Reconsidering the Obstetrical Dilemma: Correlations Between Head and Pelvic Size

The Obstetrical Dilemma (OD) theory has become canon in biological anthropology. The OD posits that i] dystocia results from bipedal mothers and encephalized infants, ii] contrasting selection for bipedality and obstetrics hinders locomotive efficiency, and iii] the contradicting requirement of the fetus being small enough to pass through the birth canal yet being cognitively advanced enough to cling to its mother after birth. Females, theoretically, exhibit deficient gait efficiency for the sake of successful childbirth. An obstetric advantage theory has been posited where taller individuals with a larger head size have larger pelves. If the distance between the acetabulae increases as an obstetric advantage, it would be necessary for there to be a concomitant increase in femoral neck length to maintain equivalent locomotor efficiency. This study tests that individuals with larger cranial circumferences have wider pelves and, in turn, longer femoral neck lengths. The cranium, pelvis, and femur of a modern sample of 100 individuals were assessed (49 females and 51 males) at the Sam Houston State University’s Applied Anatomical Research Center. Cranial circumference encompassed the widest points of the cranium. Pelvic and femoral metrics include anterioposterior diameters of the pelvic inlet, midplane, and outlet as well as the transverse diameter of the pelvic inlet, bi-iliac diameter, bi-acetabular diameter, femoral head circumference, and femoral length. Three variables were computed: i] pelvic inlet shape, ii] lateral iliac flare, and iii] skeletal effective mechanical advantage. Results show statistically significant correlations between cranial circumference and the anterioposterior diameters of the pelvic inlet and midplane for females as well as bi-iliac diameter and lateral iliac flare for males. No significant difference was found in pelvic inlet shape for either sex in this sample; nor was there a significant correlation in either sex between femoral length, a correlate of stature, and pelvic inlet shape. There was no significant association among cranial circumference and pelvic inlet shape nor femoral neck length. The proposed hypothesis fails to be supported. Results also show no convincing evidence of significant deficient locomotive ability for females, as theorized by the OD