  Social categorizations divide people into "us" and "them," often along
continuous attributes such as political ideology or skin color. This division
results in both positive consequences, such as a sense of community, and
negative ones, such as group conflict. Further, individuals in the middle of
the spectrum can fall through the cracks of this categorization process and are
seen as out-group by individuals on either side of the spectrum, becoming
inbetweeners. Here, we propose a quantitative, dynamical-system model that
studies the joint influence of cognitive and social processes. We model where
two social groups draw the boundaries between "us" and "them" on a continuous
attribute. Our model predicts that both groups tend to draw a more restrictive
boundary than the middle of the spectrum. As a result, each group sees the
individuals in the middle of the attribute space as an out-group. We test this
prediction using U.S. political survey data on how political independents are
perceived by registered party members as well as existing experiments on the
perception of racially ambiguous faces, and find support.
