A new study found no connection between hip width and efficient locomotion, and suggests that scientists have long approached the problem in the wrong way. "This idea, that pelvic width for birth and pelvic width for locomotion are connected, is deeply ingrained in this discipline," said the first author of the stud. "Everyone thinks they know this is true...but it's wrong, and it's wrong for two reasons. First, the way we had modeled the forces involved didn't make sense. Second, we found that you can't predict, from the width of the pelvis, how much energy someone is using, so we've been looking at this biomechanical problem entirely wrong."

Source: Solving the obstetrical dilemma: Wider hips don't make locomotion easier, so why is labor so hard?