My poor cousin Dan. A middle-aged joker with a surprising spiritual bent, he'd been single a long time when he met… well, let's call her Lynn. It was great, he said, to be intimate with someone again. They saw each other every day, and within a couple months they were living together. But even on their first date, Dan remembers, there were signs: all the talk about the Lexus or Infiniti she wanted; all that food she ordered and didn't eat. Nevertheless, he felt good about the agreed-upon plan for sharing his place, which he owned. Lynn would pay $500 for rent, about half of what she'd been paying on her own, and they'd split everything else.

Within a year, though, things changed. The first time Lynn said she had no money to contribute that month—despite her midlevel job for a heath insurer that had her jetting around the Pacific Northwest—Dan said it was fine. "Stuff like that happens," he told her. "Just don't make a habit of it." But she did, all the while coming up with unconvincing excuses involving travel reimbursements that never showed up. "I thought, is she doing drugs?" Dan remembers. "Gambling? No, that wasn't her style. I'd beg her to give me a good reason. I wanted a reason more than I wanted the $500."

In the end, he called it quits, and, in the process, learned the truth: For years, while her debts mounted, Lynn had been buying things—mostly clothes, which she'd pack away and never wear. When she finally moved out of Dan's house, he found six fur coats stored in one closet. Another room was full of brand-new items of all kinds, "literally from the floor to the ceiling." She never came back to pick it all up.

"It helped," Dan confessed, "when I first heard the term 'compulsive shopper.'"

Not that he, an inveterate non-shopper, could really identify—but he could sense that they'd broken up over something far more painful and psychologically ingrained than poor money management skills. "One time she said to me, 'I feel like I'm nothing,'" Dan remembers, flooded with sympathy. "It was like when she bought something, she was trying to increase her presence in the world."

In this divorce-happy age, conventional wisdom targets financial issues as one of the leading causes of marital breakups. But my cousin's story—extreme though it is—suggests that if you peel back the layers, you almost always find that there's much more than money disturbing the peace. In fact, the only study on divorce that used real-time data (as opposed to relying on couples' memories of their rifts) found only a tiny correlation between financial problems and failed marriage. At the beginning of the 12-year study, conducted by consumer economist Jan Andersen of California State University in Sacramento, all the participants were married.

At various points in time, the more than 2,000 randomly chosen couples answered a long slate of questions about their marital stability, concerning everything from their health to their church attendance to, of course, their financial habits. When Andersen looked closely at the results, he got a surprise: Only a tiny number of couples that logged negative responses on the money questions wound up divorced.