A now-obsolete law called the “Breach of Promise to Marry” once allowed women to sue men for breaking off an engagement. Back then, there was a high premium on women being virgins when they married — or at least when they got engaged. Surveys from the 1940s show that roughly half of engaged couples reported being intimate before the big day. If the groom-to-be walked out after he and the bride-to-be had sex, that left her in a precarious position. From a social angle, she had been permanently “damaged.” From an economic angle, she had lost her market value. So Breach of Promise to Marry was born…

Let’s think like an economist. An engaged couple aren’t all that different from a borrower and a lender. The woman is lending her hand in marriage to the man, who promises to tie the knot at a later date. In the days of Breach of Promise, the woman would do this on an unsecured basis — that is, the man didn’t have to pledge any collateral — because the law provided her something akin to bankruptcy protection. Put simply, if the man didn’t fulfill his obligation to marry, the woman had legal recourse. This calculus changed once the law changed. Suddenly, women wanted an upfront financial assurance from their men. Basically, collateral. That way, if the couple never made it down the aisle, she’d at least be left with something. And that something was almost always small and shiny. The diamond ring was insurance.