In this thesis individual and collective decision making is being researched. For individuals prone to procrastination or impulsivity, this work suggests that researchers and therapists should focus on the discrepancy between intentions and beliefs. For example, addicts could keep intention-belief diaries, where they specify how they want to behave, and how they think they will.Collective decision making is also intricate, as selfish free riders often take advantage of cooperative individuals’ efforts. This research shows that groups tend to select leaders who can force free riders to cooperate. Such self-organized hierarchies enable groups to achieve better outcomes