The Social Brain Hypothesis was proposed by British anthropologist
Robin Dunbar, who argues that
human intelligence did not evolve primarily as a means to solve
ecological problems, but rather as a means of surviving and
reproducing in large and complex social groups
Some of the behaviors associated with living in large groups include
reciprocal altruism, deception and coalition formation. These group
dynamics relate to Theory of Mind or the ability to understand the
thoughts and emotions of others...
Tool use and exploration may be just side effects of social skills. How did
our brains get so big? Researchers have put forward a number of possible
explanations over the years, but the one with the most staying power is an
idea known as the social brain hypothesis. Its chief proponent, psychologist
Robin Dunbar of Oxford University, has argued for the past two decades that
the evolution of the human brain was driven by our increasingly complex
social relationships. We required greater neural processing power so that we
could keep track of who was doing what to whom.
Our expanded brains could have been practical for other things, of course,
such as innovations in tool use and food gathering. Most researchers,
including Dunbar, agree that these hypotheses are not mutually exclusive.
Whatever the reasons for the very large human noggin, there is a lot of
explaining to do, because big brains have a lot going against them.
The oversized Homo sapiens brain let us take over the planet, build cities,
send space probes to Mars, and do all the other marvelous things that we
humans are so proud of. But none of these things makes us much better at
reproducing, and in terms of evolution, that’s really all that matters. It’s
not so obvious why Darwinian natural selection should have favored the
brain’s dramatic expansion given the huge costs. Although the human brain is
only about 2 percent of total body weight, it siphons off about 20 percent
of our total calorie intake; this overall percentage varies little whether
we are engaged in hard mental tasks or just zoning out.
Why do we have big brains? …Many explanations for the evolution of primate
intelligence relate to the challenges of finding food. Monkeys and apes need
big brains to keep track of widely distributed, patchy and unpredictable
foods like fruit. Or maybe they need enhanced intelligence to extract food
embedded in a tough shell or to collect termites hiding in a mound.
Critics of such arguments have pointed out that these problems are not
necessarily unique to primates. As an alternative, in the late 1980s,
scientists suggested primates have big brains because they are highly social
animals. Primates are not the only mammals that live in large groups, but
monkeys and apes stand out, in general, for having very intense social
relationships. In fact, watching a group of monkeys is kind of like watching
a soap opera: Individuals have friends, but they also have enemies. They
team up to form coalitions to overthrow their foes, but they also reconcile
after a fight. They yield to the leaders of their group, but they also sneak
off to engage in clandestine affairs when no one’s looking.
If you’re going to be involved in all of these social maneuverings, you
need to be able to keep track of all sorts of social information—how you
relate to others in the group, how third parties relate to one another—but
more importantly, you need to be able to use that information to your
benefit. And to do that, you need a big brain. That’s the basis of the
Social Brain Hypothesis (PDF).
Humans Evolved Big Brains to Be Social? Some scientists think humans and
other primates evolved big brains in response to the social challenges of
living in large groups
Almost all primates live in groups with an observable and definable social
hierarchy, and humans aren’t an exception. We may overlook it in our day to
day lives, but every so often it becomes evident that we interact best when
we understand the pecking order. The social brain hpyothesis argues that the
cognitive demands of living in complexly bonded social groups selected for
increases in executive brain…
…the striatum showed activity in a situation where a rise or fall in rank
was a possibility as much as it did to the monetary reward. The stratium is
a critical part of the brain where dopamine is regulated, and a previous
study investigated the genetics of dopamine and the linkage it had to
agressive social behaviors. Overall, this observation implies that social
status is highly valued in our subconscious minds, even as much as
money.
Another interesting observation involved subjects that were presented a
‘superior competitor’ in the game. When that happened, it triggered
activity in, “an area near the front of the brain that appears to size people up –
making interpersonal judgments and assessing social status. A circuit
involving the mid-front part of the brain that processes the intentions and
motives of others and emotion processing areas deep in the brain activated
when the hierarchy became unstable, allowing for upward and downward
mobility.”
Also when the player preformed better than any superior competitors,
another area towards the front of the brain which controls planning was
activated. In contrast, when the player did worse than an inferior
competitor different activity was shown in centers of the brain associated
with emotional pain, frustration, and stress…
…players who were at the top of the hierarchy, not only did they say they
had a more positive experience but more activity was associated in the
emotional pain circuitry when they perceived an outcome that could drop them
down in rank.
These results kinda thwart any Utopian anarchists out there. This data
shows that our brain’s hierarchical consciousness seems to be ingrained in
the human brain, so much so that there are distinct circuits activated by
concerns over social rank.
