The article is devoted to a problem of correctness of interpretation of prosocial decisions observed in experimental studies in economics as an undoubted evidence of a prosocial motivation and considerations which take into account other side's interests among people making these decisions. The results of the main research confirmed the hypothesis that decisions about equal distribution of money in the situation of ultimatum transaction cannot be viewed as a disproof of axiom of egoism of classic economic game theory. It was shown that such prosocial cooperative decisions in the vast majority of cases were determined by aspiration to care exclusively for one's own interests maximizing one's own benefits and minimizing one's own losses.