© chaddlin/iStockphotoPinky promise: The longer your ring finger is compared to your index finger, the higher statistical likelihood of promiscuity.
Why humans seem to be an exception among mammals on the matter of sexual relationships has long been a puzzle.
Other mammalian species are emphatically polygamous or monogamous as a group.
But as everyone knows anecdotally, Homo sapiens do not fall into one neat category or the other.
Everyone knows of couples that are sexually faithful, but also of those that are not.
What has been lacking are the statistics to show these differences, which is a key step to explaining them.
Now a team of UK scientists say they have found just that.
"We observed what appears to be a cluster of males and a cluster of females who are more inclined to 'stay,' with a separate cluster of males and females being more inclined to 'stray' when it comes to sexual relationships," says study co-author Rafael Wlodarski, an experimental psychologist at the University of Oxford.
In their study, published today in the journal Biology Letters, Wlodarski and colleagues compared two potential indicators of sexual behaviour.
One source was an online questionnaire on sexual habits, completed by 585 North American and British respondents between the ages of 18 and 63, who on average were nearly 25.
The other was data obtained from 1314 British men and women -- an investigation based on something known as the '2D:4D' ratio.
Finger length
What lies behind the 2D:4D idea is that the length of one's ring finger indicates the level of the hormone testosterone to which one was exposed in the womb.
The longer your ring finger is, compared to your index finger, the higher the likely concentrations of foetal testosterone.
This in turn has been linked to a higher statistical likelihood of promiscuity.
Put together, the datasets showed that 57 per cent of men were more likely to be promiscuous, and 43 per cent faithful.
This balance inversed among women -- 47 per cent fell within the 'stray' category and 53 per cent in 'stay'.
Taken alone, the 2D:4D study, based on a purely physiological characteristic, yielded higher 'stray' numbers for both men and women -- 62 per cent and 50 per cent respectively.
The higher number of 'stay' candidates in the questionnaire study may be explained by the influence of life experience and culture.
This very difference underlined the need for caution in interpreting their results, say the researchers.
"Human behaviour is influenced by many factors, such as the environment and life experience," says study co-author Professor Robin Dunbar of the University of Oxford.
"What happens in the womb might have only have a very minor effect on something as complex as sexual relationships."
How can different sexual behaviours be explained?
Seen through a Darwinian lens, sex with multiple partners boosts the chances of offspring -- of passing on one's genes.
A long-term sexual relationship requires more personal investment. But it increases chances that the offspring that results from the sex will survive.
"This research suggest that there may be two distinct types of individuals within each sex, pursuing different mating strategies," say the scientists.
Source: AFP
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