> COOPERATION THROUGH REWARDS AND PUNISHMENT

Punishment seems important in human culture and evolutionary history. When Darwin began considering the evolution of morality, he reflected on a possible role for the "fear of others acting in unison" and "the fear of punishment" (M Notebook, p. 151; 1871, p. 92). However, punitive behavior cannot be assumed. It costs extra effort or resources. Humans, nonetheless, accept personal cost to ensure group benefits in anonymous experimental situations. Moreover, others respond to their punitive actions (Fehr and Gächter 2002). A norm of cooperation can be learned and enforced through punishment.

Punishment of selfish behavior seems present in all human cultures. They include not only different nations on different continents and Oceania, but also cultures with widely divergent environments, economies (from foraging and pastoralism to industrialism) and residence patterns (from nomadic to sedentary) (Heinrich et al 2006; Herrmann, Thoni and Gächter 2008). Most important, perhaps, negative sanctions are found in small mobile hunter-gatherer cultures — similar to our Paleolithic ancestors — where they help maintain egalitarian societies (Boehm 1999).

Selective interaction and punishment may combine. When given the option, human subjects prefer to join groups functioning cooperatively through sanctions over groups where they are "free" to be selfish but can reap only limited benefits (Güreck et al 2006). Cooperative groups are thus not necessarily at a relative disadvantage. Indeed, mathematical models indicate that being able to choose between such groups (or not join either) may have been critical to the origin of punishment-based cooperation (Hauert et al 2007). As suggested by Hardin (1968), the tragedy of the commons may be solved by "mutual coercion, mutually agreed upon." Again, social-level dynamics can affect how individuals act.