I've already written of taking the bus in Los Angeles westbound on Sunset Boulevard from Hollywood to UCLA in Westwood. Continuing in that direction and reaching the terminus at the Pacific Ocean will put you in Pacific Palisades, one of the most homogeneously white locations in Los Angeles County – 89 percent Anglo. But if, instead, you took Sunset Boulevard eastward all the way to the end, you would reach another demographic extreme. As the name on the street signs changes from Sunset Boulevard to Cesar E. Chavez Avenue and then to Avenida César Chávez, you will find yourself in Boyle Heights and East Los Angeles, 97 and 95 percent Latino, respectively. These neighborhoods are by some measures the least diverse places in Los Angeles County – almost entirely Latino and overwhelmingly of Mexican heritage.

A bird's-eye view of the Los Angeles Basin's Latino population shows them in all but the most exclusive sections of the city, but also shows a steady increase in Latino density as you move away from the ocean, culminating in the homogeneous bastion of East Los Angeles. (See Figure 8.1. In this and in the other maps in this chapter, black dots represent the exact location of individual Black voters and gray dots represent the exact location of individual Latino voters (see below for how I do this).)

Visit East LA and Boyle Heights and you will likely hear nothing but Spanish. But look at the street signs and you will see names that seem out of place, such as Eagle, Wabash, and McBride. These names are there, of course, because these neighborhoods weren't always Latino. Built up in the first half of the twentieth century, they were primarily Anglo, with a large Jewish population in Boyle Heights.

I began this book with the “L” train in Chicago, crossing the boundaries between Black and white. The train made a journey that most Chicagoans, because of the social and psychological space between the two groups – reinforced by segregation – would not make. On the “L” train, we see the great divisions in Chicago. This journey could be repeated for other cities, of course. A bird's-eye view of most of the world's cities large enough to be diverse shows social boundaries mapped onto geographic boundaries. Not all such boundaries are as extreme as Chicago's, with its hyper-segregation between Black and white, but the connections between groups and place are still present. And for most of us, these connections between groups and place are how we bringmeaning to the cities. In Boston, Southie and theNorth End are just names until we associate them with Irish and Italian, respectively. Those geographic boundaries, I argue, don't just coincide with the social boundaries – they reinforce them.

In Figures 1.1 and 1.2 on pages 10–11 (see also Figure 5.2 and Figure 5.3 on pages 132–134), I tried to convey the impact of this social geography by treating the city as a giant puzzle which could be rearranged by an experimenter to disrupt the fit between groups and space by altering the spatial continuity of group boundaries. I claimed that this would break the connection between groups and place and reduce the salience of groups, which in turn would reduce group-based bias.

Of course, in real life, we can't add or remove segregation to alter a group's spatial continuity. The only way, so far, we have been able to test the effects of segregation was through thought experiments, as in the maps, and through statistical comparisons across place (cross-sectional comparisons in the language of statistics). This runs us headlong into what statisticians call the Fundamental Problem of Causal Inference, because the same city cannot be both segregated and unsegregated at the same time. If we compare different cities, theremay be something different about those cities or about the people in those cities – something other than social geography – that is causing differences in group bias.

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.

Rafael and Jose were awake every day before 4 AM to make their way across Boston to South Station, the city's main train station. There they would catch an outbound train to the wealthy suburbs to the west of the city, arriving in the village of Grafton before 6 AM. Then they would wait on the platform.When the next inbound train arrived, they would head back to Boston.

According to hundreds of people to whom I showed their photographs, Rafael and Jose looked handsome, intelligent, and friendly. In my personal interactions with them, I found them so. But other rail passengers at Grafton station were uncomfortable and seemed not to want them there.

How do I know this? Because I surveyed these other passengers and their responses were unambiguous. Why were they uncomfortable with these handsome, intelligent, and friendly-looking young men? Because they were speaking Spanish.

Rafael and Jose were at this train station because they were part of an experiment I conducted in the summer of 2012. My goal was to make people think that their neighborhood was experiencing demographic change due to immigration; that is, I wanted to change the social geography of their community. My research team placed pairs of people working for me – “confederates,” in the language of experimental research – at train stations to wait for the same train at the same time each morning during rush hour. These confederates were native Spanish speakers. A train time had been paired with another train time at the same station and, randomly – by the proverbial “flip of a coin” (actually by a pseudo-random number generator on my computer) – one time was assigned to treatment and one to control. The train time assigned to treatment was visited by the confederates; the other was not.

In the summer of 2013, I stared out the window of a fourth-floor apartment in Neve Yaakov, a Jewish neighborhood in Jerusalem. Looking deep into East Jerusalem, in an area much of the world considers occupied Palestinian land, I could see the gray slabs of the snaking concrete wall erected by the Israeli government to separate Israel from the West Bank. At the kitchen table across the room sat an Israeli woman, an immigrant from Kazakhstan, working at a computer while her infant slept close by in a crib.

