We investigate the dynamic relationship between residential choices of individuals and resulting long-term aggregate segregation patterns, allowing for feedback effects of macrolevel neighborhood conditions on residential choices. We reinterpret past survey data on whites' attitudes about desired neighborhoods as revealing large heterogeneity in whites' tolerance of black neighbors. Through agent-based modeling, we improve on a previous model of residential racial segregation by introducing individual-level heterogeneity in racial tolerance. Our model predicts, in the long run, a lower level of residential racial segregation than would be true with homogeneous racial tolerance. Further analysis shows that whites' tolerance of black neighbors is closely associated with their overall racial attitudes toward blacks.population heterogeneity | Schelling model | Guttman scale | Detroit Area Study | segregation index R acial residential segregation is an enduring social phenomenon in American society that has a negative impact on the black population (1). One proximate cause of this phenomenon is whites' widespread attitude of preferring not to live in the same neighborhoods with blacks (2, 3). Although there are signs that racial residential segregation has lessened in recent decades, it remains very strong in many American metropolitan areas today (4). A common measure of segregation is the dissimilarity index (denoted as D), with 0 representing no segregation and 1 representing complete segregation. According to the latest data from the 2010 US decennial census, blacks and whites are still severely segregated in many large metropolitan areas, with D exceeding 0.7 in Detroit, Milwaukee, New York, Newark, Chicago, Philadelphia, Miami, Cleveland, and St. Louis (5). If racial segregation results from whites' racially based residential preferences, do these very high levels of racial residential segregation indicate whites' strong opposition to having blacks as neighbors? This is not necessarily the case. In a highly influential work, Thomas Schelling (6, 7) demonstrated that even a very mild ingroup preference for one's racial group not making up less than 50% of the population in a neighborhood could lead to a high level of racial segregation in the aggregate through a dynamic process. When a white family moves from one neighborhood to another, for example, it changes the racial composition of both the origin neighborhood (i.e., making it slightly less white) and the destination neighborhood (i.e., making it slightly whiter), and these changes in neighborhood racial composition could cause other families to move in response. Thus, severe racial segregation may result even though the population does not have a strong race-based preference.Demonstrated with coins and graph paper, Schelling's model was simple. However, it proved powerful in illustrating that small individual preferences can lead to the unexpected emergence of severe segregation in a population. Schelling's model contained two important features that make ...