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Summary of The Pleistocene Social Contract

Book By Kim Sterelny

Book Summary by Haggeo

I have a book review forthcoming in the quarterly review of biology. However, that is only about 500 words long. To write it, I had to summarize all the sections of the book in my own words and then decide what to include. That larger summary is below. I hope you enjoy and I recommend you read Sterelny's book.

Chapter 1. The Pleistocene Social Contract

1 Building Cumulative Culture

1.1 Methodological Preliminaries

The chapter explains that the book will be about the “, elaboration and interaction of two very distinctive features of our lineage: our dependence on cooperation and our dependence on culture…” (1). It is an explanation starting with hominems (6 mya= million years ago). He considers the concern that this type of work is a “just so story,” but he dismisses it because the account makes predictions and contact with existing data and theory.

He then flags three controversial claims of the book: 1. Our ancestors were cooperative hunters since long ago (1.8mya). 2. About 100-150 kya, cooperation changed from collective action to reciprocation. 3. Intergroup violence did not play an important role in human history until about 12kya.

Sterelny then says that the argument of the book depends on the idea of cognitive plasticity–the idea that we can acquire new skills and repurpose cognitive circuits. This leads to changes in behavior, which changes the relevant forces of selection, which can then lead to changes in biology, and this process is reiterated. Sterelny says that he develops from here two novel claims, one about cooperation and another about culture, but he doesn’t say what he means.

Next he introduces the four phase model for the book: 1. Suppression of dominance hierarchy. 2. Foraging cooperation changed from immediate return mutualism to direct and indirect reciprocation. 3. Residential groups became more networked, hence becoming larger. 4. The reestablishment of hierarchies. Sterelny believes that norms, mores, and collective action problems led to the fourth phase.

Sterelny believes, with the majority of theorists, that culture is cumulative. But he rejects the need for a special cognitive adaptation for high fidelity learning. This is because longer juvenile periods provide ample time for learning. Next, Sterelny makes a distinction between a cultural item that has been incrementally improved and an item whose knowledge of it is outside one’s zone of latent solutions (ZLS). Something is inside one’s ZLS if one can learn about it via individual learning.

Cumulative cultural information allows one to be in a total state of knowledge that is outside of one’s ZLS even if each individual item is within the ZLS. And this means that we don’t need to focus exclusively on incremental improvement.

1.2 Culture and Cooperation

The generation of benefit problem is the challenge to explain the benefits of a particular form of cooperation and how that cooperation could have evolved. The distribution of benefit problems concerns how to stabilize cooperation from the threat of free riding–how to ensure cooperative benefits are distributed so that further cooperation is incentivized.

Sterelny notes that while there is disagreement on details, there is consensus on the idea that by mid-Pleistocene, hominins have come to rely on cooperation, expertise, and technology. This is because hominin lifeways depended on the capture of rich but difficult to capture resources. Examples include using tools to find deep underground storage organs of plants, cooperative hunting, or taking prey from predators. These practices involve tools, cooperation, and culturally preserved and transmitted knowledge. Cultural learning and extraction of difficult resources would have positive feedback loops with life history. Capturing difficult resources can support a longer life. A longer life can support longer time to engage in cultural learning.

1.3 The Prehistory of an Unusual Ape

Sterelny begins with a chart of epochs. The miocene is from 23 to 5.3 mya. The pliocene is from 5.33 to 2.58 mya. The pleistocene is from 2.58mya to 11,700ya. The holocene is 11,700 ya to present. There is a caveat in that the further back we go, especially with the miocene, pliocene, and into the mid-pleistocene, much is not much known, due to missing evidence.

The standard story is that encephalization is underway by 2 mya with homo erectus. The erectines took off within 200k years: expanding through africa and eurasia, asia, and later into europe. Fire was domesticated around 800 kya and some basic tools around 1 mya. A common phenomena in research is that tools and lifeways are discovered earlier than believed. For example, many researchers dated various tools and non-utilitarian symbols to about 50 kya, only to find them around 250 kya.

Sterelny draws out three points from his brief overview of hominin history. First, behavioral and morphological divergence has been rapid and spectacular. Second, the divergence is not even. Third, morphological and behavioral changes are not correlated. For example, until about 250k years ago, new technologies seem to require 1 million years. Also, technologies do not correlate with hominin species. New hominin does not entail new technologies. Moreover, marked changes in encephalization do not correlate with marked changes in behavior or tools. The last 100 thousand years have seen remarkable innovation with little change in encephalization.

1.4 The Growing Footprint of Cultural Learning

One view holds that cultural learning depends on genetic adaptations for cultural learning. Another view holds that cultural learning depends on cultural adaptations for cultural learning. Sterelny focuses on the latter view, though he emphasizes that humans structure the learning environment.