This particular view of the wall – called the “separation barrier,” “security fence,” or “apartheid wall,” depending on who is speaking – stuck vividly in my memory because it seemed so appropriate, given the reason that I found myself in this woman's home. On the laptop my colleagues and I had provided for her, she was deciding whether or not to share money with an Arab and another Jew, who had also been given money and asked to share it with her. She was playing a “Prisoner's Dilemma” game, the classic game of cooperation designed to measure trust. Scholars argue that it captures the dynamics of trust necessary for the collective action which, in turn, is necessary for societies to function successfully. I had come to Israel to collect data from subjects like this woman in order to understand how social geography, the space between us created by walls and governments, affects our ability to cooperate – to solve the dilemmas of collective action represented by this game.

Out the window, I could see one of the starkest examples of man-made segregation you will find. This was what I had come to Israel to understand: when people segregate themselves, often intentionally, can a society still function well? Can a segregated society reach the levels of cooperation necessary to care for the young, old, and sick and provide for the common welfare? Or does social geography – the mighty demagogue – stop us from doing these things?

I had been to Israel before. In late summer 2011, I found myself on a hilltop in an olive orchard outside the town of Bir Zayt in the Occupied West Bank.

In 2008, the first time that every American voter had a chance to vote for a Black man for President, there was a striking pattern. In some counties, white voters cast far fewer votes for Barack Obama than they had for previous Democratic candidates. For example, in Floyd County, in eastern Kentucky, white voters cast only about 49 percent of their votes for Obama, while, only four years earlier, they had cast 63 percent of their votes for John Kerry. This shift away from voting Democrat was seen in many counties across the United States. In fact, in many counties, Obama received a smaller share of votes than had any of the previous four Democratic candidates.

Of course, there are many reasons this could have happened. But given that the troubled presidency of GeorgeW. Bush and the 2008 economic crisis meant it was a favorable election year for Democrats, it seems likely that race may have played a role – that white people were driven away from their traditional party loyalties by a Black man on the ballot. We might imagine that if we had gone to these counties during the 2008 election, we would have found petty demagogues using race to stir up fear by implanting negative stereotypes of Blacks and of a Black president into the minds of white voters.

Some of these demagogues did exist, but in this book, we are searching for something more systematic, something that will affect voters’ attitudes and behavior regardless of the actions of any particular politician. Indeed, if we zoom out and look for patterns in the data, we see that in 2008, the demagogue was not any local politician, but rather geographic space itself. Looking across the country, we see that white voters in the most-segregated counties were between five and six percentage points less likely to vote for Obama than white voters in the least-segregated counties.

Why does social geography affect our behavior? Why can it lead to group-based bias? Why is it so powerful that it can overcome the forces of the modern world that work to close the spaces between us?

In this chapter, I will answer these questions. It is the centrality of both groups and space in the mind that helps to explain why group-based biases – taking forms as disparate as voter turnout and violent conflict – are affected by social geography. Groups are a central part of the way our minds organize the world. Space is, too. Within our minds, space acts as a demagogue, shaping our mental images of groups.

Not long ago, I boarded a flight from Boston to Washington, DC. Across the aisle was a light-skinned Arab woman in a hijab, the Muslim headscarf, and her Black Muslim husband. Despite all my best intentions as a liberal – one married to a Muslim woman, no less – that simple blue scarf turned my mind to every negative stereotype about the danger of Muslims on planes.

When our own prejudices are laid bare, we realize how easily negative stereotypes can be brought to the surface. This is an awful, awful way to react to somebody. I have in-laws who detest flying because they are aware of the prejudices their fellow travelers have about them.

Near the end of the short flight, I looked over and saw the woman unwrapping her headscarf and thought to myself, “She must not be very religious if she will adjust her scarf in public.” But rather, it turned out, as far as I know, that she wasn't Muslim or Arab at all. She was just a non-Muslim woman with a scarf around her head, tucked in a certain way that made it look like a hijab. This event stuck in my memory because it was remarkable to feel the accessibility of my stereotypes change so quickly, away from those associated with a Muslim woman and toward those associated with a non-Muslim, “American” woman married to a Black man.

In this book, in order to study the influence of context, I have moved across space and time. One way to experience a change in context on a tremendous scale, across both space and time, is to travel south across the United States toward the Mexican border. Start at a city in the Midwest, say Chicago, and as you travel south, with each mile toward the border, the Anglo proportion of the population will decline and the culture and people will become more Latino. Figuratively, you will be moving through time as the cities become not only less Anglo but also generally less dense, with the older, more compact cities of the Northeast and Midwest giving way to the newer, sprawling, car-based cities of the Southwest. In a Midwestern city like Chicago, where so much social science – including parts of this book – has been focused, you are in the urban past. When you reach a city like Phoenix – low-density and only 120 miles from the Mexican border – you are in the urban future.

Chicago and other big cities of the Northeast and Midwest represent not only the urban past but also, with their large Anglo white and African American populations, the demographic past. The populations of these cities were a result of the previous great migration – northward and westward – of African Americans from the South. Now, different currents are flowing: Anglos are moving south and west and Latinos are coming north. The demographic future of the United States is found in the meeting of Anglos and Latinos in these sprawling, fast-growing communities near the Mexican border.

Keep pushing south from Phoenix on Interstate 19 through Tucson and into the emptiness of the beautiful Sonoran Desert and, right before the road terminates, it will bend sharply east and put you on the dusty streets of Nogales, Arizona, a community straddling the border with Mexico. Here, you will find the extreme conclusion of demographic change.