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The Social Intelligence Hypothesis
Lily Johnson-Ulrich
Michigan State University, East Lansing, MI,
USA
Synonyms
Machiavellian intelligence hypothesis;Social
brain hypothesis;Social complexity hypothesis
Definition
The social intelligence hypothesis is a scientific
hypothesis proposing that social complexity was
the main selective force shaping the evolution of
sophisticated intelligence and large brains in
extant animals.
Introduction
The social intelligence hypothesis (SIH) is a pop-
ular hypothesis that purports to explain the evolu-
tion of large brains and sophisticated cognitive
abilities. The SIH proposes that social complexity
is cognitively demanding and is thus the key
selective pressure affecting brain size and, by
extension, intelligence (Dunbar 1998; Humphrey
1976; Jolly 1966). The SIH was originally devel-
oped to explain the large brains and intelligent
behavior of primates compared to other animals
(Dunbar 1998). It has also found support across
many mammal and bird species to explain varia-
tion in both brain size and cognitive abilities
(Dunbar and Shultz 2007; Shultz and Dunbar
2010). Though often pitted against “ecological”
hypotheses, which emphasize the role of diet and
environmental challenges in brain evolution, pro-
ponents of the SIH often claim that the two are not
mutually exclusive. Instead, they propose that
ecological problems are solved socially, not indi-
vidually, and that ecological selection thus acts
indirectly on brain size through selection for
social intelligence (Dunbar and Shultz 2007).
More conservative forms of the SIH simply posit
that more complex social groups favor specialized
social cognition. But ultimately, the SIH suggests
that the primary function of intelligence is in the
social domain.
Origins of the SIH
The SIH was originally formulated based on the
observation that primates in general, and great
apes in particular, have unusually large brains
and uniquely complex social systems. The origins
of the social intelligence hypothesis are usually
credited to Humphrey (1976). Though Chance
and Mead (1953) and Jolly (1966) had previously
developed similar ideas, they were not as widely
read or detailed as Humphrey’s(1976) work.
Chance and Mead (1953) suggested that male-
male competition for access to mates in primates
may have been a strong driving force in the
#Springer International Publishing AG 2017
T.K. Shackelford, V.A. Weekes-Shackelford (eds.), Encyclopedia of Evolutionary Psychological Science,
https://doi.org/10.1007/978-3-319-16999-6_3100-1
evolution of the human cortex. Years later, after
observing that lemurs have quite complex socie-
ties, but do not show the intelligence displayed by
simiiforme primates (monkeys and great apes)
when dealing with the physical world, Jolly
(1966) suggested that social complexity preceded
and defined the nature of general intelligence.
Independently, Humphrey (1976) suggested that
the social realm offers far more opportunity for
cognitive challenges than the physical world. He
observed that, although primates did not need to
be particularly strong innovators, their survival
required a high level of practical knowledge
about their environment and that this knowledge
was acquired socially. This type of society, with
mixed age and sex members, creates a degree of
complexity where social intelligence can have
large payoffs, ultimately leading to the develop-
ment of culture and human intelligence.
Eventually, with advent of empirical data relat-
ing social complexity, social intelligence, and
brain size, the SIH became the most popular
hypothesis for the evolution of intelligence. Over-
all, the SIH makes four key predictions: (1) social
complexity is the main selective force behind
enlarged brains and general intelligence, (2) com-
plex social groups select for greater social intelli-
gence than less complex groups, (3) social
intelligence may be transferred for use in the
physical domain, and (4) large brains and intelli-
gence are primarily used for solving social
problems.
Social Intelligence in Primates
Primates possess many sophisticated socio-
cognitive skills (Barrett et al. 2003; de Waal
1982; Seyfarth and Cheney 2012). For example,
in complex primate societies, keeping track of
relationships among other individuals is critical
for access to mates and food resources. Two social
cognitive skills of particular interest are recogni-
tion of third-party relationships and theory of
mind. Not only do primates recognize other indi-
viduals in their group and understand their own
relationships to those individuals, but they also
recognize the relationships between group mem-
bers to whom they themselves are unrelated; this
entails recognizing third-party relationships. This
skill is particularly interesting because, although
remembering as many as 100 other individuals in
a primate group may be difficult, the number of
pair-wise relationships that are possible, even in
small groups, is combinatorially massive and thus
intellectually challenging (Seyfarth and Cheney
2015). Theory of mind, which is the ability to
recognize mental states in other individuals, is an
advanced cognitive skill that helps predict the
actions of others. Theory of mind likely came
about as a result of social pressures in humans,
but the rudiments of this ability appear to exist in
the great apes (Call and Tomasello 2008). For a
more complete summary of primate social intelli-
gence, see Barrett et al. (2003), Cheney and
Seyfarth (1985), and Seyfarth and Cheney
(2012). Today, a vast number of social cognitive
skills have been observed in primates such as
reconciliation, reciprocity cooperation, and com-
munication to name a few. In general, the great
apes appear to outperform monkeys, which corre-
lates with the greater brain size observed in great
apes. The presence of sophisticated social cogni-
tion suggests a strong degree of selective pressure
for such skills, and these skills are related to
fitness in primates (Seyfarth and Cheney 2015).
