What if the soul incarnates in a body which is mentally handicapped?

For those who believe in reincarnation, they say that after a long series of incarnating into various body forms, a soul enters a human body and finally has a chance to work out his karmic reactions because he has already acquired a consciousness that lower forms of creations does not have. if you were born mentally handicapped, are you still creating good and bad karma or are you in some kind of a vacation from karmic reactions??
question is for those WHO BELIEVE in reincarnation please. thank you =)