The changeling narrative, whereby fairies - or elves, or pixies, or trolls - steal a human baby and leave one of their own infants in its place, is found throughout Western European folklore, dating back to pre-Christian times.

Tellingly, it is classified by scholars as a folk legend rather than a fairy tale, according to the distinction set out by the brothers Grimm themselves: "The fairy tale is more poetic, the legend is more historical … While it is children alone who believe in the reality of fairy tales, the folk have not yet stopped believing in their legends."

In other words, changelings were widely believed to be a real phenomenon, such that changeling tales are commonly set in a specified place and time, rather than the "once upon a time" of fairy stories. In some rural areas, these beliefs cast their influence well into the 19th century. All of which forms the backdrop to singer and harpist Rachel Newton's New Voices composition, premiered at Celtic Connections next Sunday, and simply titled Changeling.

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Newton will also feature on viola, alongside Corrina Hewat (vocals/harp), Adam Holmes (vocals), Su-a Lee (cello/musical saw), Lauren MacColl (fiddle) and Mattie Foulds (percussion). Besides Newton's work with all-female sextet The Shee and the Emily Portman Trio, she also won high praise for her debut solo album, 2012's The Shadow Side, with this latest project likewise indulging her taste for folk music's darker, uneasier dimensions - for the actual truths concealed within changeling legends are painful indeed.

Newton says: "The changeling is basically the explanation that people came up with for children born with physical or learning disabilities, or other behavioural problems.

"There are also songs and stories - including one I'm using in the piece, The Queen Of Elfan's Nourice - where the fairies steal new mothers away to be wet-nurses, because human breast-milk is better for their babies, and these were presumably a response to things like post-natal depression."

In the case of babies, customs dictated an array of charms and rituals to safeguard the newborn against abduction, and also of often brutal methods to make a changeling reveal its true nature (whereupon it would be spirited back to the supernatural realm and the human child returned), with the latter essentially codifying or euphemising the widespread incidence of abuse and infanticide involving disabled children.

Some of the ballads Newton has drawn on, however, unfold in a lighter vein, with parents winning back their offspring by tricking or outwitting the fairy kidnappers. She also views the body of changeling lore in a more artistic light: "A lot of the words and the emotions in these tales show how people have always found such beautiful creative ways of expressing or dealing with the unthinkable and inexplicable."

It's a process she herself has continued in the seven shorter pieces within Changeling, which include new settings of traditional ballads in both Gaelic and English; poems by Sydney Goodsir Smith and James Russell Lowell, reworked as songs, and three all-original instrumentals inspired by aspects of her theme.

"I'm really fascinated by the fact that all these magical stories came about because people took it as read that fairies existed," Newton says. "It really was the most logical explanation for so many aspects of their lives. And for me this kind of symbolism also relates to the way music works - its ability to convey things no words can explain."

Rachel Newton plays the Mitchell Theatre, Glasgow, on February 2 at 1pm