I like the way sparkling earrings lay...

I was reading a book for my work on missing persons, and I came upon an interesting comment by a New York detective of the 1930s: that when he found the body of a young woman with pierced ears,  he could assume she was foreign-born or the daughter of immigrants. He also mentions elsewhere—in relation to older cases—that ear piercing was something that had been more popular in the nineteenth century.*
Plenty of movies and photographs (and vintage stores, and grandmother’s jewellery boxes) show us that clip-on earrings were very popular from the 1930s to the 1960s, when pierced ears became standard once again. But why did the custom drop off? Was it precisely the association of pierced ears with immigrants: that the arrival of large numbers of people from southern Europe, who tended to pierce the ears of their infant daughters, made the practice seem declassé to the WASP middle classes? This is just my stab-in-the-dark guess; I’d be interested to know if any readers have more information. (It seems to have dropped from popularity far too early for blood-borne diseases to have been a concern).
We know that in the classical world, Greek sailors wore a gold earring that they could use to pay the boatman across the river Styx, and the Song of Solomon mentions earrings. There is plenty of evidence of some women in the early modern period in Europe having their ears pierced (some earrings still exist, and portraits show at least elite women had them). But like so many small details of women’s lives, particularly those relating to beauty customs, we have sketchy evidence even of recent generations.

*John Ayers, Missing Men, New York, 1932