One might wonder how cultural learning can take off without specific adaptations for it. Sterelny’s answer is that individual learning enhances social-cultural learning. Such features include executive control, working memory, and causal reasoning. And hominins were probably good individual learners since they relied on rich but difficult to capture resources.

Some of the benefits of social learning are that it reduces the search space, substitutes for the world’s signals of error, makes experimentation safer, and distributes the cost of error. Sterelny postulates that as juveniles spent more time with adults, and as adults became more socially tolerant, juveniles had more time to learn from the activities of adults. This creates feedback loops between life span, social tolerance, and difficult resource acquisition. Sterelny believes that early hominins are like chimps who only have females migrating. So, the spread of information and innovation is limited at this point. These points suggest that early hominin life is gradually becoming more culturally dependent but that innovation is still slow.

1.5 Cumulative Cultural Learning

Sterelny notes that one way for culture to be cumulative is by small, incremental improvements on a technology, but he stresses that this is just one way. We saw earlier that there are ZLS. An agent might acquire accumulated cultural knowledge by learning various things that are not possible to learn individually, even if each item is not the result of small, incremental improvements.

Sterelny then notes a few ways in which cumulative culture is important and how it is socially and cognitively scaffolded: 1. Dangerous cases of trial and error learning (mushrooms, sea navigation). 2. Cases where error is not informative (making the right clothing). 3. Cases where practices are causally opaque. 4. Cases where bandwidth is very high (foraging knowledge). 5. Cases where social norms and rituals must be learned.

There is evidence of hominin ambush hunting from as early as 1.8mya. These practices imply coordination and cooperation, both physical and informational. Members had to learn where and how to hunt, how to source, develop, and use tools.

1.6 Adapted Minds, Adapted Environments

Sterelny thinks that cumulative social learning depended on various factors. By encephalization, hominins could begin to enrich their cognitive potential. With longer life spans, juveniles have more time to learn. And this improves survival. Cognitive architectures like language and theory of mind help. With theory of mind, one can be a better teacher. Culture develops things that serve as cognitive scaffolds. Concepts help alert us to unobvious differences. Culture develops stories to code useful information.

Environments are adapted to aid learning. This might begin via leakage of adult activity and conversation. This can be done by giving novices less difficult activities. Population size matters. Size means specialization can be supported. Specialists are more likely to innovate. Larger community sizes means collective defense, and in turn this means information sharing. Information sharing increases redundancy and reduces risks with errors in sender-receiver signals.

1.7 Overview

The early picture of hominin life is one increasingly dependent on cumulative culture, but it is one where technological innovation is static and shaky. Innovation takes long periods. They often disappear and reappear thousands of years later. The next chapters present the resources of stability and new sources of challenge to that stability.

2. The Pleistocene Social Contract
2.1 Free riders and Bullies

Cooperation is profitable, but there is always temptation to cheat, as a free rider and as a bully. Bullying is more destabilizing. Free riding means others pick up the tab, but the return may still be profitable. Bullying means one can monopolize the rewards. All chimp species engage in dominant structures that are not open to cooperation. So Sterelny thinks that an explanation of hominin cooperation is in order. Moreover, he thinks that mechanisms stabilizing cooperation are different when we take into account size. Small societies require different mechanisms than do large ones.

2.2 Curbing Dominance Hierarchies

On the assumption that non-cooperation is a viable alternative, cooperation needs to be explained. Sterelny notes an early form of cooperation is one where profits are immediately and equally distributed. Collective defense is like this. Collective hunting is like this too. Everyone who participated will be there. Anyone who takes more than a fair share will be seen by everyone, and everyone is interested in sanctioning this cheater. Recuiting third party support to sanction cheaters is not needed.

So Sterelny believes that an early form of cooperation was established based on immediate return mutualism. All that is required is anger and resentment towards cheaters. The use of weapons, in out bursts of anger, can mean that even the alpha are in danger. At this point, one need not appeal to norms to stabilize cooperation.

2.3 An Economy of Reciprocation

Sterelny believes that indirect reciprocation became important in early hominin lives. Hunting does not always work, so we should have fallback resources. Perhaps early hunters only hunted in bloom, when females were sure to gather alternative resources.

In the next stage of hominin evolution, hominins greatly expanded their resources. They depended more on river and sea resources. With the projectile revolution, they could catch more. Small hunting parties became more effective than large scale ones. All of this is economical. One is better served by diversifying.

