Most people who believe in faeries, and would even go so far as to seek their company, believe them to be creatures of light and love, who want nothing but the best for their human counterparts. Many Wiccan's will actually go so far as to invite them to join in with a ritual.

However, the undeniable truth about the fae is that historically they were creatures that were feared and loathed. They were blamed for many things that went wrong and were avoided at all costs.

An article I came across had this to say about their history:

"Like today’s aliens, fairies were believed to have deployed many devious methods to abduct human babies. In one story from Denmark, a troll kidnaps a pregnant woman and replaces her with a doppelgänger to cover his crime. The tale begins when a blacksmith working at his forge late one evening sees a troll driving a woman along the road. The blacksmith uses a red-hot iron to scare away the troll. When he takes the rescued woman to his house she immediately gives birth to twins. Thinking the woman’s husband will want to know what has happened, the blacksmith goes to their home the next morning. Much to his surprise he finds the man in bed with a woman who is the exact image of his wife. The blacksmith, knowing that this woman is a fake provided by the troll, kills her with a blow from his axe; after which, the man is reunited with his wife and the two new additions to his family. [5]

In the Orkney Islands they sometimes call the fairy folk ‘trows’. Trows are ugly little creatures, thought to live in the ancient mounds on the islands. Their homes are large, well lit, lavishly decorated and always contain plenty of fine foodstuffs. Generally, the trows are mischievous creatures that like to play tricks on people; they were mainly feared for their tendency to steal human babies. The trows’ own children were weak and sickly, so at any opportunity they would swap them for healthy human infants. To prevent this, women were carefully guarded by family and friends, both while they were pregnant and after giving birth.

When trows abducted adults (or animals) they would cover their crime by replacing them with exact replicas. These replicas were called ‘stocks’ and were usually left in the person’s bed (or animal’s stall) to hide the act. [6]

In many regions, it was thought that an infant was particularly vulnerable to being replaced by a fairy changeling before it was christened. To ward off this danger, open scissors would be placed on the baby when it was left alone; alternatively, a cross made out of mountain ash or a red thread placed round its neck were probably safer options. In Ireland, according to George Waldron, you can get rid of a changeling baby by putting a mixture of oatmeal and boiling digitalis in its mouth, or by putting it on a red-hot shovel; needless to say, neither of these courses of treatment should be tried at home! [7]

If parents believed their child was a changeling, they had an excuse to beat, whip and torture it to force the fairy parents to return the real human infant. In the 16th century, it was feared that some humans would actually help the fairies to steal babies; as a result, midwives had to swear not to substitute babies or to use magical incantations during childbirth. As David Sivier put it:
“These beliefs have the function of explaining the occurrence of deformed children and assuaging the grief felt by their parents when they eventually pass away. After all, if the children were really malicious spirits, and not the couple’s own children, then there was no point in grieving over their deaths. On the contrary, if the creatures were evil, their final demise should be a cause of celebration.” [8]

Today, instead of fairies, it’s often the authorities who are blamed for swapping babies. One such case in the 1990s involved a couple in London, who had a Downs Syndrome baby they would not accept as their own. The mother had given birth to four healthy babies previously, and could not believe that her fifth was any different. Both mother and father thought that the hospital staff had swapped this baby with their healthy one. [9]"

Bibliography:

Doppelgänger Effect - Doppelgängers in old fairy lore, modern science fiction and ufology
Nigel Watson / Images: Jonathan Burton
November 2007

http://www.forteantimes.com/features/articles/903/doppelgnger_effect.html