>>2292375Honestly even Harukasan is shown to not be retarded so much as she is wild
In the manga it shows all the rest have been around civilization for a while when they are picked up whereas harukasan is the only one to come from somewhere isolated from people.
In the more recent chapters for example it shows that Ritsuko has had some success training her to be less retarded in general which would imply it is more lack of human contact
