Groups very similar to the ingroup provoke more intergroup bias than dissimilar ones do.

Evidence

Distinctiveness threat is another type of threat related to social identity. According to social identity theory (Tajfel & Turner, 1986), people seek membership in positively distinct groups and threats to ingroup distinctiveness are therefore aversive. Intergroup comparisons with a very similar outgroup threaten group uniqueness and distinctiveness (Branscombe et al., 1999), therefore very similar groups may prompt more competitive intergroup comparisons leading to higher levels of intergroup bias. In this regard, Roccas and Schwartz (1993) manipulated the degree of intergroup similarity and found that as similarity increased, high identifying group members showed an increase in intergroup bias along dimensions relevant to the ingroup. Additionally, on some resource allocation tasks there is more discrimination against members of similar relative to dissimilar outgroups (Diehl, 1988). — Blake M. Riek, Eric W. Mania, and Samuel L. Gaertner, Intergroup Threat and Outgroup Attitudes, 59

Categorization

• XREF: Connects to the ‘narcissism of small differences’ idea; boundary-blurring similarity provokes the sharpest boundary assertion. — connects to: Similarity of an outgroup raises bias only among highly identified ingroup members. — Blake M. Riek, Eric W. Mania, and Samuel L. Gaertner, Intergroup Threat and Outgroup…, 59
• QUESTION: This is the closest experimental analogue to the reader’s question about whether boundary-blurring internal enemies increase identity assertion — but the finding concerns similar outgroups, not internal enemies per se.

Related

• Threat from similar outgroups drives more bias than threat from dissimilar ones. (similarity 0.95)
• Similarity of an outgroup raises bias only among highly identified ingroup members. (similarity 0.93)
• Recategorizing outgroup members as ingroup members diminishes perceived threat. (similarity 0.86)
• Groups only compare themselves with relevant out-groups, not all available ones. (similarity 0.86)
• Ingroup bias rises when outgroups are similar and ingroup identification is strong. (similarity 0.86)
• Distinctiveness threat may precede and amplify symbolic and realistic threats (similarity 0.86)
• In-group bias intensifies against similar rather than dissimilar out-groups (similarity 0.85)
• Recategorization into a common identity can backfire and increase subgroup bias via distinctiveness loss. (similarity 0.84)
• Group-based identity forces precede realistic and symbolic threat perception (similarity 0.84)
• A shared superordinate identity can backfire by fueling subgroup in-group projection. (similarity 0.83)
• Threat to distinctiveness drives more complex identity structures, but evidence is sparse (similarity 0.83)
• Perceived threat to group distinctiveness reduces social identity complexity (similarity 0.83)
• Even explicitly random group assignment produces discrimination stronger than interpersonal similarity (similarity 0.83)
• Distinctiveness threat shifts behavior more than judgment, though its effect size is tiny. (similarity 0.83)
• In-group identity is defined only through contrast with out-groups (similarity 0.82)