“Starting in 1935 in Indiana, U.S. states started altering their laws to abolish the action for breach of promise. Women responded, by Brinig’s account, by requiring a down payment from their fiancees in the form of an expensive ring—which forfeited if the fiancee terminated the engagement. Think of it as a performance bond. Brinig looked at data on diamond imports and concluded that the demand for diamonds started to rise about 1935, four years before the Ayer marketing campaign that is usually given credit for creating the demand for engagement rings. The evidence also suggested that the custom began declining once premarital sex became widely accepted.”