The Social Brain Hypothesis: Are our brains hardwired to deal with
socialhierarchies?
Primates have unusually large brains for body size compared to all other
vertebrates. The conventional explanation for this is known as the “social
brain hypothesis,” which argues that primates need large brains because
their form of sociality is much more complex than that of other species
(Byrne & Whiten, 1988). This does not mean that they live in larger
social groups than other species of animals (in fact, they don’t), but
rather that their groups have a more complex structure.
Summary: Primate societies are unusually complex compared to those of other
animals, and the need to manage such complexity is the main explanation for
the fact that primates have unusually large brains. Primate sociality is
based on bonded relationships that underpin coalitions, which in turn are
designed to buffer individuals against the social stresses of living in
large, stable groups. This is reflected in a correlation between social
group size and neocortex size in primates (but not other species of
animals), commonly known as the social brain hypothesis, although this
relationship itself is the outcome of an underlying relationship between
brain size and behavioral complexity. The relationship between brain size
and group size is mediated, in humans at least, by mentalizing skills.
Neuropsychologically, these are all associated with the size of units within
the theory of mind network (linking prefrontal cortex and temporal lobe
units). In addition, primate sociality involves a dual-process mechanism
whereby the endorphin system provides a psychopharmacological platform off
which the cognitive component is then built.
Robin I. M. Dunbar - The Social Brain Hypothesis and Human
Evolution
A key driver of brain evolution in primates and humans is the cognitive
demands arising from managing social relationships. In primates, grooming
plays a key role in maintaining these relationships, but the time that can
be devoted to grooming is inherently limited. Communication may act as an
additional, more time-efficient bonding mechanism to grooming.
Chimpanzees had differentiated social relationships, with focal chimpanzees
maintaining some level of proximity to almost all group members, but
directing gestures at and grooming with a smaller number of preferred social
partners.
Pairs of chimpanzees that had high levels of close proximity had higher
rates of grooming. Importantly, higher rates of gestural communication were
also positively associated with levels of proximity, and specifically
gestures associated with affiliation (greeting, gesture to mutually groom)
were related to proximity. Synchronized low-intensity pant-hoots were also
positively related to proximity in pairs of chimpanzees.
Further, there were differences in the size of individual chimpanzees'
proximity networks—the number of social relationships they maintained with
others. Focal chimpanzees with larger proximity networks had a higher rate
of both synchronized low- intensity pant-hoots and synchronized
high-intensity pant-hoots.
These results suggest that in addition to grooming, both gestures and
synchronized vocalizations may play key roles in allowing chimpanzees to
manage a large and differentiated set of social relationships.
Gestures may be important in reducing the aggression arising from being in
close proximity to others, allowing for proximity to be maintained for
longer and facilitating grooming.
Vocalizations may allow chimpanzees to communicate with a larger number of
recipients than gestures and the synchronized nature of the pant-hoot calls
may facilitate social bonding of more numerous social
relationships.
As group sizes increased through human evolution, both gestures and
synchronized vocalizations may have played important roles in bonding social
relationships in a more time-efficient manner than grooming.
Introduction
Primate sociality is frequently characterized as being especially
complex in its nature, and primates have unusually large brains for
their body size when compared to other mammals. The “social brain
hypothesis” proposes that the complex social world of primates is
especially cognitively demanding, and that this imposed intense
selection pressure for increasingly large brains (Byrne and Whiten,
1988; Dunbar, 1998).
Group size in primates is strongly correlated with brain size, and
specifically with neocortex size in relation to the rest of the brain,
but exactly what makes larger groups more complex than smaller groups is
poorly understood (Dunbar, 2003). The complexity of primate social
groups depends on the complexity of individual relationships between
animals, because the social system itself is an emergent property of
these micro-level interactions (Hinde, 1966).
Thus, to understand the complexity of social groups, a detailed
understanding of how primates interact with others to build and maintain
social relationships over time is required, as this is at the heart of
what makes primate life socially complex (Dunbar and Shultz,
2010).
Other species also come together in large groups (e.g., grazing
ungulates such as wildebeest), but these are aggregations of animals,
with less stable group membership and thus less stable social
relationships between individuals (Haddadi et al., 2011).
