CHIMPANZEE WARFARE
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First documented Gombe National Park in
January, 1974
Eight males formed a fighting party
Crossed border of their usual range
Attacked isolated male for 10 minutes as a group
Individual assumed to have died
First time coalition observed raiding neighboring
territory and resulting in lethal attack on member
of their own species
HUMANS AND CHIMPANZEES
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Unique pattern of aggression shared with no other
known species
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Caused some rethinking of warfare as solely a
cultural or social process
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With regularity coalitions voluntarily choose to
initiate coalitional aggression
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Tooby and Cosmides explored evolutionary
aspects in frequently cited article
CONTEXT OF THEORY
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Darwin proposed warfare influenced human brain
evolution
Group selection proposed by Alexander (larger
group gains resources)
Individual/genic selection argued by Durham
(survival and reproduction improved inclusive
fitness) ; theory was criticized for lacking economic
motives, not answering rarity of warfare or male
predominance
Tooby and Cosmides answered with the theory we
read
INDIVIDUAL AGGRESSION VS. COALITIONAL AGGRESSION
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Large gap between group level behavior and
individual adaptation
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How does group aggression function adaptively?
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How is group aggression consistent with individual
selectional thinking?
INDIVIDUAL AGGRESSION
COALITIONAL AGGRESSION
~Fight
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Gain resources/promote
fitness
All species
Common
No cooperation
~War
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Gain resources/promote
fitness
Chimps and humans as
killers
Rare
Cooperation
SURPRISING FEATURE OF
WAR IS COOPERATION
ANALYSIS OF COOPERATION EXPLAINS…
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Adaptive obstacles
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Rarity among animals
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Appeal of war
ADAPTIVE OBSTACLES
“It is difficult to see why any sane organism,
selected to survive and genetically propagate,
should seek so actively to create conditions of
such remarkable personal cost and danger”
~Tooby and Cosmides
RARITY AMONG ANIMALS
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Coalitional aggression evolved because it
promoted fitness
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Why is coalitional aggression not MORE
common?
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Elephant seals/Hanuman langurs don’t use
coalitional aggression
APPEAL OF WAR
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Males seem less concerned about individual
somatic welfare than females
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Universal in humans, prized and glorified
Evolution of Cooperation and
Coalitional Aggression
Advances in evolutionary biology and game theory
show that cooperation can exist if:
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There is ecological stability for enhanced payoffs
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Individuals can discern cooperators from noncooperators
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Individuals can exclude cheaters from the group
benefit
(Axelrod, Hamilton, Trivers, Smith)
Modern Simulations Ask…
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Are gains from trade available?
What is the amount of repeat interaction?
What is the availability of information about others
behavior?
These criteria provide an Evolutionary Stable
Strategy (ESS) for cooperative interaction within a
group environment
Cooperation depends on sophisticated cognition
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The phylogenetic distribution of sophistically
advanced species is rare
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Such specialized information processing cannot
be culturally learned, or be a product of general
intelligence
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Must be adaptively designed systems specialized
for these functions (Darwinian Algorithms)
MULTI-INDIVIDUAL COMPLEXITY
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With more than two individuals the cognitive
problem becomes more complex
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Must track performance and levels of participation
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Modifying individual behavior for acceptance by
group (Consensus)
Evolution of specialized cognitive mechanisms to
regulate coalitional participation
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Direct product of evolution
Generates behavior for the formation of coalitions,
maintenance of organization, and war
Must understand the evolutionary game theoretic
structure of multi-individual cooperation, and the
cognitive mechanisms that produce solutions to
these game theoretic preconditions
Must examine the problem at both of these levels
simultaneously for a coherent exploration
“Evolutionary considerations indicate that for recurrent,
evolutionarily important situations, the psyche will evolve
specialized information processing procedures, Darwinian
Algorithms, which can handle these situations with special
efficiency. These programs should organize information into
adaptively meaningful units, focus attention on adaptively
important environmental circumstances, call up special
inference networks, and so on, allowing the animal to solve
such recurrent problems using procedural knowledge built up
over thousands of generations”
~Cosmides and Tooby
TASK ANALYSIS OF SUCH PROGRAMS
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Decisions regulating coalitional formation
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Participation
