The authors distinguish between emotions that operate primarily in dyadic relationships and emotions that operate in a significant manner in collective contexts. The authors examine the evolutionary role each emotion and cite research about ways these emotions might contribute to the creation and maintenance of cooperative behaviors: "This chapter is premised on the claim that human cooperation is profoundly shaped by, and perhaps only possible because of, emotions. We will examine the manner in which different emotions shape behavior in cooperative contexts…Although framed within an evolutionary psychological perspective, our goal is not to present definitive evidence of the validity of this particular approach, but rather to spur future investigations of the role of emotions in cooperation. Toward that end, on an emotion-by-emotion basis we will both briefly describe a variety of existing findings and present a number of hypotheses, specifying discrete, testable predictions whenever possible."

Emotions that are primarily dyadic include romantic love, gratitude, anger, envy, jealousy, guilt righteousness and contempt. Romantic love is seen as a means of overcoming a barrier to the kind of cooperation we see in parenting -– the temptation to defect in the short term on a relationship that requires a long-term investment. "A number of investigators have suggested that some emotions can be understood as mechanisms design to commit people to behavior that yields long-term payoffs, thus overcoming the temptation for short-term defection. Romantic love, a universal human emotion that underpins pair bonding, appears to be such a mechanism."

Where romantic love is about how one feels about another person, gratitude addresses how one feels about somebody's behavior, and can be an emotional currency that binds one to reciprocity. "Gratitude focuses both attention and a positive, affiliative orientation on a party who has supplied the actor with a substantial benefit. In the context of its initial elicitation, gratitude seems to prompt the actor to recognize a valuable interaction partner and subsequently signal a willingness to reciprocate."

Why do people get so angry when someone cuts ahead of them in a queue or in traffic? This is clue to the evolutionary advantage of anger as a means of protecting ones own interests, but when it comes to the thus-far unexplained human propensity to punish cheaters, even at a cost to themselves, anger might be instrumental in conferring advantage to a group that requires monitoring and sanction of free riders in order to maintain a public good or create an institution for collective action: "If gratitude is elicited by receipt of a benefit, its opposite is anger, elicited by actual ar attempted exploitation or harm. More formally, anger is the response to the infliction of a cost. In addition to showing an "irrational" willingness to reward generosity, subjects in behavioral economics experiments also show an eagerness to punish uncooperative partners…Together, these results clearly demonstrate that even within the confines of finite anonymous games, angry individuals often place paramount importance on harming the transgressor, and are willing to incur substantial costs in order to do so."

Six Degrees begins in the beginning. Stanley Milgram's initial small world studies are analyzed. His findings in seeing if a group of people in Nebraska can get a letter to someone in Massachusetts are scrutinized. Milgram left a puzzle. Mathematically, six degrees of separation can be shown and intuitively it is appealing. But do social networks actually work that way?

Initially, Watts steps into the world of pure mathematic theory. Graph theory and random graphs are employed to build potential worlds in which connections can be made. These tools are detailed and their histories explained.

Watts and his colleagues then take the science to new levels, by introducing sociology, epidemiology, economics, and business models into this new multi-disciplinary science. Immediately, each new field of study brings with it new insights into network dynamics.

This convergence of disciplines reveals the social, transportation and technological networks that make up our world. These networks are, ultimately, made up of individuals. Individuals in turn relate back to the networks and define how they operate.

Socially, people relate to their network by clustering. Clusters are logical organizations of network elements. In a social context, we might cluster in terms of a religion, a favorite author, a school we are attending or an affinity for a type of food. Some of these have very close physical distance, while others have a social distance with members spread out over a large area.

Networks of this type are, to various extents, “scale-free” networks. If graphed these networks roughly follow a classic power law trend where the level of connectivity between two nodes in a network increases dramatically as more nodes are connected. Real-world scale-free networks tend to have highly connected hubs which rapidly, purposely, and efficiently transmit pertinent or pervasive content from one location to another. In social circles, these are networkers. In the airline network these are hub airports. In traffic they would be freeway interchanges.

Due to this architecture, the Internet and modern air transport have combined to greatly decrease the role of proximity in our social networks. This has had great impacts on commerce, tourism, cultural sensitivity and other social factors. However, it has also led to great risks in the transmission of diseases, sensitivity to distant economic fluctuations, and rapid spread of misinformation.

These dynamics create a type of network that Duncan calls simultaneously robust and vulnerable. Their strength and weakness is that, with rapid transmission from cluster to cluster, anything can move quickly from one location or group to another. He uses the example of Toyota, whose network of suppliers was organized in such a way as to quickly compensate for and recover from a potential economic catastrophe.

