The thesis investigates the influence of communication messages on individuals' willingness to cooperate. The influence of the type of communication messages and the tendency toward ingroup bias on the change in personality levels of modesty, honesty, and greed in decision making was examined. Results indicated that a message model that promotes a rational approach to decision making and a sense of group belonging can significantly influence changes in individuals' levels of honesty and greed, which may also affect the long-term sustainability of societies. A review of the literature revealed that, at the time of writing, the influence of communication messages on changing individuals' levels of personality, as well as on expanding the domain of in-group bias, had not been studied in detail. The paper provides an overview of the influence of different emotions on decision-making in people, based on the innate need to belong to social groups, the ability to imagine oneself in the position of other people, the ability to put long-term benefit before short-term and the phenomenon of ingroup bias. The theoretical framework for the application of game theory to the study of complex social relationships is also presented, as is the tool of evolutionary simulation of social development. Ways to create and model messages for rational and irrational strategies were discussed, and message models for research purposes were created based on the results. A total of 569 respondents participated in the research conducted in preparation for this paper, and four phases of research were conducted: Survey research on empathy level, experimental research from the field of game theory using the Ultimate Bargaining Game, survey research on individual personality level, and evolutionary simulation of the development of society. The paper contains two figures, two diagrams, 11 graphs, and 215 tables, as well as 189 bibliographical references.