Williams syndrome is a genetic learning disability characterized by extreme language fluency and volubility, good awareness of appropriate body language and emotions. Surprisingly, these signs of intelligence are paired with a subnormal IQ score between 40 and 70, emotional lability, lack of motoric dexterity and difficulties with mathematics. Typically, patients have elfin features: a wide mouth, a turned up nose and slightly bulgy eyes, supra valvular aortic stenosis (SVAS -- thickening of the arteries), high calcium levels at birth, hypersensitivity to sound and a generally retarded development.
The victims lack a piece of DNA on Chromosome 7. Their brains are 80% normal size, and have clusters of neurones in unusual spots. The victims are charmingly sociable people; parents worry about overfriendliness to strangers.