These Americans were born into an earthquake. During the 1800s America’s population exploded from 5 million to 75 million. By 1900 nearly as many people lived in New York City as had lived in the entire country during the Revolution. The nation went from a rural backwater to an industrial behemoth — producing more than Britain, Germany and France combined — but every decade the economy crashed. America saw the kind of wild change we see today in China, and in a new society with little to stabilize it.

For rootless 20-somethings, each national shock felt intimate, rattling their love lives and careers. Many young adults could not accept that their personal struggles were just ripples of a large-scale social dislocation. So each New Year’s, they blamed themselves. In a Jan. 1, 1859, entry in her journal, 19-year-old Mollie Dorsey, stuck on a Nebraska homestead in the middle of a recession, castigated herself for not being “any better than I was one year ago.”

Romance worried them above all. Today some fret about the changing institution of marriage, but we are used to such adjustments; 19th-century Americans were blindsided when the average age of marriage rose precipitously, to 26 — a level America didn’t return to until 1990. In a world where life expectancy hovered below age 50, delaying marriage until 26 was revolutionary.

Cities brimmed with bachelors and unmarried ladies in their mid-20s, once a rare sight. In their New Year’s reflections, men and women noted that their parents had had children by their age. One typical Union Army soldier wrote home wondering, “Do you think I will be married before I am thirty?”

This social change brought personal turmoil, especially for young women. Marriage meant love and family, but in a society that discouraged ladies from working, young women were dependent on their husbands. Remaining single meant economic and legal instability, and the perception of childishness. When the mother of one diarist, Emily Gillespie, scolded the Midwestern farm girl by saying, “you are twenty years old and not married yet,” it hardly mattered that Emily was in line with her generation.