While more economical, this stage requires indirect reciprocation, and this brings with it unique challenges. First, different individuals or groups will have different rates of success. The question emerges on how to treat the less successful. Second, resources have a degree of incommensurability. How should one resource be traded for another? How about when everyone had to take some of yours today because they were unsuccessful? Third, information about contribution is no longer public. So it is harder to track who is free riding and who is contributing. Fourth, when one feels cheated, one has to pay the price of doing something about it. And, who is to blame, since reciprocation is indirect? Fifth, resources deplete at different rates. This means that different groups of society are ready to move on at different rates. Some foraging societies have groups leave for long periods of time. This opens problems with sexual fidelity, and it means face to face interactions are gone.

2.4 Making Reciprocation Work: Gossip

Since early foragers were dealing with actions displaced in space and time, gossip comes into play by keeping track of people’s reputations. In turn, reputation matters a lot in these earlier societies. Material capital is not as prevalent in mobile societies, so social capital becomes that much more important. There is informational capital, but that can also be tied to social capital. You get to know things if others approve of you.

2.5 Making Reciprocation Work: Norms

Explicit, social norms solve the problem of ambiguity. Social norms can tell everyone the details of how to share. Another problem explicit norms solve is third party motivation. If people internalize norms, then they will be motivated to sanction norm violators. Sterelny thinks we will, in this view, end up with prosocial norms, not for selection pressures, but rather because human cognitive-motivations are pro-social (a potential circular argument?).

How are norms internalized? Why are human minds evolved to internalize norms? Sterelny considers two answers. One answer is that internalizing a norm is the best way to showcase one’s good reputation. The best way to appear trustworthy is to be trustworthy. A second answer is that norm internalization begins in skills. Skills meet certain standards, and one has to embody the standards. The thought is that norm internalization evolved from pride in performance. Sterelny suggests that hunting, since it is both a skill and a social level interaction, is a bridge between the two domains.

2.6 Making Reciprocation Work: Ritual

Rituals have experiential effects. These serve to reinforce and legitimize norms. Often mythical stories emerge to support ritual. Increases in ritual suggest an increase in norm and social stress. Through various rites of initiation, music, dance, mind altering substances, and so forth, ritual helps promote group identity.

3 Cooperation in a Larger World
3.1 Cooperation Between Bands

Sterelny begins by noting some contrasts between foraging societies and chimp societies. First, foraging societies are open to movement. Second, bands are part of larger wholes. Third, kinship systems are more complex, diverse, and open. Kinship across bands is a reason for trust, and cooperation.

Bands come together to share information, to facilitate marriages, strengthen ties, resolve conflicts, engage in trading. Sterelny sees the complexity of large scale projects in hunting and resource extraction and concludes that these had to come late, as they depend on rich collaboration and accumulated ecological knowledge. Sterelny thinks that this all requires extensive and elaborate solutions to coordination. Once large communities can form, then so can intricate forms of coordination, including, unfortunately, war.

3.2 The Origins of an Open Society

Sterelny believes that as foraging bands grew, and as they became dependent on capturing prey, they began to splinter. The ranges of predators are large. However, it remained beneficial for large bands to occasionally come together, for collective defense, informational cooperation, access to different mates, and so forth. At this point, it is not necessary for collectives to see themselves as the same people. The collaborations are ad hoc. When chimp societies split, they become hostile. Sterelny believes this didn’t happen to hominin life because of the development of kinship systems in conjunction with selection pressures against aggression or selection for prosocial motivations. If so, this implies that the roots of open society are early–coinciding with hunting and the origins of prosocial motivations.

With development of kinship systems (and practices and rituals that support it), we can begin to see pathways into hostile land. Early excursions into hostile land required a fallback community–one that can accept you as their own. Migration into hostile lands was probably slow with many returns home without success.

3.3 Cooperation Culture and Conflict

A common account of group cooperation begins with the above observation that chimp societies are hostile. From here, it is thought that groups that were better able to coordinate for war were better suited to survive. And this applies to the cultural practices and norms that come with cooperation.

Sterelnly raises some objections to this view. First, it makes the origins of multi-levels society mysterious. Second, the territories of hominins are much larger territory than those of chimps. This lowered the probability of conflict, and it made patrolling territory harder. Fourth, chimps attack when the odds are 4-1. With weapons, the one person may still be doomed, but the costs inflicted on the four may still be grave. Finally, the conflict view ignores the benefits of peace.

Next, data does not show that Pleistocene foragers were not resource stressed, as one would expect during war or as a motivation for war. There is no evidence of war, either in bones found or in military tools. Evidence of war begins to emerge at the end of the Pleistocene when sedentary lifestyles were emerging. Sterelny concludes that intergroup conflict was not a significant selection factor shaping the evolution of early hominins.

