Among the reasons that the scientific study of society is both difficult and exciting is that society never stops changing.

This book is about what happens when different groups of people live close together – sharing small towns, large cities, states, and countries – yet remain separated in geographic space and, as a result, separated in psychological, social, and political space as well. In ways that have not previously been made clear, this property of being close, yet far, penetrates our psychology and affects our thoughts, behaviors, and collective well-being. It paradoxically repels us from the groups to which we are already close. These repelling forces have shaped behavior, as far as scholars can tell, for all of human history. Yet increasingly, especially in the West, people from different groups that were once widely separated are moving into closer and closer contact, changing the context in which many of us live. This makes those repelling forces more important.

Moreover, these forces influence our democratic institutions. They affect whom we vote for and whether we will share our resources with groups other than our own. Recent events remind us of why this is important.

I finished this book on October 20, 2016. On November 8, Donald Trump was elected President. Although not my primary focus, in revising the manuscript for publication, I added analysis showing that Trump's election appears to have been aided by the very prejudices I explore in this book. My contention in this book is that people living in certain places were affected, probably much more than they realize, by specific aspects of the social geography in which they live. That is, their attitudes and behaviors (such as voting for Trump versus voting for Hillary Clinton) were affected by the size of another group (in this case, Latinos), by the nearness of that group to where they themselves live, and by the extent to which the two groups live in segregation. Thus, in areas where the Latino population had recently grown, previously Democratic non-Latino voters shifted their support to Trump, a candidate who centered his campaign around anti-immigrant demagoguery.