Why do people perceive ingroup homogeneity on ingroup traits and outgroup homogeneity on outgroup traits?

People tend to perceive ingroup homogeneity on ingroup stereotypical traits and outgroup homogeneity on outgroup stereotypical traits (e.g., Kelly, 1989; Simon, 1992a; Simon & Pettigrew, 1990). If it is assumed that people use homogeneity ratings to indicate the extent to which groups possess traits, then this stereotype effect may be interpreted as an expression of perceived trait possession (i.e., ingroups possess ingroup stereotypical traits and outgroups possess outgroup stereotypical traits). If it is further assumed that research participants abide by the conversational norm of appropriate quantity (e.g., Bless, Strack, & Schwarz, 1993), then this stereotype effect should be significantly reduced following prior expressions of perceived trait possession. A literature review and two minimal group experiments (Ns = 75, 104) supported this prediction. This evidence is discussed in relation to the outgroup homogeneity effect and self-categorization theory.