Transcript A cooperative species

A Cooperative Species*
Herbert Gintis
Santa Fe Institute
Central European University
University of Siena
*Material based on Samuel Bowles and Herbert Gintis,
A Cooperative Species: Human Reciprocity and its
Evolution (Princeton University Press, 2011)
Princeton
University Press
2011
Two Propositions on the Nature
of Human Cooperation
I will advance two propositions.
First, people cooperate not only for self-interested
reasons, but also because they are concerned about the
well being of others,
try to uphold social norms,
and value behaving ethically for its own sake.
People punish those who free-ride on the cooperative
behavior of others
even when they cannot expect to gain anything from it
other than the pleasure of hurting someone who has
done wrong.
Two Propositions on the Nature
of Human Cooperation
Second, we have these moral sentiments,
because our ancestors lived in environments in which
groups of individuals who are predisposed to cooperate
and uphold ethical norms
tended to survive and expand relative to other groups,
thereby proliferating these pro-social motivations,
both culturally and genetically.
Mutualism and Altruism
Cooperation may be mutualistic; i.e., an activity that
confers net benefits both on the actor and on others.
Cooperation may also impose net costs upon the
individual, in which case we say that it is altruistic.
By contrast to mutualistic cooperation, altruistic
cooperation would not be undertaken by an
individual whose motives were entirely selfregarding
and thus did not take account of the effects of his
actions on others or the norms of the group.
Mutualism and Altruism
The evolution of cooperation that is mutualistic or
involving only kin altruism---sacrifice on behalf of
close relatives---are easily explained by natural
selection
because such behaviors help proliferate one’s genes
irrespective of whose body they are in
including the genes that induced the helping
behavior.
Cooperation could also have evolved because of
reciprocal altruism, a form of mutualism in which I
help you today in the expectation that you will help
me at a future date when I am in need.
Mutualism and Altruism
Kin altruism, mutualism, and reciprocal altruism
(which is really long-run mutualism) are popular
among biologists and economists alike
and explain many forms of human cooperation,
particularly those occurring among close kin or in
very small groups (two or three individuals).
Mutualism and Altruism
These models fail to explain two facts about human
cooperation:
that it takes place in large groups of non-kin, and
it occurs in interactions
where there is no repetition, and
where there is complete anonymity (i.e., there are
no reputational gains from behaving prosocially)
Only a model in which humans have social preferences
that motivate them to participate and to punish the
transgressors of social norms can explain human
cooperation in large groups of non- kin.
The Evolution of Social Preferences
Early modern humans inhabited the large, mammal-rich
African savannah and other environments in which
cooperation in acquiring and sharing food yielded
substantial benefits at relatively low cost.
The slow human life-history with prolonged periods of
dependency of the young also made the cooperation
of non-kin in child rearing beneficial.
The Evolution of Social Preferences
As a result, groups that fostered cooperation in
hunting, child-rearing, punishing,
prevailing against hostile neighbors,
and sharing truthfully transmitted information
had significant advantages over other groups.
Prosociality in general and altruism in particular were
successful for three reasons.
The Evolution of Social Preferences
First, human groups devised ways to protect their
altruistic members from exploitation by the selfinterested.
Prominent among these is the shunning, ostracism, and
even execution of those who violate cooperative
norms.
Prominent also are the egalitarian practices that limit
hierarchy and inequality, including the sharing of
food and information, within the group:
There is no “big man” in a hunter-gatherer group.
The Evolution of Social Preferences
Second, humans adopted prolonged and elaborate
systems of socialization
that lead individuals to internalize the norms that
induce cooperation,
so that contributing to common projects and
punishing defectors became objectives in their own
right rather than constraints on behavior.
Allied with norm internalization are the social
emotions, such as shame, guilt, pride, and empathy
that are at best rudimentary in almost all other
creatures.
The Evolution of Social Preferences
Third, between-group competition for resources and
survival was a decisive force in human evolutionary
dynamics.
Groups with many cooperative members tended to
dominate less cooperative groups,
thereby gaining both through genetic and cultural
selection.
The Evolution of Social Preferences
Throughout human history, group extinction, costly
group dispersal, and ostracism have been powerful
mechanisms supporting the evolution of human
cooperation.
The extraordinarily high evolutionary stakes of
intergroup competition
and the contribution of altruistic cooperators to
success in these contests
meant that sacrifice on behalf of others, extending
beyond the immediate family and even to virtual
strangers in one’s group, could proliferate.
The Science of Morality
The relationship between self-regard and moral
sentiments has been illuminated in recent years by
behavioral game theory.
The experiments confirmed that self-interest is indeed a
powerful motive, but also that other motives are no
less important.
Even when substantial sums of money are at stake,
most experimental subjects are fair-minded,
generous towards those similarly inclined, and nasty
towards those who violate these pro-social precepts.
Let us review some of this evidence.
