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’.


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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Sexually despotic men rewrite history

Alfred L. Kroeber (left) with Ishi, the last member of California’s Yahi people, in 1911. Wikimedia Commons
Alfred L. Kroeber (left) with Ishi, the last member of California’s Yahi people, in 1911. Wikimedia Commons

“Heredity”, opined the pioneering cultural anthropologist Alfred L. Kroeber in 1915, “cannot be allowed to have acted any part in history”. I have yet to encounter a crisper expression of the view that biological explanations have no place in the study of society and history. Kroeber’s words have resonated through the social sciences for a century, divorcing nurture from nature, social from biological, at considerable harm to our understanding of society and what it means to be human.


Fortunately, many 21st Century anthropologists, economists, neuroscientists, geneticists, sociologists and thinkers so busily inter-disciplinary that they defy dusty departmental labels, are consigning the hoary distinction between nature and nurture to the past. Likewise the distinction between heredity and history is steadily dissolving as the where intimate links between evolutionary fitness and major historic transitions come into view.

Last week’s European Journal of Human Genetics, for example, carried a fascinating article led by evolutionary geneticist Patricia Balaresque exploring the signature of historic population expansions in the distribution of Y-chromosome genotypes of men alive today.

The Y of who, what and when

The the human Y chromosome represents a tiny portion of the genome, including the genes that trigger a foetus to develop into a male, rather than following the default female pathway. Every now and then a small change occurs in one of the less important parts of a Y-chromosome’s DNA. Such a change is passed to a man’s sons, those sons’ sons, and so on. That means there’s quite a bit of variation in these Y-chromosome sequences in any human population.

Marco Polo at the Kublai Khan. Miniature from the Travels of Marco Polo c 1298.
Marco Polo at the Kublai Khan. Miniature from the Travels of Marco Polo c 1298.

So if a man happens to have many sons, who each go on to have many sons and so on, one might detect a sudden surge in the frequency of the Y-chromosome sequence borne by that line of men (patriline). In 2003, a large team from Oxford University detected evidence of just such an event. Across much of Asia, one particular Y-chromosome sequence was carried by 8 percent of all men. In a paper pithily titled “The Genetic Legacy of the Mongols”, they famously fingered Genghis Khan as the chief suspect.

The pattern of variation within the lineage suggested that it originated in Mongolia approximately 1,000 years ago. Such a rapid spread cannot have occurred by chance; it must have been a result of selection. The lineage is carried by likely male-line descendants of Genghis Khan, and we therefore propose that it has spread by a novel form of social selection resulting from their behavior.

Where by ‘behaviour’ they mean more than the Great Khan’s triumph in unifying the Mongols and establishing the largest continuous empire history has ever known. For Khan was as much about the establishment of a genetic dynasty as a political one. According to one disputed quote, he once said:

The greatest joy for a man is to defeat his enemies, to drive them before him, to take from them all they possess, to see those they love in tears, to ride their horses, and to hold their wives and daughters in his arms.

Or words to that effect.

Such a genetic legacy is far more than the work of a single man. It is likely that Khan’s uncles, brothers and cousins, played a substantial role, too. And his direct male descendants spread both his empire and his genetic legacy. Ghenghis’ grandson, Kublai Khan, married four main Empresses, but, according to Marco Polo’s Travels was attended to by hundreds of beautiful young women, working five at a time in three-day shifts. And a much greater number of women, recruited to the palace but, after a second round of screening, not deemed perfect enough for the Khan himself, were bestowed on Kublai’s nobles, many of whom would have been relatives sharing his patriline and Y-chromosome.

Khan not the only one

The Mongol expansion is far from the only such event to leave a genetic signature. A 2005 paper identified a particularly successful lineage that expanded about 500 years ago in Northeastern China, possibly through the lineage who established the Quing Dynasty.

Last week’s findings report on a survey of 5321 men from 127 Asian populations, testing for evidence of similar population expansions. Belaresque and her collaborators identified eleven such events, including the ones tied to the expansions of the Mongols and the Qing dynasty. And some of those events date back as far as 2100 BCE.

The earliest expansions, between 2100 and 300 BCE, are associated with the flourishing of agriculture in the Fertile Crescent, East India, and South East Asia. As agriculture took hold, elites accumulated wealth and influence beyond anything our hunter-gatherer ancestors could have conceived. These represent perfect conditions for sexual despots: male rulers who take many wives, keep numerous concubines or mistresses, and have many offspring. They also establish successions that favour their male descendants, handing them the wealth and power to become sexual despots themselves.

Another expansion began around 1100 CE in the Near East and expanded to the South East Indian coast. This might be a signature of the “rapid expansion of Muslim power … after the establishment of a unified polity in the Arabian peninsula by Muhammad in the 7th Century and under the subsequent Caliphates”.

The Mongol and Qing expansions, and another from Northeast China beginning no earlier than 850 CE, were not associated with the establishment and spread of agriculture, but rather with nomadic, pastoral lifestyles made possible by the domestication of horses. Pastoral nomads ruled the steppes for thousands of years, thanks to their horse-powered mobility and fighting ability.

They established several empires, giving rise to hierarchies, elites and patrilineal reproductive despots like the Khans and the Qing. And their Silk Road trade corridor facilitated westward expansion of their genetic dynasties.

Heredity and History

Even in the rather well-studied case of Genghis Khan we are well beyond certain idenfification of the individual progenitors whose success sparked each expansion. Innovative analyses and lucky ancient DNA finds may yet do so for some cases. But the ability to detect great tides of patrilineal descent in societies of various types offers far more interesting possibilities than compelling personal narrative.

The evidence shows that with great power and wealth can come great evolutionary fitness. The tools are now falling into place to assess how much the psychological adaptations that shape reproductive success have given history its shape.

This is one tale where History certainly represents HIS Story. The fact that new Y chromosome sequences can spread so fast and so wide when history’s tide turns suggests that a very small number of sexually despotic men can leave massive numbers of descendants. But each man who traces his descent back to Genghis Khan or another such super-ancestor through an unbroken male line has sisters who do so too, save for the very last branch in their family tree. If 2% of people in Asia descend on a male-only lineage from the same male ancestor as Khan, then how many times do they descend from him through at least one female ancesor?

Each of us descends many times over from a great many sexual despots. It would be Kroeber-like wilful ignorance to be think we don’t also inherit many of the genes that biased their behaviour toward the accumulation of power, the vanquishing of rivals and reproductive despotism.

And when I say “we”, I don’t only mean men. Every man has a mother. Every descendant of Ghenghis Khan is also a descendant of his mother Hoelun.

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