Stable scale-free networks do not rely on a rigid hierarchy to provide direction in times of crisis. Rather, the structure of the network itself can rapidly respond to an unforeseen situation.

Their network was arranged in such a way as to foster and reward communication. This communication helped cope with ambiguous or unplanned situations. Rather than paralyzing Toyota while people waited for a decision from a rigid hierarchy, the contractors in the network were able to analyze the calamity and provide a rapid response to it.

As mentioned above, this robustness also rapidly transmits malicious content as well. The Melissa Virus, SARS and Ebola are analyzed to show why the network did or did not transmit them and, when it did, how they eventually died out.

Watts ends this book by summarizing that the multidimensional nature of social distance is sometimes counterintuitive and subjective. People can feel close in a network sense to people they are physically distant from and, conversely, socially distant from people physically nearby.

He continues by warning that social and physical distances have shrunk. People can quickly travel from place to place and economies are highly interdependent. The sheer number of dependencies in the modern world may yield surprising results from seemingly insignificant actions.

He finishes by showing the stability of our networks with the example of how New York adapted to the 9-11 attacks. The City bounced back to semi-normal operations within a week. During the disaster, the best laid plans of emergency operations staff were scuttled by the utter unavailability of facilities and services designed to copy with disasters. The network will provide.

Humans have taken the cooperative arrangements that benefited organisms and species at the biological level to the cognitive and social levels: the capacity to play cooperative social games that benefit all was a driver of the evolution of human intellectual capacity; increased intellectual capacity manifested in both the concrete sphere of tool-making and the abstract sphere of social relationships. Once enhanced cognitive capabilities made complex social arrangements like status, reputation, gossip, persuasion, punishment, alliance possible, human social capacities became a tool for ratcheting up cooperative game-playing capacity.

Certain technologies push human societies to reorganize at a higher level of cooperation. As an example, Wright offered the Shoshone, a Native American tribe that lived in a territory with no big game to hunt but an abundance of jackrabbits at certain times of year. Because of their stark environment, the Shoshone normally existed at a simple level of social organization, with every extended family foraging for itself. When the rabbits were running, however, the families banded together into a larger, closely coordinated group, to wield a tool too large for any one family to handle or maintain — a huge net. Working together with the net, the entire Shoshone hunting group can capture more protein per person than they could working apart. Wright declared that "The invention of such technologies — technologies that facilitate or encourage non-zero-sum interaction — is a reliable feature of cultural evolution everywhere. New technologies create new chances for positive sums, And people maneuver to seize those sums, and social structure changes as a result."

Wright noted that people who interact with each other in mutually profitable ways are not always aware that they are cooperating; he cited evolutionary psychologists to assert that unconscious underpinnings of cooperation — like affection and indignation — are rooted in genetic traits:

"… natural selection, via the evolution of 'reciprocal altruism' has built into us various impulses which, however warm and mushy they may feel, are designed for the cool, practical purpose of bringing beneficial exchange."

"Among these impulses: generosity (if selective and sometimes wary); gratitude, and an attendant sense of obligation; a growing empathy for, and trust of, those who prove reliable reciprocators (also known as "friends"). These feelings, and the behaviors they fruitfully sponsor, are found in all cultures. And the reason, it appears, is that natural selection "recognized" non-zero-sum logic before people recognized it…Some degree of social structure is thus built into our genes."

"In the intimate context of hunter-gatherer life, moral indignation works well as an anti-cheating technology. It leads you to withhold generosity from past nonreciprocators, thus insulating yourself from future exploitation; and all the grumbling you and others do about these cheaters leads people in general to give them the cold shoulder, so chronic cheating becomes a tough way to make a living. But as societies grow more complex, so that people exchange goods and services with people they don't see on a regular basis (if at all), this sort of mano-a-mano indignation won't suffice; new anti-cheating technologies are needed. And, as we'll see, they have materialized again and again — via cultural, not genetic, evolution."

The cultural innovations that reorganize social interaction in light of new technologies are "social algorithms governing the uses of technology." Wright called these social methodologies "metatechnologies.". In the Middle Ages, the metatechnologies of capitalism — currency, banking, finance, insurance — pushed the hierarchical machinery of feudal society to transform into a new way of organizing social activity, the market. "The metatechnology of capitalism then combined currency and writing to unleash unprecedented social power." Wright claimed that the emerging merchant class pushed for democratic means of governance, not out of pure altruism, but in order to be free to buy and sell and make contracts. Throughout this process, powerful people always seek to protect and extend their power, but new technologies always create opportunities for power shifts, and at each stage from writing to Internet, more and more power decentralizes: "I mean that new information technologies in general — not just money and writing — very often decentralize power, and this fact is not graciously conceded by the powers that be. Hence a certain amount of history's turbulence, including some in the current era."