Honesty
Why would a human be unconditionally honest, as
opposed to being honest only where it pays off and
ruthlessly dishonest when one can get away with it?
Numerous experiments indicate that most human
subjects are willing to sacrifice material reward to
maintain a virtuous character, even in one-shot
encounters with perfect anonymity.
Honesty in the Laboratory
Gneezy (2005) used a two-player game played once,
under completely anonymity.
Call the two players Bob and Alice.
Bob is shown two options. Option A pays him $5 and
pays Alice $6. Option B reverses the payoffs.
Alice can’t see the options, but she must choose one
based on Bob’s advice (which she is free to ignore).
Bob then chooses one of two sentences to tell Alice:
“Option A will pay you more than Option B,”
or
“Option B will pay you more than Option A.”
Alice then chooses either Option A or Option B, and the
money is paid.
Honesty in the Laboratory
Before making her choice, Alice is informed that
one option is better for Bob, and the other is
better for her.
Gneezy (2005) found that 83% of Bobs told the
truth.
Moreover, 78% of Alices believed their Bob, even
though they knew that Bob had an incentive to
lie
(provided Bob believed that Alices would likely
follow his advice---which he did indeed believe).
Honesty in the Laboratory
People make trade-offs among values and between
values and material rewards. Morality is not
categorical.
If the options were A:(5,500) and B:(500,5), many
more Bobs might lie.
In Gneezy’s experiment with options A:(5,15) and
B:(6,5), more Bobs told the truth (lying is very costly
to Alice).
In Gneezy’s experiment with options A:(5,15) and
B:(15,5), more Bobs lied (the cost of honestly was
very high).
Honesty in the Laboratory
Many such experiments show that people value virtue
for its own sake.
People are generally not selfish, even when they can
behave selfishly with complete impunity.
Of course, some fraction of people are completely
selfish (usually about 20% in behavioral game
theory experiments).
If society does not find a way to punish the free-riders,
cooperation will very often unravel, as we shall see.
Parochial Altruism
Altruism – conferring benefits on others at a cost to
oneself
and parochialism --favoring ethnic, racial or other
insiders over outsiders –
are commonly observed human behaviors that are
well documented in experiments.
In fact, they are closely interrelated in the evolution of
Homo sapiens.
Parochial Altruism
Parochialism and altruism are both puzzling from an
evolutionary perspective because they reduce the
individual’s personal payoffs
by comparison to others who behave purely selfishly.
Parochial Altruism
We consider an explanation: parochial altruistic behavior
contributes to success in between-group competition.
However, neither altruism nor parochialism alone is likely to
evolve.
Parochial Altruism
While within-group altruism and parochialism could not
have evolved singly, they could have co-evolved, each
providing the exceptional conditions allowing for the
evolutionary success of the other.
This is why other-regarding preferences are often
conditional on group membership, and may involve
negative as well as positive sentiments toward the wellbeing of others.
The Big Questions
Was war sufficiently common and lethal enough to
allow the proliferation of an altruistic trait?
Can we explain the co-evolution of such an altruistic
trait along with hostility towards ‘outsiders’?
Is There a Hamilton’s Rule for Groups?
The probability that in a single generation a group
engages in a contest for survival (κ)
The degree of genetic diversity between groups that
come into conflict (r)
The contribution of altruistic behavior to the probability
that a group will win a conflict (b)
The fitness cost of the altruistic behavior (c)
From all of the above, we can derive a “Hamilton’s
Rule” for groups (approximate):
2κbr>c
Ethnographic (black dots) and archaeological (black squares)
evidence on mortality and (white dots) genetic differentiation
Table 1 Fraction (δ) deaths due to warfare (or other violence) among hunter-gatherers: archaeological evidence
Site, source
Approx date (yrs
before present)
Citation (complete citations in
(20))
δ
British Columbia
5500BP-334
Cybulski (1994)
0.23
Nubia (site 117)
14-12000
(23) Wendoff (1968)
0.46
Nubia (near site 117)
14-12000
Wendoff (1968)
0.03
Ukraine (Vasylivka)
11000
Vencl (1991)
0.18
Ukraine (Volos’ke)
Epipalaeolithic
Danilenko (1955)
0.17
S. California
5500-628
Lambert (1997)
0.04
Central California
3500-508
Moratto (1984)
0.05
Sweden (Skateholm I)
6300
Price (1985)
0.07
Central California (SCL 674)
2415-1773
Andrushko et al (2005)
0.08
India (Uttar Pradesh)
3140-2854
Clark (1977)
0.27
Central California (SCI-038)
2240-238
