Evolutionary accounts of intergroup interactions often emphasize threats posed by outgroup individuals. This emphasis suggests the plausibility to essentialist intuitions but ignores the benefits of between–group interactions both at the individual, and group, level. For example, immigration fosters population growth, and larger populations can support greater technological complexity and defense. We develop an optimization and game theoretic model to understand the conditions under which outgroup exclusion evolves. The first model considers the payoffs to group size under different levels of population pressure. The second game-theoretic model considers the differential fitness of residents and recent migrants, adding benefits to cultural coordination. Combined, the two models illustrate two contrasting mechanisms that allow outgroup aversion to evolve—one that relies on guarding territorial borders and the other for avoiding costs within a population. While population pressure, resource abundance, and coordination benefits affect both mechanisms of outgroup aversion, they evolve in different parts of the parameter space. Unexpectedly, outgroup aversion can evolve in both resource abundant and scarce scenarios. However, we also suggest these should be conditional, rather than fixed, responses. Given these findings, we advocate work beyond ingroup-outgroup distinctions and toward the dynamics of group affiliations.