Humans behave altruistically more than any other species. Ethnocentrism, though not universal, is common in humans. Humans are also one of the few species that practice war.  In many cultures valor in war and sacrificing yourself for others are considered admirable.  Why is this?

Altruism, is the tendency for an individual to sacrifice themselves for others.  It can result in reducing the fitness of the individual making the sacrifice. This behavior should be deselected by natural selection. The tendency to be hostile towards outsiders, hereafter referred to as parochialism, results in the individual giving up opportunities for cooperation with outsiders. Parochialism also should be deselected by natural selection. So both altruism and parochialism should be weeded out by natural selection. Why then, are they so prevalent?

It turns out that parochial altruism could have emerged and proliferated among early humans because our ancestors lived in environments in which competition for resources favored groups with substantial numbers of parochial altruists willing to engage in hostile conflict with outsiders on behalf of their fellow group members.

The emergence of altruism and parochialism could have been due the environment in hunter and gatherer times, when humans competed for limited resources. Competition between groups for resources could have selected for groups that won wars. Prior to the climate stabilization that allowed for the development of agriculture, there was much variation in climate and that likely made it very hard to find enough resources. Humans likely had to move great distances to find food, bringing them in conflict with other group competing for the same resources.Groups with more altruists and parochials would have had a greater chance of winning. This explains why both altruism and parochialism is common in humans.

Without individuals that are hostile towards outsiders and willing to sacrifice themselves for the group, a group is less likely to go to war and less likely to win a war.  Both hostility towards outsiders and altruism are crucial to winning a war.

In this kind of environment, groups with less parochial altruists then lose out to groups with more parochial altruists. Yet if the population of parochial altruists dwindle in all groups then there are fewer wars and greater gains from exchange. Hostility towards outsiders is necessary for war. However without the will to sacrifice yourself for the group hostility towards outsiders is not enough to have wars. People willing to sacrifice their life for the group are necessary for war.

Yes that is right altruism contributes to war.  Altruism, the virtuous trait of sacrificing yourself for others has a dark side.  Without altruism there might not be any wars.  Also war selects for altruism so without war there might not be any altruists.

Jung-Kyoo Choi and Samuel Bowles made an evolutionary game theory model to explore this, in the paper The Coevolution of Parochial Altruism and War.

Their simulation assumes people are either altruist or non-altruist and parochial or tolerant, making four types of people: parochial altruist, parochial non-altruist, tolerant non-altruist, and tolerant altruist.  

A parochial altruist will fight when they encounter another group. A parochial non-altruist will not risk their life fighting but will not cooperate with outsiders. Both tolerant types will try to trade and engage in mutual aid with the outsiders instead of fighting.

I can imagine what the different types would say when encountering another group:

Parochial altruist, “Let’s kill those outsiders”

Parochial Non-altruist, “After you,” or “Go get them!”

Tolerants, “Maybe we could trade with them rather than kill them.”

In the simulation, groups are paired up randomly. The more parochials there are among the groups the more chance the encounter will be hostile. If an encounter is peaceful the tolerants benefit from cooperation, but parochials do not benefit. If the encounter is hostile, and there is a large imbalance in the number of fighters (parochial altruists) in each group then there will be war.   The number of fighters on each side determines the probability of victory.  If there is war, some fighters on each side will be lost and the losers will lose non-combatants proportional to the power disparity. The lost members are repopulated by offspring of randomly chosen mates from the victorious group. This inter-group interaction advantages parochial altruists, since groups with more parochial altruists will more likely win wars and repopulate the losing group. Then a public goods game is played within each group.  Only the altruists contribute, but the benefit is larger than the contribution and is shared with everyone in the group equally, even non contributors. Then the individuals reproduce in proportion to their payoffs, with the next generation the same size as the last generation.  Then the parental generation dies off leaving the groups the same size as before.

You can download their simulation from their Artificial Histories web page. It is the one under The Coevolution of Parachial Altruism and War. You can play with the variables and see what has an effect on the outcome.

The more parochial altruists (signified by the gray line in the simulation graph)  the more war and the less parochial altruists the more peace and gains from exchange.

Population Frequencies (Blue for Altruists, Red for Parachials, Gray for Parochial Altruists)

Population Frequencies (Blue for Altruists, Red for Parachials, Gray for Parochial Altruists)

It is fun to play around with the simulation. You can change variables and see how you can cause war or peace to breakout.

In order for the parochial altruist strategy to be successful group size must be small, migration low, and exchange benefit low.  Also they found that egalitarian redistribution can help the altruist population, such that fewer wars are needed to select for parochial altruist.  This could explain why many hunter and gatherer tribes are egalitarian.  It helps slow down the selection process within the group so that the weeding out of altruists is slowed.

It is important to understand that, in the field of biology group selection is seen as an unlikely method for selection, because there would need to be great genetic differences between the ‘victorious tribes’ and the losers. Also wars would have to be frequent and have many fatalities.

However, it is likely that groups might have had encounters with groups significantly genetically different because human groups were small, allowing greater variation between groups.  Also, archaeological evidence indicates that wars were frequent and lethal in hunter and gatherer tribes. These conditions might allow group selection to work.  

This model helps explain how altruism and parochialism might have emerged in humans despite both altruism and parochialism being poor strategies for individuals.

For more details read the paper, watch the video lecture, and play with the simulation.  All are linked to in the references below.

Now, ever since the stabilizing of the climate around 11 thousand years ago and the advent of agriculture humans no longer live in a zero sum game.  Humans don’t have to just rely on what nature provides, and fight over it.  Humans can now produce much more than what nature provides.  Another human is more than just one more mouth in competition for limited food supply.  Other humans are potential partners in exchange gaining from the division of labor. Today benefits from exchange are many times more valuable than in paleolithic times.

So some of the conditions that selected for parochial altruists have been in decline for the last 10 thousand years.  That means parochialism and altruism might become less dominant. The good news is this could mean less war, but it also means institutions that rely on altruism for success may find their success waning.



The Coevolution of Parochial Altruism and War by Jung-Kyoo Choi and Samuel Bowles [Simulation]

Lecture, Altruism, Parochialism and War: Rambo meets Mother Teresa, by Samuel Bowles [Slides]