This research supports the idea that selection has
favored the evolution of social intelligence.
Social Correlates with Brain Size in Primates
Interest in the SIH grew with the advent of
research that correlated specific measures of
social complexity with measurements of brain
size in primates. Group size was the first quanti-
tative measure of social complexity, and it was
correlated with a quantitative measure of brain
size (Dunbar 1998). Group size was chosen as
the key measure of social complexity because
primates interact individually, in potentially
fitness-affecting ways, with every member of
their groups, and because it is an easy number to
obtain for most species. The ratio of the neocortex
to the rest of the brain was preferred in much of the
research on the SIH because body size is a much
more evolutionary labile trait than brain size.
Other measures that were also related to neocortex
ratio include tactical deception, grooming clique
size, rates of social play, type of mating system,
2 The Social Intelligence Hypothesis
and the presence of long-term stable social bonds
(Dunbar 1998; Dunbar and Shultz 2007). Rates of
social learning have also been associated with
multiple measures of brain size (Reader and
Laland 2002).
Defining Social Complexity
Most of the aforementioned studies relating brain
size to some aspect of primate sociality also pur-
ported to identify the factor that makes primate
groups socially complex. Group size remains the
most popular measure of social complexity across
all species despite its shortcomings, and no other
measure has quite been able to replace it, despite
varied proposals regarding what makes an animal
society complex. Other measures of social com-
plexity that have been studied are fission-fusion
dynamics, complex alliances, presence of a linear
dominance hierarchy, transactional interactions,
or even crèches (for a review and critique, see
Bergman and Beehner 2015). de Waal and Tyack
(2009, p. 1) suggest that a commonality underly-
ing various research on social complexity is the
idea that individual animals must “keep track of
interactions with other individuals with whom
they must compete and cooperate.”Complex
groups are those in which there are many such
individuals; simple groups are those in which
individual members are largely anonymous.
Primates certainly appear to possess sophisti-
cated social intelligence, but social intelligence
itself might be the cause of social complexity,
which creates a level of circularity (Gigerenzer
1997). However, others argue that one must
include social intelligence in the definition of
social complexity, because social complexity is
hypothesized to be cognitively demanding
(Bergman and Beehner 2015). Thus, when
researchers attempt to define social complexity,
they should attempt to identify the cognitively
demanding features of a social group. Using this
logic, Bergman and Beehner (2015, p. 204)
suggested that social complexity should be
defined as “the number of differentiated relation-
ships that members of a species have with con-
specifics.”This definition provides an objective
and easy to quantify metric where cognition is
clearly required to differentiate group members.
Unfortunately, no definition of social complexity
has been agreed upon to date; this is a major
shortcoming of the SIH because social complexity
is the key independent variable of the SIH.
Summary: Primates Are Specialized for Social
Intelligence
Overall there is good evidence supporting the SIH
in primates. Primates possess sophisticated social
intelligence, many measures of social complexity
correlate with brain size, and primate intelligence
appears to be highly social in nature. Specific
neurons and multiple brain regions have been
found in primates that are specialized to respond
to social features of the environment; this supports
the idea that social complexity among primates
favored “social brains”(Brothers 1990; Frith
2007). In addition to possessing sophisticated
social intelligence and complex societies, pri-
mates excel at technical skills which require gen-
eral intelligence (Byrne 1995; Reader et al. 2011).
However, the causal relationship between these
variables is unclear, and there is a dearth of evi-
dence suggesting that social complexity leads to
general intelligence in addition to social intelli-
gence. In sum, primates appear to have social
brains, and it also appears that their social com-
plexity does relate to social intelligence, but this
social intelligence may be more modularized than
the SIH predicts. Research on the SIH is limited
outside of primates, and support is much more
mixed in non-primate taxa.
