Changelings were once a fairly common part of European folklore, and are still pretty widely known, even if not that many people actually believe in them anymore. Basically, a changeling was a child that a fairy or troll would leave in exchange for a human baby they stole. Why do fairies want to steal human babies? Sometimes it’s just to be mean and petty, and Scottish folklore sometimes has it that fairies need the human babies to send to Hell in place of their own kids. Other times it’s less nasty, however, with supernatural beings wanting humans for interbreeding, because their own gene pool is too shallow. While some changelings aren’t much different from humans, and in some cases are healthier and live longer, many are sickly or deformed. Changeling legends might well have been used to explain babies who were abnormal in some way, whether physically or mentally. There are also tales of a fairy simply leaving a piece of wood disguised as a baby, these being known as fetches. This is also an Irish term denoting a ghostly double of a person that would appear as a premonition of their death. Irish folklorists were really trying their best to make “fetch” happen.
There are several different methods given for getting rid of a changeling. Some legends say getting it baptized is enough, or that iron will drive them away. Maybe the reason fairies and trolls want humans for breeding is so they can breed out that debilitating iron allergy. Others claim a changeling will return to Fairyland if you throw it in the fire or mistreat them on purpose, although you’d think that could easily backfire. Selma Lagerlöf wrote a book in the early twentieth century where being kind to a troll child was what resulted in the mother getting back her real son. The message that it’s okay and even preferable to abuse a child if it isn’t your own is certainly a terrible one. Martin Luther, who thought changelings came from the Devil, was totally okay with this.
The Wikipedia entry reports that claiming a baby was a changeling was used as a legal defense for infanticide in the past. In the case of Bridget Cleary, it was used to defend the murder of a grown woman, and the killers were only convicted of manslaughter. I’ve heard there’s a band called Burning Bridget Cleary, although I don’t know anything about them. Wonder if she’s related to the woman who wrote the Ramona books. (Yeah, I know that’s actually her married name, but tell me I’m not the only one who thinks of her when reading the name “Cleary.”) A less violent way to make a changeling reveal its true form and return to the fairies is to cook a meal or brew beer in an eggshell, to which the creature would say, in essence, “Now I’ve seen everything!” There are also accounts of parents journeying into Fairyland to retrieve their missing children.
As this essay states, these stories are usually told from the parents’ point of view, but there’s some more modern literature that examines the changelings themselves. As someone who never really felt normal growing up, I can see the appeal of this. Catherynne Valente’s The Boy Who Lost Fairyland features a troll and a fetch who grow up in human families and eventually return to Fairyland. Eloise Jarvis McGraw’s The Moorchild has a changeling as the protagonist, and I recall one of the stories in Sylvia Townsend Warner’s Kingdoms of Elfin being about one who grew up to study fairy biology and eventually realized he was one himself.