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.