This chapter covers behavioural effects. Unintended behavioural effects occur when recipients or affected persons respond to an external intervention in unintended ways due to psychological factors that were insufficiently taken into consideration when the intervention was designed. This chapter covers three kinds of behavioural effects: rebound effects, backfire effects, and motivational crowding-out effects. Rebound effects occur when the improvement made possible by an external intervention increases (instead of reduces) the undesired behaviour because of an unintended and often unexpected behavioural response. Backfire effects occur when social progress of members of a marginalized group due to an international intervention leads to a counterreaction by members of the dominant group. This backfire effect plays out at the micro-level (individual), whereas the backlash effect (Chapter 3) intervenes at the macro-level (society). Motivational crowding-out effects occur when an external intervention incentivizes certain behaviours but actually undermines the intrinsic motivation for engaging in those behaviours.