Altruism (helping others at a cost to oneself) may evolve via group selection if the cost of altruism to the individual is compensated for by growth differences among groups when (1) there is high genetic variation among members of different groups; (2) more altruistic groups grow faster and (3) between-group migration is low. Nevertheless, group selection may not fully explain the actual evolution of helping behaviour if between-group migration was sufficiently common to have reduced between-group genetic variance. Lethal intergroup competition, which amplifies such growth differences between groups, appears to have been frequent in humans&#x27;; ancestral environments and could bear importantly on the evolution of altruism. Here we show that between-group migration and resulting genetic similarity can promote the evolution of costly helping behavior in the context of lethal intergroup conflict, albeit by selection at the individual level and not by group selection. The standard group selection models do not capture such basic elements of lethal intergroup competition as the possibility of an individual&#x27;s altruism being critical to the group&#x27;s success when that possibility is inversely proportional to genetic variation among members of the competing groups

John Orbell

Oleg Smirnov

English

Nature Precedings

1 
Genetic similarity promotes evolution of cooperation 
under lethal intergroup competition 
Oleg Smirnov1 & John Orbell2 
1Department of Political Science, Stony Brook University, Stony Brook, NY 11794, USA 
2Institute of Cognitive and Decision Sciences, University of Oregon, Eugene OR 97403, 
USA 
Altruism (helping others at a cost to oneself) may evolve via group selection if the 
cost of altruism to the individual is compensated for by growth differences among 
groups when (1) there is high genetic variation among members of different 
groups; (2) more altruistic groups grow faster and (3) between-group migration is 
low.  Nevertheless, group selection may not fully explain the actual evolution of 
helping behaviour if between-group migration was sufficiently common to have 
reduced between-group genetic variance.  Lethal intergroup competition, which 
amplifies such growth differences between groups, appears to have been frequent 
in humans’ ancestral environments and could bear importantly on the evolution of 
altruism.  Here we show that between-group migration and resulting genetic 
similarity can promote the evolution of costly helping behavior in the context of 
lethal intergroup conflict, albeit by selection at the individual level and not by 
group selection.  The standard group selection models do not capture such basic 
elements of lethal intergroup competition as the possibility of an individual’s 
altruism being critical to the group’s success when that possibility is inversely 
proportional to genetic variation among members of the competing groups. 
 
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Lethal intergroup competition appears to have been frequent in humans’ ancestral 
environments1,2 and increasing scholarly recognition3 that such competition could bear 
importantly on the evolution of altruism is valuable.  Whether by individual or 
multilevel selection4-6, altruism can evolve if and only if it advances the altruist’s 
reproductive success7-9.  Warfare certainly can have implications for an individual’s 
success, both as a consequence of what happens at the group level (victory or defeat) 
and of consequences for the individual, whether cooperator or free-rider.  If altruists’ 
reproductive gains from war-related events at the group level are greater than their 
personal losses from war-related altruism, then the latter attribute can evolve10,11.  The 
standard model of group selection3 suggests that this is most likely when groups are 
broadly different in altruism as derived from the Price equation10,11.  The feasibility of 
that model depends critically on the answer to the following empirical question: Were 
genetic differences between early human groups great enough to allow for the evolution 
of altruism via group selection? 
One implication of the group selection model of lethal intergroup competition3 is 
that paying a cost to help one’s group survive in lethal intergroup competition may 
provide direct reproductive benefits for an individual and, therefore, that a trait 
supporting such altruism can evolve without group selection.  The expected benefit 
from cooperation is greatest when: (a) group extinction rates are high, (b) contending 
groups are small, (c) cooperation differences are important for the probability of group 
survival, and (d) when the contest is competitive, viz., when the genetic difference 
between the groups is small.  Evolution of a trait can, of course, only happen in the 
context of variation on that trait, and that requirement has been developed in standard 
models of group selection.  In the case of lethal conflict among human groups, however, 
it seems that selection favours cooperation when between-group variation on that trait is 
small. 
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In a group selection model, the population consists of two types of individuals: 
cooperators and defectors (see the Methods section for technical details of the model 
and analysis).  Cooperators pay a cost and provide a benefit for the group while 
defectors free-ride.  The population of cooperators and defectors consists of several 
subpopulations, or demes, competing with each other at the group level.  Any two 
demes have a positive probability of a contest between them.  In the event of such a 
conflict members of one group survive while members of the other group die out.  The 
probability of individual survival, defined as λ , depends on the proportions of 
cooperators and defectors in two groups, deme size, and an exogenous parameter μ , 
which is inversely proportional to the “influence of altruists on deme survival”3;  
(Methods).  Identical probability of deme survival can be found in the Bowles model3.  
Figure 1 provides an intuitive illustration of λ  for three cases of μ . 
 
