Since 1990, class has become an increasingly reliable predictor of family patterns, Professor Carbone said. College-educated Americans are now more likely to get married and stay married than those with only a high school diploma, a change from 20 years ago, she said, when differences were much smaller.

That trend has been particularly important for rural areas, which have fallen further behind urban ones in education, according to census data. Just one in six rural residents have college degrees, far fewer than in cities, where one in three do. Nationally, there were about 121 million married adults and 26 million divorced people in 2009, compared with about 100 million married and 11 million divorced people in 1980.

Education drew a dividing line for Nancy Vermeer, a 52-year-old resident of Sioux County. She had married her high school sweetheart, a young man from a farming family. He never went further than high school, but she went on to college, and later earned a master’s degree. He worked in a window factory. She became a music teacher. He gambled. They grew apart. Eventually, he asked for a divorce.

“I grew more confident,” Ms. Vermeer said. “We were totally different people.”

When Ms. Vermeer divorced in 2002, she became the first teacher in her Christian school to do so. Divorce was more common than it had been in past decades, but she still felt judged, so she developed habits to keep a low profile, like going to the grocery when no one she knew would be there.

“There’s a perception here that you need to be perfect,” said the Rev. John Lee, a young pastor who has tried to encourage change in Sioux County by taking on taboo topics like divorce and mental illness in his sermons.

Image Wayside Chapel in Sioux Center. Much of the town seems more suburb than village. Credit... Jenn Ackerman for The New York Times

“Cars are washed, lawns are mowed in patterns and children are smiling,” Mr. Lee added. “When you admit weakness, you invite shame.”