leave the mother's body. It is thought that this shorter gestation period is an adaptation to ensure the survival of mother and child because it leads to altriciality. Neonatal brain and body size have increased in the hominin lineage, and human maternal investment is greater than expected for a primate of similar body mass. The obstetrical dilemma hypothesis suggests that in order to successfully undergo childbirth, the infant must be born earlier and earlier, thereby making the child increasingly developmentally premature. The concept of the infant being born underdeveloped is called altriciality. Humans are born with an underdeveloped brain; only 25% of their brains fully developed at birth as opposed to non-human primates where the infant is born with 45–50% brain development. Scientists have believed that the shorter gestation period can be attributed to the narrower pelvis, as the baby must be born before its head reaches a volume that cannot be accommodated by the obstetric canal. Social assistance Human infants are also almost always born with assistance from other humans because of the way that the pelvis is shaped. Since the pelvis and opening of birth canal face backwards, humans have difficulty giving birth themselves because they cannot guide the baby out of the canal. Non-human primates seek seclusion when giving birth because they do not need any help due to the pelvis and opening being more forward. Human infants depend on their parents much more and for much longer than other primates. Humans spend a lot of their time caring for their children as they develop whereas other species stand on their own from when they are born. The faster an infant develops, the higher the reproductive output of a female can be. So in humans, the cost of slow development of their infants is that humans reproduce relatively slowly. This phenomenon is also known as cooperative breeding. Malleable cranium Humans are born with a very malleable fetal head which is not fully developed when the infant exits the womb. This soft spot on the crown of the infant allows for the head to be compressed in order to better fit through the birth canal without obstructing it. This allows for the head to develop more after birth and for the cranium to continue growing without affecting the birthing process. Challenges to the obstetrical dilemma hypothesis The obstetrical dilemma hypothesis has had several challenges to it, as more