0.2 0.4 0.6 0.8 1 
0.2 
0.4 
0.6 
0.8 
1 λ
jp  
 
Figure 1 | Probability of survival λ  of members of deme j  for three cases: 0.1μ =  (solid), 
0.4μ =  (dots), 0.8μ =  (dashes).  The horizontal axis jp  represents a proportion of 
cooperators in deme j .  The proportion of cooperators in the other deme is assumed to be 
0.5qp = .  The two demes are further assumed to be of equal size.  For small μ , cooperators 
have the greatest effect on deme survival when the groups are genetically similar, j qp p≈ . 
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An individual’s switching from defection to cooperation has two effects on that 
individual’s fitness--the private cost of cooperation and the increase in the deme’s 
probability of survival resulting from that cooperative act.  The net effect of switching 
from defection to cooperation, defined as FΔ , is positive when the fitness benefit from 
increased deme survival is greater than the cost of cooperation.  In this case, genetic 
variation will favour cooperation.  Figure 2 shows FΔ  for the familiar three cases of 
μ . 
Selection favours cooperation when μ  is small and when the groups are 
genetically similar, i.e., an individual’s switching from defection to cooperation has the 
greatest effect on the probability of his or her deme’s survival.  Thus, the smaller is the 
genetic difference between competing groups, the greater is selection on cooperation.  
This result is opposite to standard models of the evolution of cooperation via group 
selection in which the evolutionary success of cooperation is based upon growth 
differences.     
The intuition behind this result is simple.  If groups are substantially different 
genetically—that is, if they have different fractions of cooperators—then the stronger 
group is very likely to survive and the weaker group is very likely to die out.  Then an 
individual’s privately costly cooperative behaviour in either group will be unlikely to 
affect the outcome and selection will favour selfishness—which, in the long run, would 
support between-group leveling in the incidence of cooperation.  Once groups become 
genetically similar, selection will favour the cooperative trait since the behaviour 
associated with that trait will be more likely to affect the probability of one’s deme 
surviving, thus of individuals carrying that trait surviving.  The effect of μ  is also 
intuitive since μ  is inversely proportional to the “influence of altruists on deme 
survival”3; as 1μ →  such influence becomes very small and FΔ  becomes negative. 
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5 
        
  
10 20 30 40 50 60
0.1 
0.2 
0.3 
0.4 
0.5 
0.6 
,i jFΔ
jA  
0.1μ =  
0.4μ =  
0.8μ =  
 
