This study attempts to understand the wishful thinking effect through a group approach. We build a group decision making model that tries to explain how a group dynamic can lead the group to interpret and recall data so that they correspond to more favorable beliefs about the group future prospects. Group members have to take one single common decision determining the group effort level in a joint production. We find that (a) An agent's incentive to enter into denial when others are in denial is higher than an agent's incentive to enter into denial when he is alone; (b) An agent's incentive to enter into denial when others are realist is lower than an agent's incentive to enter into denial when he is alone; (c) An agent's incentive to enter into denial increases with the riskiness of the project; (d) the presence of a leader can either increase or decrease the incentive to enter into denial depending on assumptions made on costs. We build an experimental protocol allowing to try to test the previous model and validate or not its predictions.