Network forms of organization, with reciprocal patterns of communication and exchange, are alternatives to hierarchically or market based governance structures; they are more suited to describing companies involved in an intricate latticework of collaborative ventures with other firms over extended periods of time.

Hierarchies are suited to transactions that involve uncertainty, recur frequently, and require substantial “transaction-specific investments”. Markets are suited to exchanges that are straightforward, non-repetitive, and require no transaction specific investments.

These “alliances” aim at creating indebtedness and reliance over the long haul: your current collaborator will be your competitor in other domains (or in the same domain) over time. In markets, the strategy is to drive the hardest possible bargain in the immediate exchange. Commitment is low.

Network organizations are more social than markets and hierarchies, they are dependent on relationships, mutual interests, and reputation. They are less guided by a formal structure of authority. Successful networks involve complementarity and accommodation. Reputation, friendship, interdependence, and altruism are integral. The most useful information comes from people you have dealt with in the past rather than from the formal chain of command.

Conflicts are resolved by haggling in markets; administrative fiats in hierarchies; norms of reciprocity and reputational concerns in networks.

Markets offer choice, flexibility, and opportunity. Prices determine production and exchange. Hierarchies are well-suited for mass production and distribution. Networks are more flexible than hierarchies. Transactions occur through networks of individuals engages in reciprocal, preferential, mutually supportive actions.

Reduction of uncertainty, fast access to information, reliability, and responsiveness are paramount concerns that motivate participants in network organizations.

Know-how, the demand for speed, and trust are critical components of successful network organizations.

Examples of network forms:

• Craft industries (construction, publishing, film and recording industries)—facilitated by loyalty to the profession and to project teams. Informal trading of proprietary expertise is common among members of a profession in different organizations.
• Regional Economies and Industrial Districts (German textiles, Silicon Valley)—Rich array of support services. Pooling resources on basic research through consortia and trade associations. Encouragement by local government. Proximity to centers of higher education.
• Strategic alliances and Partnerships (Oil and gas, chemical and pharmaceuticals, commercial aircraft)—share risk and expense. Cooperative relationships with suppliers. Gain fast access to new technologies or markets. Needs to deal with anti-trust concerns.
• Vertical disaggregation. (Downsizing, outsourcing)—Increased flexibility in responding to technological change and commodifcation of products.

Know-how, the demand for speed, and trust are critical components of successful network organizations.

• Know-how and detailed knowledge of the abilities of others who possess similar or complementary skills thrive in networks. Exchange of competencies is more likely to occur in networks; exchange of tangible resources is more likely to occur in market transactions or among units in a hierarchy.
• Demand for speed is facilitated by a network’s strengths in offering fast access to information, flexibility, and responsiveness to changing tastes. Information flow through a network is freer and richer than in more tightly controlled markets or hierarchical organizations.
• Trust develops when there is a high probability of future association. There is a higher probability of cooperation and also a willingness to punish those who do not cooperate.

The theory that ancestral hominids were hunters has achieved considerable popularity. The authors note that there is considerable evidence that ancestral hominids were more likely a prey species. There are three major lines of evidence:

• Modern humans are occasionally prey species for certain predators, especially for tigers in parts of modern India and Bangladesh … people wear masks on the backs of their heads in order to appear alert to potential stalkers.
• Modern apes and monkeys are frequent prey species for a wide variety of carnivores, a fact which does not appear in publications about the animals themselves but which is widely noted in works covering the diet of the predators themselves.
• Ancestral hominid fossils show the marks of various predators – tooth, claw, and talon marks – that are consistent with predation.

The authors demonstrate the prey-nature of early hominid ancestors. Their speculations as to what this means in evolutionary and behavioral terms, however, are weaker. This is because it is a long way (in both time and, potentially, place) from Australopithicus afarensis to H. sapiens … the use of tools and fire, for instance, may well have modified behavior inasmuch as the immediate ancestors of H. sapiens became a more formidable prey species.

The question as to what behaviors are derived from those ancestors which were commonly prey animals remains open for speculation and future work. That said, the authors present a challenge to those who maintain the view that humankind’s ancestors were hunters from the earliest times.

