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Human infants are strange in that they are born more helpless than the infants of other great apes. They are born with about 25-30% of their brain developed, compared to the 40-50% of other great apes. Leutenegger (1982) showed that although human infants have the largest cranial capacity of all primates, the relationship between newborn human cranial capacity and body size is not different from that of other newborn primates. The ratio increases after birth to become 3-to-5 times of other great apes. To achieve this the infant undergoes accelerated brain growth (compared to apes) called the secondary altriciality.

The leading hypothesis to explain this has been the obstetrical dilemma which suggests that there is a trade-off between keeping hips small to allow efficient upright walking, and making the brain large. A recent alternative suggests that there is no measurable handicap from having wider hips, and that the reason for the early birth is metabolic considerations (too expensive for the mother to continue feeding the fetus internally).

To me both these hypotheses seem fundamentally flawed in that they assume it is beneficial to be born with a more (or fully) developed brain. Although we know that the brain shows development, self-organization, and activity in the fetus this is not as efficient as the development possible once the organism is capable of receiving richer stimulus outside the womb. In particular, a fetus is incapable of learning in a social setting. If you subscribe to Dunbar's Social Brain Hypothesis, then this social learning is what our brain is for. Thus, it seems natural to expect that it is more adaptive for a human to be born with a smaller brain (past a certain min threshold) to allow more development and social learning outside the womb.

In a commentary on the new article, Karen Rosenberg suggests the same thing:

Maybe human newborns are adapted to soaking up all this cultural stuff and maybe being born earlier lets you do this. Maybe being born earlier is better if you’re a cultural animal.

This development of social cognition (my name; comments with historic name for this hypothesis are appreciated) approach was suggested by Adolf Portmann in the 60s, but without much evidence to support it. What is the current evidence on the development of social cognition hypothesis?


References

Leutenegger W. (1982) Encephalization and obstetrics in primates with particular reference to human evolution. In: Armstrong E, Falk D, eds. Primate Brain Evolution. New York: Plenum. 85–95

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Good question. What I think is interesting about this is the idea that the relatively short human gestation time relative to developmental completeness would be influenced by an advantage from exposing the fetus to the information-rich post-natal environment as quickly as possible. Such a generalist explanation would seem simpler than one that specifically targets social information, so I might see if there are any criticisms of the SCH from that angle. –  Christian Hummeluhr Mar 30 '13 at 14:57
    
@ChristianHummeluhr most of the information richness comes from social-interactions. In the end, since you want to use this hypothesis to distinguish between humans and say crocodiles, you need to account for some part of the environment that is salient to humans but not crocodiles. Hence the stress on the social part (although obviously non-social environmental learning also plays a role, but the proponents of SBH would say a lesser one). See this post for a nice discussion. –  Artem Kaznatcheev Mar 30 '13 at 15:02
    
That is not an uncontroversial statement, in my view. A priori, I would assume that e.g. the optic array, or the "photon soup" that surrounds us, is a far richer source of information than social interactions, though that is obviously also a rich source of information. If you feel such a more general explanation falls outside the scope of your question about SBH, I will leave it, though. –  Christian Hummeluhr Mar 30 '13 at 15:10
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