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?


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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    $\begingroup$ 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. $\endgroup$ Mar 30, 2013 at 14:57
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    $\begingroup$ @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. $\endgroup$ Mar 30, 2013 at 15:02
  • $\begingroup$ 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. $\endgroup$ Mar 30, 2013 at 15:10
  • $\begingroup$ This is an old question, so you've probably answered it already. It's just that I happen to be reading a book right now about this. It's by Sarah Hrdy: Mothers and Others. $\endgroup$
    – Ruth
    Mar 14, 2016 at 22:41
  • $\begingroup$ @Ruth I have not found the answer for myself, so I am eager to read one. I hope you give one based on that book when you finish reading it. $\endgroup$ Mar 15, 2016 at 0:02

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Doing some research on this interesting question, I can point you towards a few articles and books which can give you a start. From my research, there are 2 possible reasons for early childbirth compared to infants of other great apes.

  1. The maternal heat stress hypothesis (Wells et al., 2012) (mentioned in your question as metabolic considerations) put forward as a challenge to the Obstetrical Dilema is a plausible reason. It is well established that the brain uses more energy than any other human organ, accounting for up to 20% of the body's total energy use.

Through secondary altricity, babies only have a third of the brain size relative to mature adults (Walker and Ruff, 1993), and in the article within Scientific American you linked to, it states that

by one estimation a human fetus would have to undergo a gestation period of 18 to 21 months instead of the usual nine to be born at a neurological and cognitive development stage comparable to that of a chimpanzee newborn.

Apart from the idea of a woman giving birth to a baby the size of a 9 to 12 month old baby being hard to imagine, a larger brain mass in the newborn corresponds to more energy needed to sustain it. It stands to reason that when pregnant, a human having 2 fully developed brains to provide energy for, will have problems.

  1. The hypothesis, variously dubbed the ‘Machiavellian Intelligence Hypothesis’ or the ‘Social Brain Hypothesis’, depending on what theorists take to be its most important features (Adolphs, 1999), is also plausible.

In support of this idea, there is a correlation between mean group size among various primate species and their neocortex volume (specifically, the ratio of neocortex volume to the rest of the brain).

Clearly, the emotional and social development of humans is extraordinarily complex, involving a multi-factorial interplay between genes, parental behaviour, and the influence of culture.

When looking at developmental issues, there are 5 models of development which theorists – and people in general – often disagree. (Miller, 2010) (Sigelman & Rider, 2012). When we are looking at social development in the context of social environments we are looking at the nurture argument of nature vs nurture. When looking at human development through other angles, along with goodness vs badness of human nature, there are other debates such as

  • Activity vs Passivity (Humans shape their environments vs environmental forces shape humans),
  • Continuity vs Discontinuity (Changes are gradual vs dramatically through life), or
  • Universality vs Context Specificity (Development is similar between people and cultures vs considerable differences between people and cultures)

Carol Sigelman and Elizabeth Rider in their book Life-Span Human Development (Sigelman & Rider, 2012) states that

Development is characterized by lifelong plasticity. Plasticity refers to the capacity to change in response to both positive and negative experiences. Developmental scholars have long known that child development can be damaged by a deprived environment and optimized by an enriched one. It is now understood that this plasticity continues into later life—that the aging process is not fixed but rather can be altered considerably depending on the individual’s experiences. For example, elderly adults can maintain or regain some of their intellectual abilities and even enhance them with the help of physical exercise like John Tatum’s swimming, a mentally and socially active lifestyle, or training designed to improve specific cognitive skills (Hertzog et al., 2009; Willis et al., 2006).


Adolphs, R. (1999). Social cognition and the human brain In: Trends in Cognitive Sciences 3(12) pp 469–479. doi: 10.1016/S1364-6613(99)01399-6

Hertzog, C., Kramer, A. F., Wilson, R. S., & Lindenberger, U. (2009). Enrichment effects on adult cognitive development: Can the functional capacity of older adults be preserved and enhanced? In: Psychological Science in the Public Interest, 9(1), pp 1–65. doi:10.1111/j.1539-6053.2009.01034.x.

Sigelman, C. K. & Rider, E. A. (2012). Life-Span Human Development. 7th ed. Belmont, CA: Wadsworth.

Walker, A. and Ruff, C. A. (1993). *The Reconstruction of the Pelvis** In: The Nariokotome Homo Erectus Skeleton. Edited by A. Walker and R. Leakey, pp 221-233. Cambridge, Massachusetts: Harvard University Press.

Wells, J. C. K.; Desilva, J. M.; Stock, J. T. (2012). The obstetric dilemma: An ancient game of Russian roulette, or a variable dilemma sensitive to ecology? In: American Journal of Physical Anthropology. 149(s55) pp 40–71. doi:10.1002/ajpa.22160. PMID: 23138755

Willis, S. L., Tennstedt, S. L., Marsiske, M., Ball, K., Elias, J., Koepke, K. M., Morris, J. N., Rebok, G. W., Unverzagt, F. W., & Stoddard, A. M. for the ACTIVE Study Group (2006). Long-term effects of cognitive training on everyday functional outcomes in older adults. In: Journal of the American Medical Association, 296(23), pp 2805–2814. doi:10.1001/jama.296.23.2805


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