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."

Social psychological research has a long tradition of interest in cooperative behavior and commons dilemmas, beginning with Von Neumann and Morgenstern's 1944 book, Theory of Games and Economic Behavior. It is convenient to summarize the way psychological studies have attacked the commons problem through nine variables (social motives, gender, payoff structure, uncertainty, power, status, group size, communication, causes, frames) divided into three groups (individual differences, situational factors of the task structure, and the perceived effects of situational factors.) Although behavior elicited in a controlled lab environment is never the same as that observed in field research, lab research is an indispensable tool for teasing out causal relations from the larger number of interacting influences. Attention to the findings of psychological research relevant to the specific instance of a commons dilemma can make the difference in generating positive collective action.

Studies on social motives have found correlations between motives and choice behavior and interpretation of others' behavior. Liebrand et al. (1986) demonstrated that "people with individualist social motives tend to interpret behavior along the might dimension (what works), whereas cooperators tend to view cooperation and competition as varying on the moral dimension (what is good or bad)." Less intuitive findings have come out of research into social rewards for cooperative behavior. Gachter and Fehr (1999) conducted a study around public goods dilemmas with an anonymous group, a group that met beforehand to establish a group identity, a group that had a chance to interact after playing, and a group that met before and after. They found that neither the second nor the third groups had significant improvements in cooperation, but that the fourth option resulted in "significantly higher levels of contribution."

Uncertainty of resource size may play a detrimental role in commons dilemmas for multiple reasons. Any factor which threatens to bring an end to the resource will decrease interest in one's reputation and therefore with the relations that support the resource's maintenance. Uncertainty also helps diffuse personal responsibility, since overusers can try to justify their actions through their ignorance of the current resource size.

Experimenters have also uncovered interesting effects from varying groups size on self-efficacy, an individual's sense of their own competence in taking effective action. Smaller groups were found by Kerr (1989) to have higher averages when testing "collective" efficacy, the sense that the group could carry out effective actions to achieve a desired outcome. In groups with a relatively low provision point, the percentage of cooperating members necessary to support the public good, small group size was associated with a sense of "collective" efficacy.

Voting can significantly improve efficiencies in commons dilemmas by acting as a form of communication. Collective learning of general information "extends to subsequent situations and enables people to coordinate their activities even in rounds when no proposals are made."

The origins of deindividuation are found In LeBon’s crowd theory (1895/1995), who proposed that the psychological mechanisms of anonymity, suggestibility and contagion transform an assembly into a "psychological crowd." In the 1950’s it was argued that deindividuation occurs when individuals in a group are not paid attention to as individuals and that additional contextual factors (e.g. reductions of responsibility, arousal, sensory overload, etc.) played a part. In the 1970’s deindividuation theory became a popular focus of scientific research, however, the empirical support for deindividuation theory was weak. In fact there was virtually no evidence for the psychological state of deindividuation. In the 1980’s formulations of the theory focused on the psychological process of reduced (private) self-awareness as the defining feature of deindividuation.

Various studies have been conducted to empirically verify the theory, but evidence for deindividuation theory appears to be mixed. A recent meta-analysis study examined 60 tests of deindividuation theory and concluded that there is insufficient support for deindividuation. Disinhibition and anti-normative behavior are not more common in large groups and crowded anonymous settings. Moreover, there is no evidence that deindividuation is associated with reduced self-awareness, or even that reduced self-awareness increases disinhibition. Overall, then, deindividuation theory does not receive sufficient empirical support. More recent research suggests that groups are sensitive to normative cues associated with the social context. Thus, whereas deindividuation theory argues that the crowd causes a loss of identity, reverting the individual to irrationality, it seems more productive to reconceptualize deindividuation as a shift from a personal identity to a social identity, shared by members of the crowd.

The idea that behavior could be the result of local group norms was considered explicitly by Johnson and Downing (1979). Participants were made anonymous by means of mask and overalls reminiscent of the Ku Klux Klan or by means of nurses' uniforms. Although compared to the control condition, participants shocked somewhat more when dressed in the Ku Klux Klan uniforms, they actually shocked less when dressed as nurses. This finding illustrates that groups are sensitive to normative cues associated with the social context.

This latest finding lends weight to the idea that group behavior is closely tied to cultural identification. Further study in which participants are deliberately selected from specific cultural types might prove interesting.

Human babies are capable of synchronizing their movements with that of others at a very early age, indicating that the neural capacity for synchrony must have occurred during gestation: "tightly synchronized interaction with others constitutes part of the maturational environment for the cerebral cortex." Our closest primate relatives can neither synchronize with others nor hold a steady beat. Ritual music and dance appear to trigger brain mechanisms that foster social bonding and so have been essential to creating the trust upon which all social interaction depends. Separate individuals who engage in mutual music-making set up neurophysiological processes that could serve as substrates for a tight coupling between multiple individuals.

Protohumans, through restructuring their internal representations of each other in the process of making music behavior, may have adapted neural circuits that evolved for other purposes into social instrumentation. The neurochemical processes associated with emotions, which may play a role in the trust-building and intention-signaling behaviors essential to social cooperation, are evoked and harnessed through rhythm and music-making rituals: "We are social creatures, we depend on our fellows. When we express emotion we are signaling something about our interior milieu. We assume that others will pick up the signal and respond accordingly. Similarly, when we pick up on the emotions of another, our nervous system will bring out changes in our interior milieu."

The brain's oscillatory circuits called central pattern generators can be internally synchronized through sonic activity; Benzon proposes that mutual internal synchronization, coordinated through rhythm and harmony, creates a literal inter-brain coupling which can then be adapted for complex social coordination: "Music thus becomes a means of communal play, of communal dreaming. It is a group activity in which interactions between individuals are as precisely timed and orchestrated as those within a single brain. The individuals are physically separate but temporally integrated. It is one music, one dance." Benzon proposes that physical mimicry of animal cries could have led protohumans to making music together, which later differentiated into spoken language.

A survey by Alan Lomax and others of the music of 233 cultures from five continents and the Pacific islands found consistent correlations with other measures of social structure and economic practice: "…a culture's favored song style reflects and reinforces the kind of behavior essential to its main subsistence efforts and to its central and controlling social institutions."

"…ritual creates a cultural space where social innovation takes place. During periods when a society is under no particular stress, ritual serves to confirm and maintain the existing order. But when the society comes under duress, ritual allows new social mechanisms to emerge. As societies grow and their structure differentiates, musicking continues to play the role it had in humankind's beginning: the forge in which new forms of social being emerge."