Marketing all the way down
The idea that a man should spend a significant fraction of his annual income for an engagement ring originated de novo from De Beers marketing materials in the mid-20th century, in an effort to increase the sale of diamonds. In the 1930s, they suggested that a man should spend the equivalent of one month’s income in the engagement ring; later they suggested that he should spend two months’ income on it.
Via Wikipedia sourced from Broke Is Beautiful
(Source: darrellsilver)