In contrast, primates live in groups with stable membership, and form
long-lasting bonds with certain individuals within the group, where they
flexibly respond to one another in repeated instances of affiliative
interaction (Dunbar, 1992b). Individual variation in the nature of these
social bonds has direct fitness consequences—for example, the sociality
of adult female baboons (as measured by grooming and proximity to
others) is positively associated with both their own (Smuts, 1985;
Palombit et al., 1997; Silk et al., 2010b) and their offspring's
survival (Silk, 2007). It is the dynamic and multi-facetted nature of
these social relationships, and the need for individual primates both to
keep track of its own relationships, and the relationships of other
group members (third party relationships), that is hypothesized to drive
the social complexity of primate life (Silk, 1999; Engh et al., 2006; le
Roux et al., 2013; Roberts and Roberts, 2015).
Thus, one of the distinctive characteristics of primate sociality is
its complexity, with complex social systems defined as those in which
individuals communicate frequently in many different contexts with many
different individuals, and repeatedly interact with many of the same
individuals over time (Freeberg et al., 2012). The fact that the
neocortex ratio correlates strongly with typical group size lends
support to the idea that the larger neocortex in primates evolved under
selection to manipulate information about social
relationships.
The social brain hypothesis assumes that cognitive processing
capacities (represented by relative neocortex size) place an upper limit
on the size of groups that can be maintained as a cohesive social unit.
Primates do not maintain equally strong relationships with all group
members, but form differentiated, stable, long-lasting bonds with both
related and unrelated group members (Pepper et al., 1999; Langergraber
et al., 2009; Mitani, 2009; Silk et al., 2010a). One of the primary
mechanisms that primates use for creating and maintaining social bonds
is grooming, which can account for up to 20% of their total daytime
activity budget.
The amount of time primates spend grooming is positively related to
group size, suggesting that when groups are large, primates have to
spend more time maintaining their social relationships than in small
groups (Aiello and Dunbar, 1993; Lehmann et al., 2007). However, the
amount of time primates can devote to grooming is limited, because of
the demands of other essential activities, notably feeding, resting, and
moving (Dunbar, 1992a).
Thus, social bonding in primates is constrained by two independent
variables—neocortex size which sets an upper limit to the number of
relationships individual primates can keep track of, and the amount of
time that is available for grooming, which is necessary to maintain
social relationships at a sufficient level to prevent the bond from
decaying (Dunbar, 1993; Lehmann et al., 2007).
If the number of individuals in a group becomes too large, it becomes
increasingly difficult for individuals to maintain social bonds with all
group members. Thus, group cohesion will decrease and the bonds will
eventually decay. For example, the probability that a baboon group will
split increases with increasing group size (Henzi et al., 1997). This
seems to be determined not by inefficient foraging in larger groups or
by predation risk, but directly by the inability of individuals to
service social relationships in the face of the inevitably limited
amount of time available for social interaction (Henzi et al.,
1997).
However, it is increasingly being recognized that in addition to
grooming, vocalizations (sounds made with the vocal tract) and gestural
communication (voluntary movements of the arm, hand, head, or whole
body; Roberts et al., 2014a,b) may also play key roles in developing and
maintaining social bonds in primates. Time constraints limit the amount
of time available for grooming (Lehmann et al., 2007), but vocal and
gestural signals are less constrained by time, and thus may offer an
important additional way to regulate social relations in groups of
primates.
Comparative analysis has demonstrated that evolutionary increases in
the size of the vocal repertoire in non-human primates were associated
with increases in both group size and also time spent grooming (McComb
and Semple, 2005). This suggests that vocal communication may play a
role in maintaining groups of primates—larger groups are more complex to
manage, and thus require a larger vocal repertoire to maintain an
increasing number of differentiated relationships.
Further, differences in the amount of time devoted to affiliative
gestural communication, but not other types of gestures, across three
macaque social systems, provides an indication that gestural
communication may be used flexibly to maintain a differentiated set of
social relationships (Maestripieri, 2005). However, systematic studies
of how vocalizations—and especially gestures—are associated with social
relationships in primates are in their infancy, despite the potential
significance of such studies for furthering our understanding of social
evolution in both primates and humans.
Chimpanzees are an excellent species to examine this question because
they have complex social dynamics. In the chimpanzee fission-fusion
social system, the association patterns change by means of the fission
and fusion of subunits (known as parties or sub-groups) according to
both the activity (e.g., resting, feeding) and distribution of resources
(Pepper et al., 1999).
Individuals thus stay in close proximity with some conspecifics from
the wider community at infrequent intervals, often weeks apart, but each
individual can recognize members of their own community and is capable
of maintaining long-term relationships with these individuals (Boesch,
1996; Barrett et al., 2003; Muller and Mitani, 2005; Amici et al., 2008;
Eckhardt et al., 2015).
