Social Brain Theory Confirmed

The social brain theory is that human brains got big mainly due to deal with the complexity of human societies.  Such as, say, signaling.  While it is the dominant theory as far as I can tell, it is nice to see some direct evidence in its favor.   New Scientist:

Geary collected data from 175 fossil hominin skulls, from 1.9 million to 10,000 years old. Then he looked to see whether brain size was best correlated with climatic variability – a crude measure of biodiversity which could indicate the complexity of hunting and gathering – or the human population size at the time, which could reflect the complexity of social interactions.  Geary’s analysis found that population size was the best predictor of brain size, suggesting that our ancestors’ need to outcompete their neighbours in order to survive may have been the strongest driver of brain growth (Human Nature, vol 20, p 67).

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  • Large brains are also attributed to sexual selection (see “The Runaway Brain”), the lifting of nutritional constraints (see “The Driving Force”) and making space for the new replicators (see “The Meme Machine”).

    All these theories seem good to me, better than many of the theories mentioned in the article. The “ice-age” theory seems highly dubious. It hits topical global-warming buttons – and seriously neglects the simple fact that temperature has always varied smoothly with lattitude.

  • Mike

    On a side note, the article mentions that inability to shed heat may have slowed increases in brain size. But then why did not humans evolve to lose hair on the scalp?

    To me there are far too many holes in the line of analysis here. For one, I have trouble equating cranial capacity with intelligence. Neanderthal had larger cranial capacity than modern humans (but perhaps they were more intelligent). But also men have larger cranial capacity (and larger brains!) than women, on average, yet my impression is that IQ between the two groups differ little if at all. Indeed, anecdotal evidence is that those people with the highest IQs tend to have weaker social skills, which calls to question what is an appropriate measure of intelligence, if we hypothesize its purpose is to negotiate social interactions.

    Also, I wonder why such a test is performed on a relatively small sample of fossil skulls, when one can compare cranial capacities — and attempt to gauge the average intelligence — of modern humans from different ancestries. There are still hunter-gatherers in this world, or at least people only a generation or two removed from that lifestyle.

  • TGGP

    John Hawks discussed another paper associating population size leading to smartness here:
    He made fun of the New Scientist article you’re referring to here:

    Richard Wrangham seems to support the lifting of nutritional constraints theory. Our stomachs shrank and our brains grew as our diet changed (thanks in part to cooking).

    • We need both a demand and a supply theory; we need to know both why larger brains were possible, and why they were desirable. The social brain theory is only about the demand for larger brains.

  • Men have bigger bodies. They do seem somewhat smarter:

    …however, some of that may be down to the lowest percentiles of males being more likely to be in asylums or prisons – and therefore not geting tested.

  • Eric Falkenstein

    It’s not clear which way the causality arrow goes: bigger brains, or bigger populations?

  • twv

    Brain size indicates intelligence, but many increases in intelligence may not correlate one-to-one with IQ. Many forms of social intelligence require a certain kind of emotional savvy, beginning with the capacity for expanded emotion and continuing with the ability to control that emotion. These abilities have selective advantages, for individual survival and survival in groups. Bigger brains allow more emotional nuance, thus greater co-operation. See the recent work of sociologist Jonathan Turner on this.

    Of course, much of this also falls in line with the MUCH earlier work of Herbert Spencer, but we are only supposed to mention him if we can make the usual ignorant sniping comments.

  • Yvain

    The proxies they use for each variable are worrying. Social interaction is measured by population density is measured by how many fossils were found nearby. This overestimates density in areas highly searched by paleontologists and highly amenable to preservation of fossils. It also doesn’t take into account the organization of hominids into bands. I would expect band size to be strongly correlated with CC (and in fact it is across different species of monkeys), but number of bands per area to be only weakly correlated.

    Difficulty of environment and biodiversity is measured by temperature variability in the 20th century. There are some big differences in 20th century climate vs. 1 million BC climate beyond acceptable noise levels, and does temperature variability measure difficulty of environment anyway?

    Anyway, I have very low confidence in the CC – population density correlation being interesting, because it seems very closely mediated by age. The age-PD correlation was -.8, and the age-CC correlation was -.85, so of course there’s going to be a high CC-PD correlation. Now, they said that PD contributed information beyond just age, but the causation’s clearly the other way; a dense population can’t cause age! They also said that age contributed no information beyond just PD, which raises a red flag for me – knowing how old a fossil is doesn’t help you figure out how big the brain is? That “PD” variable is capturing all the information we would expect from age, which is a *lot* of information.

    All in all, I agree with the conclusion they give in their paper:

    “Because each predictor correlates significantly with all the other predictors, the correlations in and of themselves provide neither evidence for the unique contribution of any single predictor to the evolution of hominid CC nor evidence for the size of the relative contributions of predictors if multiple pressures were involved.”

  • At the American Society for Human Genetics meetings later this fall, I’ll be presenting a new theory that explains this “Denser Humans/Bigger Brains” relationship.

    The link in my name heads to the conference poster listing. The title of my paper: “Scale Effects and Recent Human Evolution: Theory and Preliminary Evidence.”

    The paper Robin refers to gives me even more evidence to work with. Thanks, Robin!

  • I’m impressed by the general scientific literacy and the critical approach of the commenters in this thread.

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  • More: “Group size is a function of relative neocortical volume in nonhuman primates”

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