  
10 20 30 40 50 60 
-0.02
-0.01
0.01
0.02
,i jFΔ  
jA  
0.1μ =  
0.8μ =  
0.4μ =  
 
Figure 2 | Fitness benefit from cooperation of an individual i  in deme j , ,i jFΔ .  The 
horizontal axis jA  is the total number of cooperators in deme j .  For the figure, the total 
number of cooperators in the other deme 30qA = .  The three cases of ,i jFΔ  are for 0.1μ =  
(solid), 0.4μ =  (dots), 0.8μ =  (dashes).  The lower graph is a snapshot for 
, [ 0.02,0.02]i jFΔ ∈ − .  Other parameters used: benefit of survival 1W = , probability of a 
conflict 1κ = , cost of cooperation 0.02c = , total number of defectors in both demes 
30j qN N= = .  Selection favours cooperation for , 0i jFΔ >  which is the case when the 
fractions of cooperators in two groups are similar. 
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Note that the cases of being certain to win and being certain to lose are not exactly 
symmetric since cooperation appears to provide a greater reproductive benefit when the 
individual is in a losing group, as opposed to being in a winning group—despite the fact 
that in both cases an individual is unlikely to have an influence on the probability of his 
deme's survival. 
As Bowles3 points out, migration among groups is often said to have been 
frequent enough in the ancestral past to have reduced between-group differences beyond 
the point at which selective group extinction could offset within-group pressures against 
altruism.  In response, he points to the extraordinarily lethal character of human 
intergroup competition which could well have made selective group extinction a quite 
frequent event1,2, thus increasing the impact of between-group competition.  To the 
contrary, at the individual level, such between-group migration and the consequent 
reduction of between-group genetic differences would promote selection on the 
cooperative trait.  If empirical evidence were to show only slight genetic differences 
between groups, cooperation would still evolve in the context of lethal group conflict, 
absent group selection. 
As classically captured by the Price equation, altruism can be selected via 
between-group differences insofar as more altruistic groups grow faster.  Lethal 
intergroup competition also makes extinction of the whole group possible, meaning that 
costly cooperation can evolve if such behaviour has an “influence on deme survival”3, 
which is more likely when the contending groups are genetically similar.  The model 
proposed here does “take the altruism out of altruism”12 since selection on cooperation 
depends on the cooperator’s contribution rebounding to personal advantage.  Notice, 
however, that our result does emerge within the standard theoretical framework and is 
consistent with Bowles’ model.  In the case of lethal intergroup competition, a 
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willingness to cooperate for the sake of one’s group can evolve without group selection 
via its direct survival benefits to the individual.  
Methods 
Consider a metapopulation of two types: cooperators (A’s) paying a cost 0c >  and 
defectors, or egoists, (N’s) paying nothing.  The metapopulation consists of several 
subpopulations, or demes, competing with each other at the group level.  Two demes j  
and q  have an exogenous probability of a contest between them equal to κ .  In the 
event of such a conflict, the expected probability that group j  survives this contest is 
( ) ( ){ }0.5 1 max ,0 max ,0j q q jp p p pμ μλ = + − − −  (1) 
where [0,1]μ ∈ , and jp  and qp  are the fractions of deme j ’s and deme q ’s 
membership that are cooperators.  If jA  and qA  are the total numbers of cooperators in 
groups j  and q , and jN  and qN  are the total numbers of defectors, then 
( )/j j j jp A A N= +  and ( )/q q q qp A A N= + .  The probability of survival λ  is, in fact, 
identical to the one used in the Bowles’ model3 (supplemental materials, expression 
S13).  See Figure 1 for an intuitive illustration of λ  for three cases: 0.1μ = , 0.4μ = , 
and 0.8μ = . 
Notice that if an individual in deme j  switches from defection to cooperation 
then jp  becomes equal to ( ) ( )' 1 / 1 1j j j jp A A N= + + + − .  Consequently, 'j j jp p pΔ = −  
is the individual’s contribution to the increase in the probability of deme j  surviving 
the contest.  Assume that each member of the surviving group obtains fitness benefit 
0W > .  Thus, the fitness of an individual i  who belongs to group j  can be described as 
, ,i j i jF ca Wκλ= − + ⇔  (2)  
, ,
, , 0.5 1 max ,0 max ,0
j i j q q j i j
i j i j
j j q q q q j j
A a A A A a
F ca W
A N A N A N A N
μ μ
κ
⎛ ⎞⎛ ⎞ ⎛ ⎞+ +⎜ ⎟= − + + − − −⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟+ + + +⎝ ⎠ ⎝ ⎠⎝ ⎠  
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where , 0i ja =  if the individual is a defector, and , 1i ja =  if the individual is a 
cooperator.  Cooperators pay the individual cost c  but increase the whole group’s 
probability of survival.  In the paper, we examine ( ) ( ), , , , ,1 0i j i j i j i j i jF F a F aΔ = = − =  
which is the net fitness benefit from cooperation.  For positive values of ,i jFΔ , genetic 
variation will favour cooperation and for negative values, defection. 
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Proc. Natl. Acad. Sci. U.S.A. 103, 10952-10955 (2006). 
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Author Information  The authors declare no competing financial interests.  Correspondence and requests 
for materials should be addressed to O.S. (oleg.smirnov@stonybrook.edu). 
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Genetic similarity promotes evolution of cooperation under lethal intergroup competition

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Genetic similarity promotes evolution of cooperation under lethal intergroup competition

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