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Social Brains

We are social beings

Dr. H. C. Sinclair

on 18 September 2018

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Transcript of Social Brains

The Social Brain
The Social Brain Hypothesis
(Dunbar, 1993)
Only the Lonely...
Chronically lonely individuals have less grey matter in the left posterior superior temporal sulcus (pSTS)? (Kanai et al., 2012)
Translation: They have difficulty with processing basic social cues
Why do primates - especially humans - need larger brains than other species to do the same jobs? (e.g., hunt, survive climate change)
More than just termites & nuts
"...primates large brains reflect the computational demands of the complex social systems..."
"...the social brain hypothesis has found near universal acceptance as the best explanation for the evolution of extensive variation in brain size among mammals
(van Schaik, et al., 2012, p. 277)."
The Brain is Social
Facebook on the Brain
“…the number of friends an individual declares on a web-based social networking service reliably predicted grey matter density in the right superior temporal sulcus, left middle temporal gyrus, and entorhinal cortex. (Kanai et al., 2012, pg. 1327)”
Areas for social perception (particularly understanding social intentions) and associative memory (for relationships).
Social group size is correlated with neocortex volume across primate species
Dunbar's number...
Built off of the "Machiavellian Intelligence Hypothesis"
The neocortex ratio of a species is the ratio of the size of the neocortex to the rest of the brain.
The larger your social network, the greater the volume of your amygdala (associated with the emotional coding of social signals). - Bickart et al., 2010
Gotta have friends....
But correlation does not equal causation
Monkey business
Their analysis revealed a clear, linear relationship between the size of a monkey’s social network and an increase of neocortical gray matter in regions involved with social cognition (such as the mid-superior temporal sulcus, rostral prefrontal cortex as well as the frontal and temporal cortex).
The monkeys demonstrated an expansion of gray matter ranging from 3-8% (depending on the brain region) for each additional member of their social network.
In other words, monkeys that lived in the most socially complex group had an average increase of 20% more neocortical growth than monkeys housed individually.
Sallet et al., 2011
Egg? Chicken? Social Network
A familiar tale....
Harlow, 1958
Social Deprivation
The isolated monkeys grew up with severe emotional and behavioral problems, developing an "autistic-like" syndrome, with grooming, self-clasping, social withdrawal, and rocking.
As for humans....
"The Lost Children of Romania"
Social deficiencies
Cognitive deficiencies
Heightened Cortisol
Reduced Oxytocin
These associations are unique to online "friends"
Other Considerations
And it is more than just Machiavellian
But it doesn't just extend to primates
And dolphins & whales...(Marino, 1996)
And bats...(Barton & Dunbar, 1996)
And hyenas...(Sakai et al., 2011)

Now consider that the average individual social network size around 150 for humans
The neocortex ratio for humans is CR=4.1.
Your brain needs a social life to have a life!
In sum...
vs. the "Ecological Hypothesis" vs. ....
Mapping the Brain
Brains are expensive
"The human brain weighs about 2% of body weight but consumes 20% of total energy intake..."
"Because the cost of maintaining a large brain is so great it is intrinsically unlikely that large brains will evolve merely because they can. Large brains will evolve only when the selection factor in their favor is sufficient to overcome the steep cost" (Dunbar, 2006, pg. 179)
What's worth it?
Must always consider the costs vs. benefits, the constraints vs. facilitators - both from the environment AND the affected.
Compared to 7-8% in other primates
Expensive Tissue Hypothesis
In order to feed our growing brains, humans diverged from primates with regard to how much energy (e.g., glucose) was devoted to the brain over brawn. (Fedrigo et al., 2011).
But that still doesn't explain WHY the change in energy allocation occurred?
"...it is expensive tissue which uses up to 80% of resting metabolic rate in the newborn and continues to use almost 50% of it during the first 5 postnatal years." - Judas et al., 2013, pg. 1
Especially Expensive
During Development
Sample Question
Will it be on the test? Possibly.... and if not this, something like it.
If I were an evolutionary psychologist and I believed that human neocortex size was due to humans needing to develop the brain capacity to make tools and track down increasingly hard-to-get foods, I would endorse which theory?
A. The Social Brain hypothesis
B. The Machiavellian Intelligence hypothesis
C. The Expensive Tissue hypothesis
D. The Ecological hypothesis
Sample Question
Will it be on the test? Possibly.... and if not this, something like it.
The size of a dolphin’s brain is approximately 1603 cc on average. The dolphin’s neocortex is approximately 1279 cc. What is the dolphin’s neocortex ratio?
A. 1.25
B. 3.94
C. 39%
D. 324

And hopefully I don't need to make an argument that you need a brain...
Evolution would be so proud...
What was going on?
Ecological Dominance?
"Opportunities for meeting the world in new and diverse ways grew exponentially when these two forms of inheritance, genetic and cultural, joined forces. The human brain became intensely social. The activities and thoughts of one brain became unavoidably entangled with many others. The brain is thus much more than a structure contained inside the skull. Our brains belong to the people around us as much as to ourselves."
- Potts & Sloan, 2010
The Great (Meet & Greet) Migration
What was unique?
Big brains are costly not just energy-wise but to survival (during pregnancy, childbirth, and infancy)
Tachiano & Ichinose, 2018
The evolution of "social grooming"
Full transcript