Reciprocated social relationships are a key feature of the chimpanzee
social system and are marked by increased time and energy investment in
repeated and reciprocated instances of association and interaction
(Watts, 2006; Mitani, 2009). Chimpanzees also have social relationships
with non-reciprocated social partners or weakly bonded conspecifics with
whom they have less frequent association and interaction (Foerster et
al., 2015).
A recent study showed that the presence of reciprocated close proximity
bonds between pairs of chimpanzees (i.e., those pairs who spent larger
amounts of time in close proximity, per hour spent in the same party)
was associated with several behavioral indices. These included a longer
duration of visual attention directed at the dyad partner, a longer
duration of mutual grooming and received grooming, and a longer duration
of time spent resting and traveling, per hour the pair of chimpanzees
spent in close proximity (within 10 m; Roberts and Roberts,
2016).
Moreover, chimpanzees use a communication system consisting of gestures
(Leavens et al., 2004; Forrester, 2008; Hobaiter and Byrne, 2011;
Roberts et al., 2012a,b, 2013, 2014a; Smith and Delgado, 2013; Bard et
al., 2014) and vocalizations to maintain their relationships (Van
Lawick-Goodall, 1967, 1968; Goodall, 1986; Mitani and Nishida, 1993;
Mitani et al., 1999; Roberts and Roberts, 2016).
For instance, chimpanzees use visual gestures with strongly bonded
individuals and tactile or auditory gestures with weakly bonded
individuals (Roberts and Roberts, 2016). Gestural communication that has
previously been suggested to be important in relation to social bonds
includes gestures made when encountering each other after a natural
period of separation, in response to the threat of aggression or after
receiving aggression (Roberts et al., 2014a; Taglialatela et al.,
2015).
Vocal communication hypothesized to be important in relation to social
bonding in chimpanzees includes pant-hoot calls produced solo or jointly
with group members in conjunction with visual or auditory gestures
(Mitani and Nishida, 1993; Fedurek et al., 2013) and one-to-one calls
(e.g., low intensity pant-grunt calls produced by a subordinate
individual towards a dominant chimpanzee). Whilst it is well-known that
chimpanzees use a wide variety of gestures and vocalizations when
interacting, there have been no systematic studies of how both vocal and
gestural communication relate to association and grooming patterns in
chimpanzees.
In this study we predict that the number and strength of close
proximity relationships maintained with others (expressed as duration of
time spent within 10 m per hour spent in the same party) are associated
both with biological factors (e.g., maternal kinship, age similarity,
sex similarity, reproductive similarity; Huchard et al., 2016) and
social bonding (communication and grooming). Specifically, we
hypothesize that grooming and affiliative communication have a bonding
function through reducing the risk of aggression and therefore are
associated with close proximity. Thus, proximity bonds, grooming, and
dominance-aggression gestures will correlate, indicating a cost to
sociality. However, when affiliative communication and grooming are
included in the model, the relationship between the dominance-aggression
gestures and proximity will become weaker.
Thus, the bonds chimpanzees will have with other individuals will be
differentiated, with strong social relationships based on grooming and
affiliative communication, whereas weaker social relationships will be
based on dominance communication, as chimpanzees use different types of
behavior to maintain the different types of bonds. In addition to these
group level associations between communication and proximity, individual
chimpanzees also display a large amount of variation in the size of
their individual proximity networks.
The size of this network reflects the number of conspecifics with whom
individual chimpanzees maintain close proximity. The larger the size of
the individual proximity network, the greater the time and cognitive
demands on maintaining these more numerous social
relationships.
Thus, we predict that in smaller networks, chimpanzees will form
relatively strong ties with all network members, with frequent
interactions based on affiliative communication and grooming behavior
(Mitani, 2009).
However, as individual network size increases, the ties chimpanzees
will have with other individuals will become increasingly weak, with
less frequent interactions and an increasing dissociation between strong
and weak association networks.
These weaker, indirect ties are cognitively complex to manage, and this
is especially true in fission-fusion social systems where the frequency
of interaction between two individuals will be much lower than in other
social systems where there is a greater degree of temporal and spatial
cohesion between group members (Barrett et al., 2003).
One manner of communication that could be used to service these weak
social bonds is one-to-one gestures and vocalizations, as unlike
grooming these behaviors do not require prolonged physical contact
(Roberts et al., 2012b).
However, one-to-one communication still requires some degree of close
proximity and one-to-one prior visual attention (Roberts et al., 2014a)
or brief tactile contact and thus a relatively low number of individuals
can be bonded with at any one time.
Moreover, these interactions are cognitively complex because animals
have to remember the identities of the interactants and their past and
present relationships with them to bond in an efficient
manner.