Jurmain (2001)
0.04
Central California (Ala-329)
1500 – 238
Jurmain (2001)
0.04
Niger (Gobero)
16,000–8200
Sereno et al. (2008)
0.00
Algeria (Calumnata)
8300-7300
Chamla et al (1970)
0.04
France (Ile Teviec)
6600
Newall et al (1979)
0.12
Denmark (Bogebakken)
6300-5800
Newall et al (1979)
0.12
Table 2. Fraction of adult deaths due to warfare (δ) among hunter gatherers: ethnographic evidence
Population,regio
n
Dates
Livelihood and society
Citation (complete citations
in (20))
δ
Ache, Eastern
Paraguay
pre-contact
(1970)
foragers
Hill and Hurtado (1996)
0.37
Hiwi, VenezuelaColombia
pre- contact
(1960)
foragers
Hill et al. (2007)
0.17
Murngin
NE Australia:
1910-1930
forager including
maritime
Warner (1931)
0.21
Ayoreo BoliviaParaguay
1920-1979
seasonal foragerhorticulturalists
Bugos (1985)
0.15
Tiwi
N.Australia
1893-1903
sedentary hunter
gatherers
Pilling (1968)
0.10
Modoc
N. California
1934 sources
re ‘aboriginal
times’
sedentary hunter-gatherer
Ray (1963)
0.13
Anbara
N.Australia
1940-1960
Recently settled nomadic
maritime foragers
Hiatt (1965)
0.04
Yuki
N.California
before 1850
sedentary hunter-gatherer
Kroeber (1953)
0.27
Kato
N.California
before 1850
sedentary hunter-gatherer
Kroeber (1953)
0.01
Yurok
N.California
1830-1840
sedentary hunter-gatherer
(storage)
Kroeber (1953)
0.04
Table 3. Critical cost ( c*) of an altruistic behavior for given estimates of genetic differentiation (F) and
mortality in intergroup hostilities (δ) among Australian foragers.
Murngin
δ = 0.207
Tiwi
δ = 0.100
Anbara
δ = 0.045
Aboriginal Australian
groups F = 0.042
0.136
(0.073)
0.066
(0.035)
0.030
(0.016)
Kaiadilt-Lardiil
F = 0.081
0.209
(0.146)
0.101
(0.704)
0.046
(0.032)
If c = 0.03 in the absence or
group conflict, the behavior
would go from 90% to 10% of
a population in just 150
generations
Inter-demic genetic differentiation is measured by the FDT. Mortality in intergroup
hostility (δ) is from Table 2. b = 2. Group size is 26 which could be a coalition of 3
groups of the size considered to be typical of foragers during the late Pleistocene
Entries in parentheses are large group size t statistics.
2 κ b r = c*
Consider a population in
which individuals may be
either Altruistic or Not and
either Tolerant or Parochial
towards other groups.
Parochial Tolerant
Altruist PA
TA
Not
NT
NP
A’s contribute to the fitness of other group members at a
cost to themselves
Only the Parochial Altruists fight wars.
P’s induce hostilities and forgo the benefits of peaceful
interactions with other groups enjoyed by the T’s
Four behavioral types;
two selection processes
(multi-level selection)
Within group selection:
a) N’s payoffs exceed A’s
b) in absence of war, T’s
payoffs exceed P’s
A
N
Inter-groups interactions: (a)
hostile conflict or (b) trade,
insurance, exploiting buffer
zones
P
PA
NP
T
TA
NT
Between group
selection: Group with
fewer PA’s lose
conflicts, a fraction of
their members are
replaced by a draw the
from the winner
group
Within group selection favors N over A and in the absence of hostile
inter-group conflict favors T over N; between group conflict favors PA
over other behaviors.
The model parameter values
(per generation, where relevant)
•
•
•
•
•
•
•
•
•
•
# of groups = 20
Group effective size =26 (i.e. census size about 70 = 3
bands)
Mutation =0.005
Two loci, two alleles at each locus, full recombination
Between group island (random) migration =0.25
Benefits and costs: b=0.02, c=0.01, baseline fitness=1
Benefit from peaceful interaction: g=0.001
Between group interactions per generation: k=1
Fitness loss of losing group (non fighters) = 2.5c
Fighters’ mortality in warfare = 0.14
An empirically estimated distribution
The height of the bars
gives the fraction of a
very long period in
which we observe the
indicated pair of
population-level
frequencies of altruists
and parochials in the
population.
Conclusion
Under conditions approximating those experienced by our
Late Pleistocene ancestors, groups of parochial altruists
could emerge, and such groups would frequently engage
in and win hostile conflicts with other groups.
The model indicates that neither parochialism nor altruism
is viable singly but that warfare, altruism and
parochialism could have evolved jointly.
Cooperation against others may be the distinctive capacity
of our ancestors that explains their success in the great
exodus from Africa around 60 thousand years ago.
Conclusion
There is no evidence of a genetic basis for war, altruism
or parochialism in humans.
What we show is that if genes supporting these
behaviors were to exist they could have evolved.
Parochial altruism may be our (genetic and cultural)
legacy, but it need not be our fate.
As an especially cultural species, we may be able to
invent, redirect, and overcome, outsider hostility.