Comparisons with other humans are a central issue in everyday life. They can influence both, the perception of the self as well as the behaviour. The present study investigated the influence of a nationality-specific in- or out-group comparison on the active self-concept and, subsequently, on behaviour. Based on the self-categorization theory, it can be inferred that a member of the same or a member of another nation can lead to categorization processes on the level of social identity. Consequently, the individuality of a person becomes less important and country-specific stereotypes get activated in the active-self concept. In self- assessment as well as assessment by others, Austrian are rated higher in warmth than Germans. In the present study it was investigated if social comparison leads to stereotype-consistent self-assessment and behaviour of Austrians and Germans. A confrontation with the out-group should lead the self to move away from the out-group-stereotype to the in-group stereotype. Furthermore the potential function of the identification with the own nationality as a moderator has been examined. The sample comprised 250 Austrians and 93 Germans. The self-assessment of warmth was measured by a self-concept scale. The behaviour was operationalized in form of a test that observed how helpful the participants were. We presented three different kinds of standards (Austrian-, German- or, a control standard) in form of an audio-sequence. The assumptions could be partially verified. A link between self-concept and behaviour could only be found for Austrians. Subsequent studies should further assure a balanced relation between Austrians and Germans.