Microcephaly

Microcephaly is a condition in which your damn head is too small! It is a congenital disorder characterized by a radical deviation in size of parts or the entirety of the human head in proportion to the rest of the body. People who suffer from this condition are sometimes referred to as "Pinheads" (not to be confused with everyone's favorite dermatological train wreck, Pinhead, from the Hellraiser film series; whose head is perfectly sized by the way).

As Microcephaly is a physical abnormality, film characters who suffer from it usually appear in projects involving carnival sideshows or freak shows.