Thus, a signaling and bonding strategy of this type may not be
effective in meeting the demands of maintaining social relationships in
a large proximity network.
In contrast, a larger-scale, vocally-based bonding system, such as a
pant-hoot call, can be produced jointly by several individuals at the
same time (Mitani and Nishida, 1993). In this context, simultaneous,
rhythmically matched sound production and/or movement can replace the
need for prolonged physical contact and act as an alternative bonding
mechanism to grooming (Tarr et al., 2014).
Here we therefore predict that the joint communication enables
chimpanzees to bond effectively with the individuals beyond the size of
the one-to-one grooming and communication network.
Thus, there will be a switch from one-to-one grooming and communication
to joint communication when the chimpanzees maintain large proximity
networks. Such a communication system reduces the need for one-to-one
interactions and therefore decreases the time and cognitive demands
arising from one-to-one social bonding. How chimpanzees adjust their
patterns of communication and grooming in proximity networks of
differing sizes is thus informative of the key cognitive and time-budget
pressures involved in sociality.
Social Brain Hypothesis: Vocal and Gesture Networks of Wild
Chimpanzees
Dunbar proposed the Social Brain Hypothesis which contends that primates
have large brains because they live in complex societies; the larger the
social group, the bigger the brain. Accordingly, from the size of the
brain, the frontal lobe in particular, one might be able to predict the
optimal social group size for that animal. In a meta analysis, he related primate brain size to the average
size of the social group each species lives in. His estimate of brain
size emphasized the neocortex and he judged the size of the social group
based on the number of animals that practice social grooming together.
The study looked at 38 different primate groups and he found a strong
and remarkably linear correlation between brain size and social group
size.
Dunbar had data on humans as well and it occurred to him to ask what
the optimal human social group would be based on the data from
nonhuman primates. He fit a linear regression to the group/neocortex
ratio for non-human primates and extrapolated it to the size of the
human neocortex. Judging from the size of an average human brain, the
number of people one would have in her/his social group is predicted
to be one hundred and fifty. This is Dunbar’s number, ~150 (95%
confidence interval; 100 to 230). Anything beyond that, he suggests,
is too complicated to handle at optimal processing levels.
Support for Donbar’s number comes from unlikely places. The average
group size among modern hunter-gatherer societies is 148.4
individuals. Company size in professional armies is close to 150 from
the Roman Empire and sixteenth-century Spain to the twentieth-century
Soviet Union. Then there is the interesting story of GORE-TEX, the
company that makes rain gear and such. Bill Gore, the guy who founded the company, was quite
successful and he opened up a large factory that also continued to
grow. Then one day he walked into his factory and realized he simply
didn’t know who everybody was. He wondered whether as the company
grew, people would start to become less likely to work hard and help
each other out. He observed that after putting about 150 people in the
same building, things at GORE-TEX just didn’t run as smoothly. People
couldn’t keep track of each other and a sense of community diminished.
So Gore decided to cap his factories at 150 employees. Whenever they
needed to expand, he just build a new factory, sometimes right next
door. Things ran better this way. This famous story from the realm of
corporate sociology seems to support Dunbar’s number. Malcolm Gladwell
discusses it in The Tipping Point.
There is more. The estimated size of a Neolithic farming village is
150; 150 is the splitting point of Hutterite settlements; 200 appears
to be the upper bound on the number of academics in a discipline’s
sub-specializations. One has to ask, is this all coincidence, or is it
neurobiology?
Breaking down the Dunbar number
In her New Yorker article, Maria Konnikova points out that the Dunbar
number is actually a series of numbers. The best known, 150, is the
number of people we call casual friends—the people we’d invite to a
large party (in reality, it’s a range: 100 at the low end and 200 for
the more social of us.). From there, based on interviews and survey
data, Dunbar concluded that the number grows and declines according to
roughly a “rule of three.” The next step down, 50, is the number of
people we call close friends; you see them regularly. Then there’s the
circle of 15, the friends that you can turn to for sympathy and can
confide in. The most intimate Dunbar number, 5, is your close support
group. These are your best friends and often family members.
Cognitive Limits, Social Networks and Dunbar’s Number
The Attention Schema Theory (AST) ...suggests that consciousness arises
as a solution to one of the most fundamental problems facing any nervous
system: Too much information constantly flows in to be fully processed.
The brain evolved increasingly sophisticated mechanisms for deeply
processing a few select signals at the expense of others, and in the AST,
consciousness is the ultimate result of that evolutionary sequence. If the
theory is right—and that has yet to be determined—then consciousness
evolved gradually over the past half billion years and is present in a
range of vertebrate species...
