My informant is a twenty-two year old student at USC. She is originally from Pennsylvania and came to LA to study screenwriting. As a writer, she makes it her business to be familiar with a variety of legendary creatures from different regions and cultures; she is ethnically Jewish.
“This is one of my favorites. There was that Angelina Jolie movie about it, and everything, but that kind of sucked (laughter) so I first heard this from my grandmother sometime in high school, just kind of like, a scary story or something. But I’ve done more research and the basic story shows up across a bunch of different cultures and whatnot. So basically, we think of fairies as like, Tinkerbell, right? Well in most old cultures, fairies were not that fucking benign. Like, at all. Fairies were these sort of horrendous creatures that would sneak into your house and steal your babies and drive you crazy. Like, parents would hang iron over their baby’s crib to keep fairies from getting in and taking their kids. This whole legend grew up around that idea; when people were wondering if their kid was their kid or the fairy replacement, they’d call them ‘changelings.’ Basically, what a fairy leaves behind to mimic your child while the real thing lives on in the fairy realm. Some stories say that these changeling kids were kind of brutal, monstrous little things, and others just say they’re sort of…off, I guess? There’s a lot of variety, which makes it a cool story to build stories off of, if that makes sense. A lot of wiggle room.”
This legend seems almost mythic, in the sense that it’s a story that was created as a means of explaining and understanding the world around us. As Kieryn mentioned, the changeling story appears in a variety of different traditions; it appears here: https://www.britannica.com/art/changeling-folklore and http://www.pitt.edu/~dash/scanchange.html and elsewhere across print and digital resources. Nowadays, you hear stories about a child one day “changing” and becoming “someone else;” all of a sudden, an otherwise sweet baby grows disobedient or angry or difficult. We would usually go straight to an autism diagnosis or some other psychological explanation, but this offers a more spiritual explanation. According to one of the articles above, there were myths about how to have your child returned from the fairy world, many of which included torture. This story in particular speaks to mankind’s understanding and treatment of mental illness and disability.