The study began with an oft-cited theory called the obstetrical dilemma, which holds that humans sacrificed relative ease of childbirth for standing upright, said Colin Hendrie, a professor of human and animal ethology at the University of Leeds in the U.K. and researcher on the new study. Narrower hips are better for walking, while wider hips are better for birthing; so women's hips strike the balance between the two needs.
"There is a prediction that says childbirth is more dangerous for smaller women than larger women," Hendrie told Live Science.
Thus, women with wider hips might subconsciously know that childbirth is less risky for them and be "a lot more relaxed in their social behavior," he said.
Hendrie and his colleagues recruited women between ages 18 and 26 from the University of Leeds and areas nearby. They measured the distance between each woman's iliac crests, the bony protuberances of the hips that can be felt on the front of the body. The women then filled out questionnaires about their sexual histories.
"What we found is that women that had narrow hips had fewer sexual partners, and most of those were within the context of a relationship," Hendrie said.
The women with wider hips — more than 14 inches (36 centimeters) across — had the same number of committed sexual partners as the women with narrow hips, but also had more one-night stands and hookups in their past. The findings are detailed in the April issue of the journal Archives of Sexual Behavior.
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