Testing the SIH in Non-primates
Mammals
Group size does not explain variation in brain size
in carnivores (Finarelli and Flynn 2009), instead
the hunting of large vertebrate prey best explains
brain size (Swanson et al. 2012), nor does physi-
cal problem-solving skill relate to sociality in
carnivores (Benson-Amram et al. 2016). Exten-
sive research on spotted hyenas, which have soci-
eties that exhibit convergent evolution with those
of cercopithecine primates, provide an interesting
case study into the evolution of social intelligence
(Holekamp et al. 2007). They exhibit comparable
social cognitive skills on every measure tested,
The Social Intelligence Hypothesis 3
including the recognition of third-party relation-
ships, and compared to other hyenids, they have
larger brains, are more social, and are better at
tasks of physical problem-solving (Holekamp
et al. 2015). Taken together, the data from
Hyaenidae strongly support the SIH. However,
compared to cercopithecine primates, spotted
hyenas are small brained and poor physical prob-
lem solvers which contradicts many predictions of
the SIH when considering their convergent soci-
ality. In bats, living in stable social groups and
smaller teste size were associated with an increase
in brain size (Barton and Dunbar 1997; Pitnick
et al. 2006). Within gregarious ungulates, those
that live in stable year-round groups of at least six
individuals had the largest brains (Pérez-Barbería
and Gordon 2005). Another study found that
monogamous ungulates that lived in mixed habi-
tats had the largest relative brain sizes. Elephants
are some of the largest brained and longest lived
mammals; they are extremely social, forming dif-
ferentiated relationships with many individuals.
Like primates, elephants use tools and may have
the rudiments of theory of mind (Hart et al. 2008).
In cetaceans, pod size and relative brain size are
associated (Marino 2002). Cetaceans also exhibit
remarkable convergence with primates of cogni-
tive abilities in both social and physical domains
(Marino 2002; Marino et al. 2007). As cetaceans
are among the largest brained mammals, this con-
vergence strongly supports the SIH. A large study
that included data from both the mammalian fossil
record and extant species showed that increases in
brain size in mammalian taxa across evolutionary
time were associated with greater proportions of
species, but not all species, within those taxa
living in stable bonded groups (Shultz and Dunbar
2010). This provides some support for the SIH but
does not explain the fact that many extant mam-
malian species with large brains are not social
(Holekamp and Benson-Amram 2017). In sum,
varied measures of social complexity sometimes
correlate with brain size in mammals, but this
relationship is not consistent, and there are notable
exceptions to this relationship (e.g., bears).
Birds
Birds are capable of many of same feats of social
cognition and tool use as primates (Emery and
Clayton 2004; Marler 1996). Beauchamp and
Fernández-Juricic (2004) found no relationship
between either mean or maximum flock size in
birds and forebrain size, possibly because flock
size is not a stable trait in most bird species.
However, Burish et al. (2004) found that telence-
phalic volume fraction, a measure similar to neo-
cortex ratios in primates, was related to a
categorical measure of social complexity.
Corvids, in particular among birds, appear to
show convergent evolution with apes when it
comes to brains and intelligence. Though corvids
have smaller overall brain sizes, their relative
brain to body size ratio is almost the same as that
in chimpanzees (Emery and Clayton 2004).
In a large study that included several mamma-
lian and bird taxa pair-bonding was found to be
the strongest predictor of brain size. One explana-
tion for why pair-bonding, as opposed to group
size, is related to brain size outside of primates is
because any groupings larger than a pair-bond in
non-primates could be temporary aggregations or
involve undifferentiated relationships (Dunbar
and Shultz 2007). That is, primate relationships
might be uniquely bonded such that group size
approximates the number of bonded relationships
only in primates. However, we now know that
many species outside of primates do form stable
bonds outside of pair-bonds (e.g., spotted hyenas
or elephants). In addition, recent research that
specifically examined bird species that do live in
long-term stable groups found that stable group-
ings larger than the pair-bond were associated
with smaller brains (Fedorova et al. 2017)
contradicting the idea that long-term stable
bonds might be the main cause of social complex-
ity and larger brains.
Fish
Fish have both relatively and absolutely small
brains, but fish have been shown to exhibit social
intelligence with a diverse array of social cogni-
tive skills documented, including reciprocation,
transitive inference, coordination and coopera-
tion, and the ability to remember past interactions
4 The Social Intelligence Hypothesis
with other individuals (Bshary et al. 2014). This
research suggests that relatively simple brains can
accomplish many of the same feats seen as intel-
ligent in primates, which at the least suggests that
a closer look at the rules governing social behav-
ior may be required to measure social intelligence.
