There has recently been a sharp increase in support for anti-immigrant policies in Europe. This paper tests whether this increase can be explained by an in-group insularity (rooted in Social Identity Theory) felt by traditional natives toward immigrants, or whether it acts in tandem with perceived economic and demographic threat (rooted in Realistic Group Conflict Theory). I ran a cross-sectional OLS model that incorporated both in-group insularity and perceived threat across 14 European countries individually, and then a cross-national analysis controlling for state-level factors. The results demonstrated that in-group insularity is more powerful in explaining support for antiimmigrant policies than perceived threat.