. My point is that more often than not women’s economic dependence on men is bundled up with strong views against sexual promiscuity.

But why? Are economic dependence and anti-promiscuity morality both symptoms of the same cause? Patriarchy, perhaps? Or does one bring about the other? A new study in Archives of Sexual Behaviour suggests that economic dependence might lead to anti-promiscuity views.

Paternity no laughing matter

Visiting friends or relatives in the neonatal ward isn’t the place to crack jokes about paternity. In fact, most people, especially relatives of the new mum, go to great efforts to comment on the newborn’s likeness to the guy who thinks he’s dad.

Paternity stikes such a raw nerve with men because they can never be truly sure that they’re the father. At least they couldn’t until recent technological developments in DNA analysis made it possible.

And yet throughout our evolutionary past, some men thought they were working hard to raise their own genetic progeny where they were actually rearing the young of another. Men who were suspicious, jealous and not prepared to raise another man’s children might not have won any nice-guy prizes, but they did ensure their hard work contributed to the success of their own genes. Including any genes that disposed them to jealousy and vigilance about paternity. Unfortunately, we are all descended from many such men.

Today, humanity’s long history of insecurity over paternity can be seen in the politics of paternity testing and the undignified squabble over how many children are really sired by someone other than dad.

Sensitivities over paternity have shaped religious practices, laws, customs for the inheritance of wealth, and behavioural norms.

In their pop bestseller Sex at Dawn, Christopher Ryan and Cacilda Jethá argued that sexual jealousy and paternity insecurity are newcomers to human society, almost unknown in our species’ long hunting and gathering past where love flowed more freely and couples stayed together only briefly. The economic changes wrought by farming tied families to the land, necessitating cultural innovations to ensure wealth and land stayed within the family.

Ryan and Jethá make several mistakes, including unduly romanticising our hunter-gatherer ancestors, and viewing culture as something separate from biology. The cultural practices that surround fidelity and conception are more usefully viewed as extensions of men’s evolved paternity insecurity. And the scale of those extensions varies among places and over time.

When to worry about promiscuity

When women depend economically on their husbands or partners, then both women and men should value paternity certainty more highly. Men working hard to raise a family have plenty to lose in evolutionary terms if the children they raise are actually sired by somebody else. When men don’t do much for their partners or the offspring, they should be much more chilled about paternity, and thus much more relaxed about sexual promiscuity.

Likewise, when a woman depends heavily on a man’s labour, or the money he brings in to the household, then the cost of losing him is much greater. There are two ways she might lose him through extra-pair sex: if he has other sexual relationships he could run off with one of the other women, leaving his existing family in the lurch. But when she has extra-pair sex and gets busted, she might lose him. Or worse. Jealousy can trigger psychological abuse and violence.

In the recent paper that inspired this column, psychologists Michael E. Price, Nicholas Pound and Isabel M. Scott, from Brunel University in the U.K., sought to test the links between women’s economic dependence and both women’s and men’s attitudes to promiscuity.

From online surveys of more than 5000 Americans, Price and his colleagues showed that when the women in a subject’s social network depend economically on men, then subjects tend to judge promiscuity more harshly. And the effects weren’t spurious consequences of religion, or ethnicity or political conservatism. When they fitted these other variables into their statistical tests, the association between female economic dependence and opposition to promiscuity remained.

Price also asked whether the association arose as an artifact of geography: Texas and Utah, for example differ culturally on questions of morality and gender roles from, say, Massachusetts or California. States in which women earned more were also more relaxed about promiscuity. And this result arose out of the effects women’s earnings had on female economic independence.

Even more compelling, by comparing the attitudes of geographic neighbours, in the same or nearby zip codes, Price and his colleagues found that the association held. Irrespective of where you live, the economic dependence of the women a person’s social network predicts how they feel about promiscuity and non-marital sex.

We’ve known for some time that variation among societies in sexual morality is associated with variation in gender roles and, especially, earnings. The exciting development is how the new research suggests the patterns emerge from the bottom upward, with individual attitudes being shaped by prevailing economic circumstances in their close social network. At least in part.

I’m interested to know what the authors think about the relevance of their data to Roy Baumeister and Kathleen Vohs’ ideas on sexual economics in which women restrict the supply of sex under circumstances when they have the most to gain from a high price. This interpretation is not inconsistent with Price et al’s arguments about paternity certainty. But high female economic dependence presents exactly the kind of economic situation in which women need to drive a hard bargain in the sexual marketplace. Intriguingly, women took stronger anti-promiscuity stances, on average, than men did.

There are so many studies I would like to see done with a view to teasing out the causal relationships, and how attitudes to promiscuity change in the headwind of religion and other cultural forces. But this finding explains much about some of the trickiest ideological differences both within and among societies.

It could explain how economic changes since the Second World War paved the way for the sexual revolution. And why conservative politicians, especially in the U.S.A. seem equally hung-up on sexual liberty and the growing proportion of breadwinner moms.

And it may form an important ingredient in the ever-growing and dangerous ideological chasm between patriarchal theocracies and more gender-egalitarian democracies.

Thom Yorke singing Radiohead’s True Love Waits

P.S. I always relish seeing how other media cover research concerning issues touching sexual morality. According to “News Staff” at Science2.0, “If women want their promiscuity to be accepted they have to earn more money say evolutionary psychologists”. Keith Perry of the Telegraph reckons “Promiscuous women more likely to be tolerated if they are high earners”. And Lydia Smith, writing in the International Business Times got even more pithy, declaring “Only poor women are branded sluts”.

These were the first three links to news stories that popped up on Google. Not one headline reflects the real message of the study, but they all find a short path to slut shaming. For the record, the study tracked attitudes to women’s and men’s promiscuity.

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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.

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