Hip width and risk of birth-related trauma may play role in woman's decision to have sex

In a new study, women who were more inclined to have one-night stands had wider hips, reveals Colin A. Hendrie of the University of Leeds in the UK. He is the lead author of a study into how a woman's build influences her sexual behavior, published in Springer's journal Archives of Sexual Behavior.

The study into whether hip width or waist-to-hip ratio was a better predictor of a woman's sexual behavior was conducted among 148 women between 18 and 26 years old. The participants all had at least one sexual partner previously. Their hip width (defined as the distance between the upper outer edges of the iliac crest bones of the pelvis) was measured, as well as their hip circumference at the widest point and their waist circumference at its narrowest point. Participants also completed a questionnaire about their sexual histories, including the age at which they lost their virginity, the number of sexual partners they had had, and information about emotionally significant sexual relationships they had had.

The results show that the number of sexual partners a woman had is largely driven by one-night stand behavior. This, in turn, correlates with a woman's hip width and not waist-to-hip ratio. Overall, women in this study with hips wider than 36 centimeters (14.2 inches) had more sexual partners and more one-night stands than women with hips under 31 centimeters (12.2 inches) wide. More specifically, the women for whom one-night stands accounted for three out of every four of their sexual relationships had hips at least two centimeters (0.8 inches) wider than their counterparts in whose lives such fleeting relationships were not as prevalent.

The researchers, Hendrie and co-authors Victoria J. Simpson and Gayle Brewer, surmise that women with wider hips are more likely to engage in sex because the birth process is generally easier and less traumatic for them than for smaller-hipped women (below 31cm). This in turn relates back to how humans learned to walk upright and the subsequent development of narrower hips to make it easier to walk. In the process, female hips have become just wide enough to allow childbirth. Infants are born at a less developed stage than most other primates because of this restriction, and therefore need much more care and investment after birth from their mothers and fathers.

"Women's hip width has a direct impact on their risk of potentially fatal childbirth-related injury. It seems that when women have control over their own sexual activity this risk is reflected in their behavior. Women's sexual activity is therefore at least in part influenced by hip width," concludes Hendrie. He added, however, that statements about causality cannot be made using the current data and it remains to be seen if these conclusions can be generalized to other populations and cultures.