1. Cooperation Reciprocators, Cheaters, and Everyone Else

2. Who Can You Trust? DeBruine (2002)  Two-person sequential trust game  Manipulated resemblance of “other player” by morphing with participant’s face  Participants trusted similar faces over dissimilar ones

3. Extending the Problem of Altruism While Hamilton’s rule explains kin-selected altruism, it does not explain cooperation between nonkin How can altruistic behaviour between nonkin be explained in light of evolutionary theory? The work of Alexander, Hamilton, and Trivers provides us with an answer: reciprocal altruism

4. Reciprocal Altruism Providing a service at a cost may be selected for if there are greater reciprocated benefits in the future Cooperation between two individuals for mutual – although not always immediate – benefit Synonymous with social exchange and cooperation

5. Iterative Prisoner’s Dilemma (1) Two strategies: cooperate or defect Features of the game  Simple representation of real situations  Intelligent plays should be self-evident  Changes significantly over repeated plays

6. Numbers represent length of jail sentence R = 2 Cooperate Defect CooperateDefect S = 10 T = 0P = 5 Iterative Prisoner’s Dilemma (2)

7. Tit for Tat Alexander ran a computerized round-robin “tournament” of competing strategies in iterated Prisoner’s Dilemma Tit for Tat strategy was the most successful  Never defect first  Retaliate only after the other player defected  Be “forgiving”

8. Examples of Reciprocity in Nature Vampire bats  Regurgitate blood nonrandomly: for “friends” Baboons & aid during sexual contests Vervet monkeys & allogrooming Common chimpanzees  Alliances formed for status (with sexual benefits) after a “political” fashion

9. Social Contract Theory Fundamental capacities of negotiating & maintaining social contracts (cheater detection)  Recognition of individual humans  Historical memory of interactions  Communication of values  Model values of others  Representation of costs and benefits

10. Cheater Detection (1) Wason reasoning task  Letter on one side, number on other  If there is a “K” on one side, there is a “2” on the other KR20

11. Wason reasoning task  Beer or Coke on one side, age on other  If drinking beer, they must be over 19 years old Cheater Detection (2) 19Beer16Coke

12. Correct answers KR20 Cheater Detection (3) 19Beer16Coke

13. The case of R.M.  Bilateral damage to medial orbitofrontal cortex & anterior temporal cortex  Left & right amygdala disconnected  Performed significantly worse on social contract problems than “precaution” problems  Controls showed no such deficit Cheater Detection (4)

14. The Wrap-Up Kin selection  Based on Hamilton’s rule  Increases inclusive fitness Reciprocal altruism  Cost incurred for greater benefit in future Prisoner’s Dilemma & Tit for Tat Social contracts & cheater detection

15. Things to Come Aggression  Adaptationist perspective  Sex differences  Patterns of aggression  Context effects  Warfare