Despite considerable research on ingroup bias, there is a lack of theory to explain this phenomenon and thus a lack of understanding of the mechanism behind it. The purpose of this dissertation is to propose a new explanation of ingroup bias. In particular, the research is interested in the cognitive, rather than motivational, basis for ingroup bias. The most predominant theory of ingroup bias has been social identity theory. The theory is based on the assumption of existence of the need for a positive social identity, and is thus a motivationally oriented theory. As an alternative to social identity theory, the dissertation proposes a new approach drawn from persuasive arguments theory which is one of the major theories in the area of group polarization. A new cognitive approach contends that whilst social identity theory argues that ingroup bias originates from motivation for a positive social identity, it may not be based on such motivation or need, but rather on some cognitive process. It argues that ingroup bias may occur because of the reasoning or argumentation that people use as they think about the reasons they and others from both the ingroup and outgroup may have had for their choices, choices which were the basis for their categorization into groups. Based on persuasive arguments theory, the research investigates the effect of listing reasons, either in favor of the ingroup or outgroup's choice, in producing ingroup bias and in reversing this bias using experimental method and questionnaire. The results showed that ingroup bias differed depending upon whether subjects had a chance to think again about their choice and to list reasons for the choice made either by the ingroup or the outgroup. Therefore, a new cognitive approach based on persuasive arguments theory is proved to be valid although further research is needed.