The Williams syndrome theory

The rare condition Williams Beurens  Syndrome was listed as of interest to scientists studying domestication because the people born with it are especially gregarious. The syndrome is described as a combination of about 26 missing genes.

Further reading after quick Internet searches revealed that  people with this syndrome are exceptionally kind, highly verbally skilled and they have a tendency to be famously musical and possess perfect pitch. The syndrome is characterized by a lack of elastin in the body which gives a certain look to the features.

The Wikipedia entry suggests that these people were thought of as magical because they were so kind and trusting , friendly and musical. They might have been one source of fairy legends.