The United States, the third most populous country globally, accounts for about 4.5% of the world’s population. The U.S. population—currently estimated at 307.5 million persons—has more than doubled since its 1950 level of 152.3 million. More than just being double in size, the population has become qualitatively different from what it was in 1950. As noted by the Population Reference Bureau, “The U.S. is getting bigger, older, and more diverse.” Estimates of the United states population at the middle of the 21st century vary, from the U.N.’s 404 million to the U.S. Census Bureau’s 422 to 458 million. To develop a snapshot of the nation at 2050, particularly its astonishing diversity and youthfulness, I use the nice round number of 400 million people, or roughly 100 million more than we have today. The United States is also expected to grow somewhat older. The portion of the population that is currently at least 65 years old—13 percent—is expected to reach about 20 percent by 2050. This “graying of America” has helped convince some commentators of the nation’s declining eminence. For example, an essay by international relations expert Parag Khanna envisions a “shrunken America” lucky to eke out a meager existence between a “triumphant China” and a “retooled Europe.” Morris Berman, a cultural historian, says America “is running on empty.” But even as the baby boomers age, the population of working and young people is also expected to keep rising, in contrast to most other advanced nations. America’s relatively high fertility rate—the number of children a woman is expected to have in her lifetime—hit 2.1 in 2006, with 4.3 million total births, the highest levels in 45 years, thanks largely to recent immigrants, who tend to have more children than residents whose families have been in the United States for several generations. Moreover, the nation is on the...