Why cooperate? The common approach emphasizes material gain, expressed in evolutionary biology as fitness. In psychology, it is expressed as maximization of the expected utility. This approach is usually demonstrated in experiments by using isolated anonymous individuals. But when social dimensions are restored, a bias-to- cooperate occurs: subjects cooperate more than expected from immediate material gains. This book offers an explanation to the bias based on two hypotheses: one, that there are two types of immediate reinforcements: emotional states linked to the social dimensions of cooperation, and material gains; and two, that the adaptive value is realized by strengthening social relationships between participants that eventually lead to fitness gains. These were tested by observing the behaviors of bottlenose dolphins at the Dolphin-Reef, Eilat. Additionally, a game-theoretical framework is drawn, that explains the link between psychological processes and natural selection, and offers a testable solution to the long debated problem of why do we cooperate, and to the more recent evidence for a cooperation bias that is nevertheless adaptive.