Social Brain Hypothesis

Chapter 2: Dunbar, R.I.M Brain and cognition in
evolutionary perspective (pp. 21-46)
Social brain hypothesis: Argues that the evolution of the human brain is
largely due to social cognitive factors – in other words, human brains
evolved as social brains.
Brains are metabolically expensive tissue; largely owing to the difficulty in
creating and replenishing neurotransmitter supplies.
Evolving a large brain must mean that brain size offered an important
fitness advantage, otherwise its hard to see why it would happen.
Social Brain Hypothesis
• Jerison’s encephalization quotient (EQ): how much brain is left
over after we scale its size against body size?
• EQ for humans is around 7, for chimpanzees it is around 3-4. A
disproportionate increase in brain size in humans is attributable to
increases in the frontal lobe. Controversy over whether human
frontal lobes are larger than would be expected for a primate of our
size. Dunbar says yes.
Social Brain Hypothesis
• Evo-devo of brain evolution: simple process – extend
the ontogenetic time period for neurogenesis. In other
words, delay the “stop making (precursor) brain cells”
genetic signal. May help explain long developmental
period for humans.
• Selection for large brains
• Ecological vs. Social brain hypotheses
• Ecological: increased demands of hunting meat or
selecting ripe fruit
• Social: increased demands of living in complex social
Social Brain Hypothesis
• Dunbar argues that social brain
hypo more strongly supported
since most of primate brain
expansion is in the neocortex
which is where social
computational processes would
be taking place as opposed to
sub-cortical structures where
more ecological processing
such as cognitive mapping
would be taking place.
• Reader and LaLand (2002)
hypo that expanded neo cortex
is for more sophisticated social
learning. Neocortex size and
innovation related in primates.
Social Brain Hypothesis
• Critical social cognition process: Theory of
• Transition to second-order intentionality – 4
to 5 years of age; I know that you know
TOM in other species
• Dolphins appear not to have TOM
• Monkeys and nonprimates probably not
• Great apes, some capacity, may understand
intentional vs. accidental actions, may
understand goal and desires, but not
evidence of understanding beliefs, theories;
more complex mental states.
Social Brain Hypothesis
• Some evidence that mentalizing tasks are computationally
• Possibility that spindle cells (unique to great apes) may be
important for social computation. High density in ACC;
Connects ACC (working memory; self regulation) with
amygdala (emotional processing). Spindle cell development
tracks self-control development in children.
Social Brain Hypothesis
• Total brain volume and TOM; as brain grows larger more diverse
processing units can be integrated and directed to one task.
• Great ape social groups appear to be more complex than Old world
monkey groups (even though OWM groups can often be just as
larger if not larger in number)
• Important transition in brain evolution appears to have taken place
after split of OWM (23mybp) but before the branching off of the
lesser apes (16mybp).
• Possibility that this had to do with a genetic mutation that allowed
for faster neuronal transmission times.
• Possibility of fission-fusion social system being significant. Apes
must disperse more widely because they cannot processes unripe

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