In an academic paper they completed last year that analyzed earlier findings from the national surveys, Professors Musick and Bumpass compared responses to questions about how couples described their relationships, how often they fought and over what, and how they would envision their lives if they separated.

The research doesn’t address whether blissful 21st-century relationships are any more or less enduring than they were in the 20th century, so it may be that happy coupledom always came with a three-year expiration date. With nonmarital childbearing more common and women more economically independent, “What’s keeping people together is their love and commitment for each other,” Professor Musick said, “and that’s fragile.”

Anecdotal evidence suggests that the findings have some foundation.

Bart Blasengame, a 33-year-old freelance writer from Portland, Ore., was with his former fiancée for three years. “I felt like, by year three, we were both forcing it,” he recalled.

“It’s the whole cliché of pursuit,” he said. “Your dates are planned out like some Drew Barrymore romantic comedy with unicorns and rainbows. By year two, we were cruising along, living together, relatively happy. But from a growth standpoint things had started to atrophy. We were happy, content is a better word, but there was no spark.”

But the evolving rules of marriage provide both opportunities and pitfalls, Professor Musick said. “There may be greater potential to find fulfillment in relationships,” she said, “but that possibility and the expectations that come from it may lead to greater disappointment for some” if the expectations aren’t fulfilled.

Her bleak statistical assessment of the durability of enchantment is one of several new findings about relationships and marriage in America. In a word, the State of the Unions is precarious.