3.4 Individual Selection, Group Selection and Cultural Group Selection

This section talks about group selection. Some evolutionary theorists have been skeptical about group selection, as they think that the individual is the main unit of selection. A group of horses is fast because the individual horses are fast. Altruism was seen as a way of bringing in group selection. An altruistic individual helps the group, but the main challenge was mixed groups. Groups are composed of selfish and altruistic individuals.

Next, Sterelny notes that with the introduction of cultural learning, group selection is once again a viable option. But he notes that, based on his above argument, group selection is not required to explain the origins or stability of cooperation in the pleistocene. However, he thinks that conflict becomes important in the holocene. (In this part of the argument, Sterelny confuses group selection with group conflict).

4 Cooperation in Hierarchical Communities
4.1 The Puzzle of Farming

The puzzle is to explain farming. Life became sedentary. Wild resources were traded for domesticated ones. Farming life isn’t always that great. Foragers seemed to enjoy plenty of resources with ample leisure time. Moreover, leadership in early sedentary societies was not hereditary, nor was it enforced by formal institutions. Elites live close to non-elites, so physical take over is always a possibility. As stored goods increased, communities had to invest heavily in storage facilities and defense.

Sterelny believes farming evolved from storage based foraging. Storage based foraging evolved when there are predictable resources, variation among them, and seasons of abundance. This means that storage based foragers are semi-sedentary, they have technologies for processing and storing food, adapted to year long cycles, and are exposed to some of the social risks associated with farming. They are beginning to change risk management strategies from mobility to storage. This gives them little opportunity costs in investing in the land around them. And this begins to introduce inequality, as some are more successful than others.

Storage foraging decreases scarcity, which supports population growth. More sedentary lifestyles support fertility, as the costs of moving young around reduce, and this supports population growth. In turn, population growth supports farming labor. Storage foraging begins to erode norms of sharing, as foraging is less susceptible to luck and more dependent on skill. Storage and farming promote concepts of property rights. This then supports the concept of inheritance, as children won’t want to work land that they can’t inherit, and parents prefer to leave something to their children. Conflict flashpoints are created. There will be blurry lines to property. Resources will be tempting things to steal. As a result, investing in defense will be profitable.

Some of the last steps are that farming and storage foraging with property rights will often be the most productive places, so mobile foraging will become less rewarding. It will be wise to produce more than necessary, and the surplus can be used for social influence. The question that arises is why do the less well off consent to the monopolies of the elites?

4.2 Cooperation in an Unequal World

Sterelny notes that for the last 10 thousand years, inequality has been rampant throughout the world. Yet it persists. He thinks that this is puzzling because cooperation breaks down in the face of cheating. (What he does not make clear, however, is why inequality is a form of cheating).

Egalitarianism is policed in foraging societies, and it is enforced by the threat of non-cooperation. Egalitarianism erodes in sedentary societies. First, the natural lottery ensures some will do better than others. With the concept of property rights and inheritance, inequality will only be exacerbated. Second, a sedentary lifestyle brings conflict flashpoints, so defense will be important. Those who have a lot can use it as political gain and hence make power moves. They will invest in costly buildings and feasts to showcase their wealth. The elites will position themselves as points of contact between clans. Third, the shape of society has changed. Before, everyone knew each other and were in proximity. In large sedentary societies, ties are less intimate, and there is more distance between people. It becomes harder to mobilize against elites. Fifth, farming allows for the exploitation of slaves, who once marked cannot ascend the social hierarchy. Sixth, a middle class is unlikely to be motivated to join with the lower ranks to dispose of elites. The middle class enjoys moderate wealth and risking it may not seem reasonable.

4.3 Religion, Ritual, and Ideology

Sterelny begins with the observation that prestige or expert hierarchies form. People with talent or knowledge are paid for that knowledge with deference. At times, this came to include religious and esoteric knowledge. The wealthy are able to commandeer these inequalities and norms of deference for their favor.

4.4 Conflict, Hierarchy and Inequality

An account of inequality should explain why it is a robust mostly Holocene phenomena. Sterelny thinks his account fits in well here. Surplus farming was probably only available during Holocene climates. Clan like hierarchies, prestige hierarchies, and kinship systems had to precede elites for them to take advantage of these cultural adaptations.

Sterelny is skeptical about the efficiency of group selection. First, cultures are homogeneous, so group selection cannot effectively work on individual norms. Second, Sterelny notes that he has argued that selection can work at the individual level, altering the individual’s payoffs and costs. The first reason is puzzling to me. First, genetic selection does not work on individual genes but on organisms, so by analogy, Sterelny should be skeptical of genetic selection. Moreover, populations aren’t entirely homogeneous, so sub-cultures can compete within a culture. Second, he still thinks of selection as conflict, but some norms can be more beneficial than others.


Sterelny K. (2021). The pleistocene social contract : culture and cooperation in human evolution. Oxford University Press.


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