Scientists said Wednesday they had amassed the first evidence to back theories that people fall into two broad categories—promiscuity or faithfulness—when it comes to sex.
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 men cut out to be cads rather than dads.
What has been lacking are the statistics to show these differences, which is a key step to explaining them.
Now a team at the University of Oxford 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," said Rafael Wlodarski, an experimental psychologist and study co-author.
Wlodarski and a team looked at 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 1,314 British men and women—an investigation based on something known as the "2D:4D" ratio.
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.
Comparing the questionnaire results to the 2D:4D study gave the investigators the data they craved, they wrote in the British journal Biology Letters.
Put together, the datasets showed that 57 percent of men were more likely to be promiscuous, and 43 percent faithful.
This balance inversed among women—47 percent fell within the "stray" category and 53 percent in "stay".
Taken alone, the 2D:4D study, based on a purely physiological characteristic, yielded higher "stray" numbers for both men and women—62 percent and 50 percent 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, the researchers said.
"Human behaviour is influenced by many factors, such as the environment and life experience," said Robin Dunbar, a professor at the Oxford unit that did the research.
"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," said the authors.
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Stay or Stray? Evidence for Alternative Mating Strategy Phenotypes in Both Men and Women, Biology Letters, rsbl.royalsocietypublishing.or … .1098/rsbl.2014.0977