The Changeling Effect takes its name from the folkloric tradition of changelings – the replacement of a human child or person with a fairy, ogre or elf – common across the Blessed Isles but is more about a human reacting as if they were an Otherworld creature. Interestingly, the changeling folklore is also shared by the Scandinavian and Hispanic countries, perhaps influenced by our own experiences and folklore, while similar but not identical myths exist in West Africa and the Philippines.
In basic terms, the Changeling Effect is when a human displays discomfort around synthetic cloth, refined metals, and other processed materials such as concrete and asphalt. They cannot use modern technology such as tablet computers and mobile phones as this makes them feel ill in some vague, unspecified fashion. They may or may not also show food allergies. These symptoms are similar to The Allergy as observed in the Otherworld itself and its creatures. In less logical times, this would have been taken as a sign that the individual, whether infant or adult, had been replaced by an Otherworld imposter.
The number of people who suffer from the Changeling Effect is small at about 0.000,1% of the population or 600 people. In the majority of cases, the suffers were foundlings and have no known family history. Bizarrely, a recent study suggests that there is often a small but statistically significant increase in these Changeling foundlings within nine to eighteen months of an Expedition passing into the Otherworld. It has been suggested that the remaining Changeling foundlings may be related to the loss of Rangers on shorter reconnaissance missions or missing persons who have independently found their way into the Otherworld but this is not a hypothesis that can be tested.
The vanishingly small number of Changeling cases with known family all develop symptoms at a later stage, following misadventure in the Otherworld – whether sanctioned or otherwise. Information about triggers is scarce due to how few cases have been recorded for posterity but the advice given by Unnatural Resources is, should one find oneself in the Otherworld, to avoid the apples.