But a wider pelvis results in a skeleton less well designed for running, hence a greater risk of being eaten by predators. Another possible solution is for the child to be born sooner, hence smaller. But the earlier the child is born, the harder it is to keep it alive and the longer the period when it is helpless and so requires extensive adult assistance. The solution, as anyone familiar with the mathematics of constrained maximization would expect, is a compromise, pushing on all margins. Human infants are born premature by the standards of other mammals that produce single offspring.
Human females have wider pelvises than males and, on average, run less well.
Humans under natural circumstances suffer a substantial rate of death in childbirth. Population Equilibrium in a Primitive Society Consider what this means for the equilibrium state of a primitive society. On average, the larger an infant's skull, the more likely it is that he and his mother will not survive his birth. This equilibrium is maintained because the larger skulled infants who do survive childbirth on average have higher reproductive success than the average infant, bringing the genetic skull size of the next generation conceived back up. Putting it in shorthand, we have a population in which the genotype has larger skulls than the phenotype.