Couples choosing to skip the marriage license on the way to a shared stack of utility bills scarcely ruffle anyone’s feathers these days. But if they can’t talk about those bills without arguing, then there’s a strong chance that feathers will be flying for good.

After studying findings on the common causes for divorce—finances, sex and housework— Jeff Dew, research associate in the Department of Sociology at UVA, set out to forge corresponding explanations for the breakups of unmarried but cohabitating couples. He found that of five different types of disagreements among couples—finances, sex, housework, spending time together and the relationship with the couples’ parents—the only type that predicted whether a cohabiting couple would break up was money.

Cohabitation has become increasingly common since the 1960s, when the sexual and gender revolutions brought into questions many of the tenets of traditional relationships. According to Dew, cohabitation allowed people to generate a relationship free of the old gender stereotypes. “Whereas marriage had this historical baggage of the patriarchy, cohabitation could be this ideal, equal relationship between the sexes, who were not necessarily tied or committed to each other, but sharing their lives for the moment,” he says.

Dew mined data from the National Survey of Families and Households, which included 483 cohabitators who were interviewed once in the late 1980s and again in the early 1990s. During the first interview, couples reported on the nature, intensity and frequency of their arguments; by the second interview, many of these couples had split up.

Sharing their lives and bills for the moment was a less breezy proposition than many couples anticipated. The fault lines of arguments about money run deep. “When people criticize what we do with money, it can influence us very personally,” Dew says. That’s partly because we often have private principles about how we manage our own money—whether we dedicate it to status, security, opportunity or fun. “This approach might come from our family—I spend like my father, save like my mother—so when you criticize how I spend, you’re criticizing more than just me, you’re criticizing how I grew up.“

One reason money is such a volatile issue is that “cohabitating couples are often less financially stable than married couples, so it may be a more salient issue for them in their day-to-day goings-on,” Dew says.

In addition, the need for money is ever-present. “Bills come in monthly or weekly, so finances present a constant opportunity to fight.”

A third reason is more philosophical: the notion of fairness. Cohabitors have a high standard for financial equality, so it can be hard to figure out what’s fair. “If they decide to go 50/50 on the rent, it disadvantages the partner earning less,” Dew says. “But if they decide to go proportional, the one that earns more is disadvantaged.”

Fairness appears to determine whether a couple will stick together in both cohabitating relationships and marriages. In marriages, couples usually interpret fairness as equity—different types of work can have equivalent value. In cohabitating couples, fairness tends to mean equality—contributing the same amount and type of work and money.

The disunion between these notions is apparent in the most stable model of each kind of relationship. In cohabitating couples that stay together, the partners work equal numbers of hours at jobs outside the home. Marriages in which the wife doesn’t have a job outside the home tend to have a statistically better chance of lasting.