The traditional explanation for our nine-month gestation period and helpless newborns is that natural selection favored childbirth at an earlier stage of fetal development to accommodate selection for both large brain size and upright locomotion—defining characteristics of the human lineage. In this view, adaptations to bipedalism restricted the width of the birth canal and, hence, the size of the baby that can pass through it. Human babies are thus born when their brains are less than 30 percent of adult brain size so that they can fit through the narrow passageway. They then continue development outside of the womb, with brain size nearly doubling in the first year.

But when Holly M. Dunsworth of the University of Rhode Island and her colleagues tested this so-called obstetrical dilemma hypothesis, their findings did not match its predictions. For example, the hypothesis predicts that because the female pelvis is broader than the male pelvis, walking and running should be more energetically demanding for women than for men. Yet most studies of the energetics and mechanics of locomotion in women and men found no such penalties for having a wider pelvis, the researchers report.

Furthermore, the team asserts, to accommodate an infant at a chimplike stage of brain development—that is, a brain that is 40 percent of adult brain size, or 640 cubic centimeters—the pelvic inlet (the top of the birth canal, which is the narrowest part) would only have to expand by three centimeters on average. Some women today have pelvic inlets that wide, and those larger dimensions have no measurable effect on locomotor cost. The researchers argue that instead of fetal brain expansion being constrained by the dimensions of the pelvis, the dimensions of the human pelvis have evolved to accommodate babies, and some other factor has kept newborn size in check.