Finger length 'not a pointer' for future sexual behaviour

"How to work out if your partner is cheating on you? Check their fingers," the Daily Mirror advises. The news comes from research founded on the theory that humans are believed to display two types of mating pattern – one more promiscuous, and the other more monogamous.

Previous animal and human studies have already suggested higher testosterone exposure is associated with longer ring to index finger ratios. And high testosterone has been linked to sexual promiscuity.

To look into this further, the researchers studied two separate samples. One sample of just over 500 people completed an online survey of their sexual behaviour. A completely separate sample had their finger lengths measured.

The results from the first sample found more men tended towards the promiscuous side, and more women towards the monogamous.

The second sample's results found more men have longer ring fingers than index fingers, while roughly equal proportions of women have either similar length fingers or longer ring fingers.

From this, the researchers inferred that – contrary to the findings of the survey – a more promiscuous pattern seems to be found in both sexes. Had women been more monogamous, as the survey suggested, they apparently should have had longer index fingers.

Whatever interpretation you could reasonably take, the study proves nothing about any association between sexual behaviour and the ratio of finger length.

Comparing two entirely different data sets takes you into "oranges and apples" territory – trying to find connections between two entirely different objects.

You also have to consider how representative people who chose to complete an online survey of sexual behaviour are of the general population.

Where did the story come from?

The study was carried out by psychology researchers at the University of Oxford and Northumbria University, and received funding from the European Research Council.

It was published in the peer-reviewed journal, Biology Letters, which is a publication of The Royal Society.

The article is open access, so it can be read online for free.

The UK media appear to have taken the study at face value, choosing not to report its important limitations. Namely, this study separately analysed sexual behaviour in one group and finger length in another group, and then drew links between the two.

But from the tone of the reporting, we suspect the journalists themselves were taking the findings of the study with a pinch of salt and with their tongues firmly in their cheeks.

If the suggestion that finger length can be used as a reliable predictive method for something as complex as human sexual behaviour sounds ridiculous, then it probably is ridiculous.

What kind of research was this?

This was a study looking at sexual behaviour patterns in one study sample, and the ratio of index and ring finger length in another study sample.

It aimed to look at the patterns of distribution of the two separate factors in these two separate samples, and from this see whether there may be an association between sexual behaviour and finger length.

The researchers say, in animal terms, humans are believed to fall midway between a monogamous and polygamous species, respectively a mix of longer-term and short-term mating patterns.

They say the extent to which any individual primarily pursues a "restricted" mating strategy, favouring exclusive pair-bonds, or an "unrestricted" strategy of promiscuity, is called "sociosexual orientation".

As a broad generalisation, males are usually considered more likely to favour promiscuous patterns than females as a way of getting more mating opportunities. 

The researchers say previous studies have shown that index to ring finger ratio is influenced by the amount of testosterone the growing foetus is exposed to in the womb, as well as the density of testosterone receptors.

Other studies in primates are also said to have shown finger length is associated with mating patterns.

This research tested the theory that there are two restricted and unrestricted types of people by looking at two large data sets – one sample completed a sociosexual orientation survey, and one sample had their index to ring finger ratio measured.

What did the research involve?

A total of 595 North American and British men and women (average age 25 years) completed the online sociosexual orientation survey (SOI-R).

Preferred mating strategy was said to be assessed using the "attitude" and "desire" subscales of the SOI-R, but this was not explained further.

A separate study collected data on index to ring finger ratios on the right hand of 1,314 British men and women.

The researchers then carried out analyses looking at the modelled distribution of sexual behaviours in the North American and British sample, and the distribution of finger length ratios in the other British sample.

What were the basic results?

Sociosexual orientation survey

The researchers plotted distribution curves, where SOI-R score is plotted against density (the number of people who had that score). They did this for four separate groups: British men and women, and North American men and women.

All the groups of men and women showed what is called a "bimodal" distribution – two normal patterns of distribution. For all four groups, there was one pattern with a peak of normality at a lower SOI score (a more monogamous pattern), and a second pattern with a peak of normality at a higher SOI score (a more promiscuous pattern).   

There was a slight difference for men and women, though. For both North American and British men, the higher peak was at the higher SOI score, corresponding with slightly more men with a more promiscuous pattern.

Meanwhile, for both North American and British women, the opposite was seen – the higher peak was at a lower SOI score and corresponded with slightly more women following the monogamous pattern.

Finger length ratios

When similarly looking at the distribution of index to ring finger ratios for the other British sample, the researchers also found two normal distributions, but this time there was more of an overlap between the two curves.

For the sample of British men, the much higher peak was at a ratio of around 0.94 (ring finger slightly longer than index). There was a second much lower peak at a ratio of around 1 (similar length fingers).

For women, there was one common peak at around 0.94 and another equally common peak at a ratio of around 1.

Interpreted crudely, this means in men it is more common to have a longer ring finger than index finger, while in women equal numbers have fingers of the same length, or a ring finger longer than an index finger.

How did the researchers interpret the results?

The researchers concluded that: "This study is the first, to the best of our knowledge, to show statistically that both men and women exhibit two reproductive phenotypes of varying proportions."

But they go on to say that: "The proportional split in males slightly favours an unrestricted (short-term) mating strategy, with a 57:43 split on average, whereas females have a reversed split (47:53)."

The researchers then said that: "However, the mixing proportions in the [index to ring finger] ratio data set suggest that a slightly higher proportion of the unrestricted phenotype is present in both sexes (males approx. 62%, females approx. 50%)."

Presumably, this is based on previous research, which suggested testosterone levels are associated with finger ratios.

Conclusion

This research was founded on the theory that, as a species, humans are believed to display two types of mating pattern – one more promiscuous pattern made up of shorter-term attachments, and the other more monogamous pattern favouring longer term pair-bonds.

Traditionally, the male of a species is considered to favour the more promiscuous pattern to create more mating opportunities.

The research also centres on the other observation from previous human and animal studies, which found finger length is associated with mating patterns, and that index to ring finger ratio is influenced by testosterone levels.

The researchers used two separate samples for their study:

• The first sample suggested there does seem to be two normal mating patterns for both men and women – one more promiscuous, and the other more monogamous. However, a higher proportion of men tend towards the more promiscuous side, while a higher proportion of women tend towards the more monogamous side.
• The second sample, which assessed index to ring finger ratio, suggested more men have longer ring fingers than index fingers, while roughly equal proportions of women have either similar length fingers or their ring fingers are longer.

The researchers seem to imply from this that finger ratios actually show a more promiscuous pattern seems to be found in both sexes. This is because men tend to have longer ring fingers, which presumably ties up with higher testosterone levels and more promiscuous patterns.

However, women were found to have two equal distributions of finger length ratios. But if women were indeed more monogamous, as the other survey suggested, the researchers should have expected them to have longer index fingers.

Overall, this paints a rather unclear picture and it is hard to draw any reliable conclusions from this study.