Parochial Altruism and War: A Game Theoretic Analysis

Pleistocene North America

North America during the Pleistocene

War, what is it good for?  Apparently, altruism.  In a paper published in Science, Samuel Bowels and Jung-Kyoo Choi took a game-theoretic approach to studying the evolutionary roots of both altruism and parochialism.  They concluded that neither would have likely evolved alone, but instead co-evolved, together being a powerful combination in the survival kit  of our Pleistocene and early Holocene ancestors.


Altruism–benefiting fellow group members at a cost to oneself–and parochialism–bostility toward individuals not of one’s own ethnic, racial, or ther group–are common human behaviors.  The intersection of the two–which we term “parochial altruism”–is puzzling from an evolutionary perspective because altruistic or parochial behavior reduces one’s payoffs by comparison to what one would gain by eschewing these behaviors.  But parochial altruism could have evolved if parochialism promoted intergroup hostilities and the combination of altruism and parochialism contributed to success in these conflicts.  Our game-theoretic analysis and agent -based simulations show that under conditions likely to have been experienced by late Pleistocene and early Holocene humans, neither parochialism nor altruism would have been viable singly, but by promoting group conflict, they could have evolved jointly.


Even Darwin noted that war was a powerful tool “used” by evolution to increase  altruism and solidarity toward ones own group members.  But, there have been two major questions lingering.

  1. What is the process by which war became common enough to support the evolution of altruism in this context?
  2. What is the likelyhood that altruism itself (conditioned on group membership) contributed to the high levels of lethal intergroup conflict among humans?

Neither of these questions has been well enough analyzed and was one of reasons the authors did their study.    Empirically, both altruism and hostility are quite important to members of other groups.

The empirical importance of both altruism and hostility to members of other groups is well established.  Experimental and other evidence demonstrates that individuals often willingly give to strangers, reward good deeds, and punish individuals who violate social norms, even at a substantial personal cost (4), while favoring fellow group members over “outsiders” in the choice of friends, exchange partners, and other associates and in the allocation of valued resources (5).

They site an example of a case in Papua New Guinea, “There exists strong favoritism toward ones-own linguistic group in giving to others,”  and a higher tendency to punish those from different linguistic groups.

They use the term Parochial Altruism in reference to a person to mean that when a person engages in hostile and aggressive behavior with another group, this person incurs a mortal risk, therefore a fitness loss verses those who refrain from such aggression.

Knowing Parochial altruism exists and assuming that neither Parochialism nor Altruism would have evolved in an environment (that is survived a selection process) that favored some other trait that resulted in higher payoffs, then how DID Parochial Altruism evolve?

A Solution

One possibility is that since oiur ancestors lived in a hostile environment where resources were scarce, Parochial Altruism could have evolved and thrived because those groups with high numbers of Parochial Altruists would have been more able to engage in aggressive action and “win” on behalf of their groups.

The two most important correlates of tribal warfare are natural disasters and resource scarcity.  The Pleistocene and early Holocene (roughly 125,000 to 10,000 years ago) are known to have been times of substantial volatility.  They also coincide with the most significant periods of human evolution.

Could Parochial Altruism have evolved in such a climate?

The Game

Bowel’s and Choi’s model consists of 4 types of players.

  1. PA:  Parochial Altruists
  2. TA: Tolerant Altruists
  3. PN: Parochial Non-Altruists
  4. TN: Tolerant Non-Altruists

Note that Parochials of both types are hostile toward other groups.  But, ONLY Parochial Altruists will engage in combat.  This is because PN’s won’t risk death for the benefit of others.

Their model has two types of selection acting at once.  Intra-Group selection favors TN’s and tends to eliminate PA’s.  And, Inter-Group selection which favors PA’s via selective extinction.

In a purely risk vs. reward scenario, it makes little sense to be a PA.  While there exists two benefits to winning a war (namely 1. Greater chance of future survival, 2. Opportunity to reproduce, thereby replacing those PA’s lost in war), the risk of mortal death incured by war “offsets this direct benefit by a wide margin.”  Therefore, each PA would be better off adopting a different strategy, in terms of their own reproductive fitness.  This confirms that PA’s are, indeed, altruistic according to the traditional meaning of the term.

3 Stage Game

The game runs in 3 stages.   In stage one, when two groups A and B meet, there is a probability that they will engage hostilely.  If they do not, then the game ends.  If  their interaction is hostile, they move on to stage two.

Stage two, given that their interaction is hostile, there is a new probability that A and B will goto war.  If they don’t, they move on, game is done.  If they do, stage 3.

Stage 3, they are now at war, the group with the higher number of PA’s has a higher probability of winning.  If this group is A, then A is more likely to win a war against the PA deficient group B.   Given that A is stronger (ie, has more PA’s) there are two options:  A and B draw, and the result is simply that both groups lose a certain number of fighters (PA’s); or A wins, and still loses a certain number of fighters, but also now gains a number of replicas that make up for that loss.

From B’s perspective, given that B is weaker (has less PA’s), there is only Draw or Lose. B could get lucky and draw, and only lose some PA’s.  But, there is a higher likelyhood of a loss.  In this case, B loses both fighters (PA’s) and civilians (made up of the other types).

In the paper they are quite explicit about what these probabilities are and why they chose them.  But, the point is that not every encounter with another group is hostile, not every hostile interaction results in war, and every war is won with a higher probability if you have a large number of PA’s.


They ran this game through a number of iterations accounting for hundreds of generations.  They found that transitions from quite tolerant non-altruistic (read: peaceful) groups to bellicose parochial altruistic groups can happen very rapidly–in about 200 generations, or about 5,000 years.

The markedly higher reproductive success of predominantly parochial altruist groups when interacting with groups with fewer parochial altruists could therefore explain the rapid range expansions that are thought to be common among some late Pleistocene human groups, and thus may partly explain sthe still puzzling second great hominid diaspora that swept from Africa as far as Australia in the course of no more than 10 millennia.

This study aids in the study of why group boundaries have such a profound effect on human behavior, from an evolutionary perspective.

In conclusion they add:

We have explained how Homo Sapiens could have become a warlike yet altruistic species.  But there is no evidence that the hypothetical alleles in our model exist, or that were they to exist they could be expressed in the complex behaviors involved in helping others and engaging in lethal conflict.  Theus, we have not shown that a warlike genetic predisposition exists, only that should one exist, it might have coevolved with altruism and warfare in the way that we have described.

They make a good closing point.  Theoretical (ie, mathematical) biology doesn’t “prove” that certain things are true.  It tests the validity of certain hypothesis and ideas, thereby opening up further possibilities for empirical research.


Choi, Jung-Kyoo, and Samuel Bowles. 2007. The Coevolution of Parochial Altruism and War. Science 318, no. 5850 (October 26): 636-640. doi:10.1126/science.1144237.


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