Do you stray or stay? Humans divide into promiscuous or faithful groups

4 February 2015

Both men and women fall into two groups, one more inclined to be promiscuous and the other more inclined to be faithful, an Oxford University study has found. 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 the 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 at Oxford University’s Department of Experimental Psychology suggests that this may actually be because humans consist of a mix of people, some of who are interested in short-term flings while others would like to form long-term commitments.

The researchers analysed previously collected answers from nearly 600 North American and British people on a standardized questionnaire about social and sexual attitudes. They also looked at measurements of the right index (second) finger versus the ring (fourth) finger from 1,313 British people. 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 people’s answers tended to clump into two groups, with one group corresponding to those who were more likely to value sexual fidelity, and the other group more likely to be promiscuous.

This was true for both men and women. While they were slightly more men in the ‘promiscuous’ group, Dr Wlodarski said, ‘We need to collect more data to confirm the possibility that there are more men in the promiscuous group, as these results are still quite preliminary’.

The analysis of the finger lengths similarly found that men 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 were similar in length, making them more likely to seek long-term relationships.

Dr Wlodarski said, ‘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 very minor effect on something as complex as sexual relationships’, according to Professor Dunbar.

For more information contact: Rafael Wlodarski on [email protected].

Robin Dunbar on [email protected], or +44 (0)1865 271413.

Alternatively contact the Oxford University News Office on +44 (0)1865 270046 or email [email protected]

Notes to Editors:

  • A report of the research, entitled ‘Stay or Stray? Evidence for Alternating Mating Strategy Phenotypes in Both Men and Women’, is published in the journal Biology Letters .
  • In total, data on mating strategies was previously collected from 575 North American and British males and females. Data on 2D:4D digit ratios was obtained from photocopies of the right hands of 1,314 British males and females.
  • Mating strategies are referred to as ‘Sociosexual orientation’, and are measured using a standardised self-report measure which assesses an individual’s attitudes and desires towards ‘non-committal’ sex.
  • ‘Finite mixture model analysis’ was used to analyse the distributions of male and female mating strategies and male and female digit ratios. Results suggested that within both males and females, there were likely to be two sub-populations within each sex explaining the distribution of the data – one comprising of individuals pursuing promiscuous (short-term) mating strategies and one monogamous (long-term) mating strategies. Similar sub-populations best explained the distribution of finger ratios (and thus testosterone exposure at birth).
  • 2D:4D digit ratio refers to the relative length of the second (index) finger compared to the fourth (ring) finger. The gene cluster which affects the development of some sex differences also affects the length of the ring finger, with higher testosterone exposure in the womb related to relatively longer ring fingers. Some researchers have previously found that such early testosterone exposure may also influence human mating strategies. In our closes relatives the primates, for example, it has been found that 2D:4D digit ratio is related to primate mating systems (i.e. more promiscuous or more pair-bonded primate societies).
  • While more data is required for confirmation, the results suggest that in these North American and British populations there were slightly more ‘promiscuous’ males than ‘monogamous’ males. Females showed the reverse, with more females falling in the monogamous group than in the promiscuous group.