The vast majority of people with mental retardation have IQ levels of 55 to 69 and are considered mildly retarded. Mildly retarded children often go undiagnosed until they are well into their school years. They are often slower to walk, talk, and feed themselves than most other children.
When Tom Six began creating the Human Centipede sequels, he envisioned a trilogy that works as a movie centipede. Each sequel opens with the ending of the previous film, as the events of that film influence said sequel.
Each film has to have a human bad guy who throws a wrench into the plans of the human good guys*, because apparently giant robots aren't menacing enough on their own. And now we know who that person will be in part four: Kelsey Grammer.