Human Social Dilemmas Cooperation Between Non-Relatives Complex Evolutionary Problem Repeated Interaction, Conditional Cooperation Human Cooperation Often Non-Kin

Human Cooperation: Model Prisoner’s Dilemma (PD): Cooperate or Defect Cooperation Benefits Other Player; May Cost Cooperator Single Play: Pure Defect is ESS

Human Cooperation: Model Kümmerli et al. (2007) Proc. Royal Society B 274:2965 – PD: Cooperation always can be exploited (P > S) Consider Snowdrift Game (SD) Always Some Benefit to Cooperating

PD and SD Models 2-Player, Symmetric Games Single Play: Cooperate (C) or Defect (D) b Benefit > 0c Cost > 0b > c Assumptions Differ

SD Model Two drivers trapped on either side of snow bank Remove snow (C) or Stay in car (D) Best outcome: Opponent does all work If Opponent stays in car, better to shovel Last Assumption: Contrasts to PD

PD Model CD Cb - c- c Db0

SD Model CD Cb – (c / 2)b - c Db0

Predictions PD: Defect, Single-Play ESS IPD: Conditional Cooperation or Defect SD: Single Play Mixed ESS, Some Cooperation Stable ISD: Cooperation

SD: ESS

“Test” 96 Swiss students (29.2%: Game Theory) Random Pairs/No Knowledge of Opponent Given Pair: IPD or ISD (12 interactions) “Monopoly” Money at Iteration/Global Payoff

“Test” PD CD C D4000

“Test” SD: Cell Mean = PD Mean CD C D3000

Results Previous Knowledge: No Effect (Game) Cooperation More Frequent in ISD than in IPD ISD: Close to p* IPD: Significantly > 0 Male vs Male Interactions: Significantly Less Cooperation

Results Cooperation Adjusted to Context (PD vs SD) Strategies (TFT or WS/LS) Greater Payoffs Strategies More Common Among Females Willingness to Cooperate Greater When Cost of Interacting with Defector Lower