To the incredulous: the argument is that expensive engagement rings helped fill the cultural gaps left by the repeal of Breach of Promise to Marry laws. The supporting evidence is an apparently strong geographic correlation between repeal of these laws and increases in high-value diamond engagement rings, the latter beginning after the start of the repeal movement yet before the infamous De Beers marketing campaigns.

For the overview: https://www.theatlantic.com/business/archive/2012/04/the-str...

The original research paper: Margaret F. Brinig, Journal of Law, Economics, & Organization Vol. 6, No. 1 (Spring, 1990), pp. 203-215 (13 pages), https://www.jstor.org/stable/764797

Wikipedia page on Breach of Promise to Marry: https://en.wikipedia.org/wiki/Breach_of_promise