Dear Ms Wise, I plan to propose to my girlfriend this year, but I don’t know how much to spend on a ring. I’ve heard that I should be spending one to three months’ salary, but that’s about $37,500 for me (before tax!), which strikes me as a lot. Any advice?

The wisdom: That “rule” of spending one month’s salary on an engagement was the result of a marketing campaign by global diamond corporation De Beers in the 1930s. Over the years it evolved to two months’ salary and then three.