Evolutionary psychology has been portrayed as justifying or implying a lot of bad ideas in the 20th century, but it need not suffer from these mistaken linkages and can potentially shed light on how to build better social institutions. Although the claim has been made, evolutionary psychology is not consistent with the tenets of Social Darwinism. Whether a trait or behavior survives the process of natural or cultural selection has no bearing over our discourse on whether it is morally justified. Nor does it mean that we are determined like machines to act out these behaviors in every case, a theory termed 'behavioral determinism' by those criticizing evolutionary psychology or its earlier form, sociobiology. Any reputable biologist, or sociobiologist, would acknowledge that the fitness of a behavioral trait is dependent on the interaction between that trait and a given environment, so saying that a certain psychological predisposition in humans is the product of an evolutionary process does not mean that it is good, justifiable or useful in the world we live in. Evolutionary science stresses that fitness is fundamentally contingent. Furthermore, humans have a cultural inheritance that dictates in subtle ways how and when we should express or repress our behavioral traits, making the interaction between trait and environment even more complex. Evolutionary psychologists suggest that we pay close attention to the basic human behaviors that through cross-cultural analysis appear 'hard-wired', because it is these behaviors, such as sympathy for those in pain or identification with one's kin or tribe, that we want to either channel or suppress in order to reap the benefits of cooperation.

This article isolates four mechanisms that promote cooperation in the absence of a central authority: kin selection, cooperation for mutual advantage, reciprocal altruism, and group selection. Kin selection implies a kind of utilitarian genetic calculus, that sacrificing one's life for the right number of relatives will be favorable for one's genes. A sibling shares on average half of one's genes, so sacrificing one's life for two or more siblings makes evolutionary sense. An example of a behavior that might be explained by kin selection is the warning call of ground squirrels; a ground squirrel that notices a hawk circling will call out to warn its family, although it increases its likelihood of being noticed and eaten by the hawk. This form of cooperation requires enough brain or nose power to be able to determine who is a relative.

The second form, cooperation for mutual advantage, occurs when a particular given end (critical for survival) is easier to accomplish with a group working together. The quintessential example of this mechanism is group hunting; wolves (and our hunter-gatherer ancestors) hunt in packs because they will end up with a portion of the large game, which can be much larger than the small game they would be able to catch on their own and not have to share. This benefits of this mechanism is not as immediate or certain as those of kin selection, because the stronger hunters could potentially share nothing with the weak who helped. This article cites field studies of monkeys, lions, and fish, which show that group hunting generally only occurs when environmental conditions make it economically more efficient that hunting alone. While cooperation for mutual advantage is an important surplus-generating mechanism in nature, we should not expect this mechanism to form the basis of modern human cooperation. Modern human cooperation cannot be pared down to a single one-shot end, and it could be argued the developments of civilization we are most proud of, charity for the poor or sick, go against the logic of mutual advantage.

Reciprocal altruism looks similar to the mechanism of mutual advantage, except the benefits are spread over time rather than through a single interaction. One individual helps another individual with the expectation that in the future the gesture will be repaid. Reciprocal altruism works best when developed alongside "a large number of supplementary psychological and social institutions." Enduring reputation and social traditions such as gift-giving foster relationships of reciprocal altruism. This kind of a relationship requires a bigger brain to remember who gave you what and who has mooched off you for too long, but can generate a big societal payoff. "By allowing trade over a period of time, reciprocal altruism opens up the possibility of a division of labor and credit-based relationships. These innovations make possible the recognition of the gains from specialization, comparative advantage, and the insurance and risk-shifting elements of inter-temporal trade."

While reciprocal altruism is most compelling in small groups with face-to-face interaction, the final mechanism, group selection, treats populations as the unit of measure. Proponents of group selection argue that a population of individuals with altruistic traits would fare better than less altruistic populations, reaching the big payoffs described in the above paragraph. The traits in question could be genetically inherited or culturally inherited. Arguing for cultural group selection, "[g]roups that adopt 'better' cultural practices will again tend to grow healthier, wealthier, and more populous, gradually supplanting less efficient cultures through conquest, migration, or conscious adoption." This kind of cooperation requires even more specific conditions than the other three mechanisms. Because the scale of group selection is so much larger than the other mechanisms, it is still a controversial theory in natural and social sciences. The argument against cultural and biological group selection is based on problem of free riders without altruistic traits who might take advantage of the social surplus generated by their altruistic neighbors. While human populations have reached impressive levels of cooperation in modern societies, one can imagine natural disasters or devastating world wars that would eliminate the evolutionary strength of group selection.