That is, not all complex social behavior may be
indicative of social intelligence. However, in one
clade of fish, increased telencephalic size was
related to monogamy (Pollen et al. 2007) which,
similar to findings in birds, suggests pair-bonds
may be related to relative increases in brain size.
Invertebrates
Insects possess remarkably miniaturized brains
but can also show highly sophisticated social
skills, among other cognitive abilities. This
research suggests that brain structure or function
may be more important to social intelligence than
brain size (Lihoreau et al. 2012). Octopuses also
provide an interesting case study. They are large
brained relative to other invertebrates and show
high general intelligence; they use tools, differen-
tiate individual humans, and show advanced per-
formance on a variety of cognitive tests
(Darmaillacq et al. 2014); however they are
strictly solitary. Overall, invertebrates possess a
rich repertoire of cognitive abilities, which chal-
lenges the assumption of the SIH that complex
cognition requires a large brain (for a full review
of invertebrate cognition, see Roth 2013).
Conclusion
Overall, the SIH explains substantial variation in
cognition and brain size within primates, particu-
larly between monkeys and apes. Primates,
including humans, do appear to have brains spe-
cialized for social cognition, and primate intelli-
gence may be primarily social. Outside of
primates, both support and opposition have been
found for the SIH; it is not yet clear what the major
selective force favoring large brains and intelli-
gence might be outside of primates, though soci-
ality may have played a role. Certainly, the SIH
has drawn attention to the fact that cognition in the
social realm is both varied and complex and has a
strong impact on fitness and daily lives of animals
as well as highlighting the role sociality may have
had in human brain evolution. Humans, as well as
possessing high general intelligence, are largely
unique in the realm of social cognition with regard
to our language and culture.
However, the SIH still faces several serious
challenges in addition to the described challenges
defining social complexity. The SIH has yet to
adequately account for grade shifts between dif-
ferent taxa. When comparing brain size to body
size, or some other scaling variable, there are
marked differences in the y-intercepts or slopes
of the regression lines for different taxa. These
grade shifts can bias measures of relative brain
size and are often not taken into account when
comparing measures of brain size to measures of
social complexity. For example, orangutans are
relatively solitary compared to other apes, but
also large brained, in part because the great ape
lineage lays on a higher grade than other primates
(also see Finarelli and Flynn 2009). A recent
paper that reanalyzed the data from primates
using updated phylogenies and a larger number
of species found no relationship between several
measures of social complexity and brain size.
Instead, frugivory was the strongest predictor of
brain size (DeCasien et al. 2017). Frugivory may
require both technical skill through the extractive
foraging of fruits and seeds and increased spatial
cognitive abilities for remembering when and
where to find ripe fruit; this would tend to support
ecological hypotheses for brain evolution rather
than the SIH (Parker 2015). This finding high-
lights the fact that the SIH cannot explain general
intelligence. Though the SIH hypothesizes that
social intelligence may be used to solve physical
problems, it seems that social intelligence instead
is quite specialized; there is little evidence for
transference between social and general intelli-
gence. Though specialized social skills are related
to size of certain brain areas, there is a lack of
research that directly relates social intelligence to
brain size.
Despite these problems, the SIH currently
remains one of the most popular hypotheses for
the evolution of intelligence. Overall, there is
ample evidence to support the claim that social
The Social Intelligence Hypothesis 5
complexity, large brains, and intelligence are
strongly associated in many taxa. Just why or
how they are related, and what selective pressures
are involved when they are not related, must still
be determined.
Cross-References
▶Bird Tool Use
▶Brain Size and Intelligence
▶Cephalod Tool Use
▶Cognitive Buffer Hypothesis, The
▶Communication and Social Cognition
▶Convergent Evolution of Hyena and Primate
Social Systems
▶Convergent Evolution of Intelligence
▶Convergent Evolution of Intelligence Between
Corvids and Primates
▶Cultural Intelligence Hypothesis, The
(Herrmann et al. 2007)
▶Encephalization Quotient
▶Evolution of Intelligence, The
▶Evolution of the Brain, The
▶Extractive Foraging Hypothesis, The (Parker
and Gibson 1997, 2015)
▶General Intelligence Factor G(Reader et al.
2011)
▶Nonhuman Intelligence
▶Primate Tool Use
▶Relative Brain Size
▶Social Learning and Social Cognition
▶Social Tool
▶Technical Intelligence Hypothesis, The
▶Theory of Mind and Nonhuman Intelligence
▶Vygotskian Intelligence Hypothesis, The (Moll
et al. 2007)
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