Both men and women fall into two groups, one more inclined to be promiscuous and the other more inclined to be faithful. Unlike other species, which are inclined to be either promiscuous or faithful, both mating strategies seem to be used by humans.
The results, published in the journal Biology Letters, are from a study jointly carried out by Professor Robin Dunbar's lab at Oxford University and Professor John Manning at Northumbria University.
Previous physical comparisons between humans and other mammals suggest that humans are mid-way between being a faithful species and a promiscuous species. Research by Dr Rafael Wlodarski and Professor Dunbar in Oxford University's Department of Experimental Psychology suggests that this may actually be because humans consist of two separate sub-populations of people: one group is more interested in short-term flings, while another would like to form long-term commitments.
The researchers analysed previously collected answers from 575 North American and British people on a standardized questionnaire which assesses an individual's attitudes and desires towards 'non-committal' sex. They also looked at photocopies of the right hand from 1,314 British men and women.
From these photocopies, they measured the length of the index (second) finger and the ring (fourth) finger. The shorter the index finger in relation to the ring finger (the 2D:4D ratio), the higher the levels of testosterone that person is likely to have been exposed to while developing in the womb. This is true for both men and women. From previous work, the researchers already knew that the higher levels of foetal testosterone can go together with greater sexual promiscuity as an adult. While not predictive of individual behaviour, the length of the ring finger versus the index finger can help identify the group of people who are more likely to be promiscuous.
The statistical analysis of sexual attitudes data found that the distribution of people’s answers was 'bimodal', i.e., it clumped into two distinct groups. One group corresponded to people who were more likely to value sexual fidelity, with the other group more likely to be promiscuous. This bimodal distribution was there for both men and women.
The analysis of the finger lengths similarly found that both men and women tended to segregate into two groups. One group had a ring finger which was much longer than the index finger, suggesting that they had been exposed to more testosterone in the womb and may be more likely to seek many sexual partners. The other group had fingers which similar in length, making them more likely to seek long-term relationships.
Dr Wlodarski explains, 'This research suggests that there may be two distinct types of individuals within each sex pursuing different mating strategies. 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.'
'It is important to note that these differences are very subtle, and are only visible when we look at large groups of people: we cannot really predict who is going to be more or less faithful. Human behaviour is influenced by many factors, such as the environment and life experience, and what happens in the womb might only have a modest effect on something as complex as sexual relationships', according to Professor Dunbar.
These findings imply that human mating may be more complex than originally thought, with many environmental, physiological and psychological variables contributing to human mating behaviours.
A report of the research, entitled ‘Stay or Stray? Evidence for Alternative Mating Strategy Phenotypes in Both Men and Women’, is published in Biology Letters.