Complex sentence: The outgroup homogeneity effect, or "relative outgroup homogeneity" has been explicitly contrasted with the "outgroup homogeneity" in general, the latter referring to perceived outgroup variability unrelated to perceptions of the ingroup.
Split into simpler sentences: The outgroup homogeneity effect, or "relative outgroup homogeneity" has been explicitly contrasted with the "outgroup homogeneity" in general. The latter referring to perceived outgroup variability unrelated to perceptions of the ingroup.