Mountains of research has been done on just why some women have more sex partners than others. And, boy oh boy, the sheer range of theories on the topic would blow your mind.
A new study conducted at the University of Leeds found that women with wider hips were more likely to have more sexual partners. Importantly, their waist-to-hip ratio was not found to impact their number of sexual partners. The researchers asked 148 women aged 18 to 26 about their sexual histories and took measurements of their hips. The data showed that the women who engaged mostly in one-night stands tended to have hips that were at least two centimeters wider than those who had fewer sexual partners.
The reason for the link? According to ScienceDaily, the researchers believe 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." As the study's author, Colin Hendrie, writes, "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."
Of course, it would be extremely difficult to prove that hip width actually causes any sort of sexual behavior — and the study author acknowledge this point — rather than just representing a mere correlation. But, furthermore, it could be that something other than birth canals is at play. It could be that in this cultural moment, we simply value slightly larger hips, which could equate to more opportunities for casual sex for that group. If this were true, then it's possible that women across a variety of hip widths enjoy one-night stands, but those with slightly wider hips have more opportunities to have them. Standards of physical sexual attractiveness sometimes vary across cultures, so further studies across many cultures could help parse what's really going on here.
This is just one attempt of many to explain what scientists call women's sociosexual orientation, or the fact that some have no-strings sex more than others. One 2010 study from the University of Texas at Austin suggests that women are more likely to have one-night stands as they find their "biological clocks running out." Again, it could be that older, "low fertility" women — defined in this study as women ages 27 to 45 — aren't having more one-night stands because their eggs are going bad. But, rather because they're, well, older and more sexually experienced. They might just know what they like sexually and be better able to vocalize it to a partner, making one-night stands more pleasurable.
Some experts think promiscuity could be a heritable trait, while others draw a connection between a woman's sociosexual orientation and whether or not her parents got divorced when she was very young. Other research (in bird mating) indicates that uneven male-female ratios make females more likely to "cheat" on their partners or switch partners many times. Then there's the study that claimed tall women are more likely to cheat on their partners possibly because they have more testosterone than other women. If tall women are, in fact, proportionally larger than their shorter counterparts, then they would be likely to have larger hips (in absolute terms). So, it could be that the presence of testosterone is at work in the results of the University of Leeds Study, not ease of birthing.
Of course, even though these theories tackle the question from nearly every conceivable angle, almost none of them address the issue of how we view (and stigmatize) casual sex in women. And, importantly, most of the existing scholarship is only concerned with heterosexual sex.
Of course biology, genetics, and hormones affect our desires and behaviors (sexual or otherwise). But, it's got to be more complex than that. It's also important to not explain female sociosexual behavior in a way that removes agency or desire from the equation. But, whether it's attributed to wide hips or a yen to procreate, there's a seeming drive among the (male-dominated) scientific community to distill female promiscuity down to some singular, biological source. Women have to be emotionally damaged, or running out of time, or in possession of "birthing hips" to want to have some non-committed sex. These are pretty simple ideas for something as complex as sexuality.