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Cost and benefit allocation
SELECTION FAVORS PARTICIPATION
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Psych mechanisms regulating individual
participation
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Evolution of adaptations for coalitional aggression
require four essential conditions
RISK CONTRACT FOR WAR
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Increase in sexual access to females outweigh
losses due to death
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Certainty of victory
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Fair allocation of benefits
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Veil of ignorance about who will live or die
NO DECREASE IN AVERAGE REPRODUCTION OF MALE
MEMBERS OF COALITION
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With polygyny, access to females can be limiting
factor for male reproduction
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Female capacity doesn’t go unused due to male
mortality
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Each individual who dies loses, but each survivor
gains to same extent
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Death of males will not decrease average
reproduction of the members of the coalition
FEMALES ARE DIFFERENT
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Net reproduction drop is in direct proportion to
females killed
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Females much less likely to participate
CERTAINTY OF VICTORY
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Larger group attacking smaller
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Perceive so many on your side guarantees
invincibility
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In chimpanzees, always larger group attacking
smaller or individual
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Optimum level of participation sensitive to
probability of success
DON’T KNOW IN ADVANCE WHO WILL DIE
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Random distribution of risk of death
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Death cannot seem certain or people will defect
FAIR ALLOCATION OF BENEFITS
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Get more the more you put in
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Raises issues of enforcement and cheaters
PROBLEMS OF ENFORCEMENT
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For coalitions to stably evolve and function, the risk contract
must be enforced by some or all of its members
Whom is contributing and sharing cost?
Some members of the group must exclude cheaters from
sharing in the benefits, or eliminating benefits
Excluding members from group participation may be costly
to the group
Forced inclusion, punishment, retaliation for nonparticipation,
favored in intense coalitional participation
Active recruitment and enforcement of coalitional
participation
If enforcers gain at the expense of members, than such
behavior will be selected against
Mob activity seems motivated by fear of nonparticipation as
well as attraction to the coalitional goal
EVOLUTION OF ENFORCEMENT
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Benefits of coalitional action are not inherently a
public good
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Vary depending on life history variable, kin factors,
and other social variables
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Some may have more at stake than others
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Their costs will be offset by greater marginal
returns (individual benefit)
“the leader of the largest
descent group is invariably
the headman of the
village”….”the leaders are the
very individuals who decide
whether killings are
revenged”
~Chagnon 1988
EVOLUTION OF ENFORCEMENT
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Contingent threat: “obey me or else…”
Low cost and reusable if no one “takes him up on
it”
A person with a one bullet gun maintains a
credible threat, so long as he doesn’t demand too
much of the players
If their benefit from participation diminishes due to
asymmetric group contributions, than they will
revolt…”the natives are getting restless”
EVOLUTION OF ENFORCEMENT
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Individuals with strongest incentives to enforce
coalitional behavior will recruit additional members
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By threat or exertion of pressure to actively
enforce coalitional norms
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Getting someone else to do the dirty work
Cosmides and Tooby argue:
These and other potential solutions to the problem of
enforcement, when combined with plausible
assumptions about Pleistocene conditions, strong
selection would have existed for the growth of an
adaptively designed coalitional psychology, in a
manner not requiring recourse to either group
selection, cultural processes, or gene-culture
evolution
Importance of exploring the evolutionary dynamics and
cognitive foundations of coalitional aggression
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Political complexity of post-Neolithic state systems cannot be
directly reduced to models based on interband conflict in the
distant past
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Wars occur that few people want, fueled by political systems
based on coercion of the unwilling
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Modern group, political, religious, and mob phenomena
CRITICISMS
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If mortality are too high, it decreases the size of
your coalition, then vulnerable to victimization.
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Low (1990) questioned whether theory answers
why chimpanzees and human are unique?
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Shaw and Wong (1989) argued that propensity for
warfare equally prevalent in women and men,
women playing a supportive and defense of
offspring role.