Figure 2.

Non-contingent experiments (a) follow a fixed script to ensure Player B's behaviour (offers and emotional expressions) is guaranteed independent of Player A's prior behaviour. By contrast, contingent experiments (b) incorporate mechanisms that respond conditionally to Player A's prior (Player B's expressions and choices to cooperate are conditioned on Player A). Each approach can treat one or more of these scripts/mechanisms as independent variables. DV, dependent variable. (Online version in colour.)