Researchers have productively tested identity fusion theory, aiming to explain extreme progroup orientations. However, the strength of effects, types of measurements, and study contexts have varied substantially. This first meta-analysis (90 studies from 55 reports, 106 effects, N = 36,880) supported four main conclusions based on the available literature: (1) identity fusion has a strong and positive but very heterogeneous relationship with extreme pro-group orientations; (2) its effect is significantly stronger than that of social identification; however, some evidence suggests that this difference is primarily observed in published rather than unpublished studies; (3) the verbal identity fusion scale has the best explanatory power; (4) identity fusion is most strongly associated with extreme collective action, followed by a willingness to sacrifice oneself, fight or die for the group, and outgroup hostility. We discuss the findings' implication for identity fusion theory. Based on the literature's limitations, we highlight avenues for future research.

Identity fusion reflects a visceral feeling of oneness with a group that predicts extreme pro-group orientations. While the theory has been tested extensively, several questions have not been conclusively answered. Here, we present the first meta-analysis of the associations between identity fusion and pro-group orientations (k = 57, N = 36,880, 106 effects). Identity fusion had a strong positive association with extreme pro-group orientations, which was significantly stronger than between social identification and these outcomes. The observed relationships were relatively robust across various countries and, interestingly, the effects were strongest for fusion with an outgroup. The verbal scale assessing identity fusion was the most predictive, and extreme forms of collective action were most strongly correlated with fusion, followed by sacrifice- and die orientations and outgroup hostility. In sum, the present meta-analysis provides evidence for identity fusion being robustly associated with extreme pro-group outcomes across contexts and settings.