Her father arranged for her to be lobotimized because she was promiscuous and Joe Kennedy feared she'd so something that might hurt his sons' images and keep them from ascending in politics. PBS's American Experience did a special on lobotomies.
I remember seeing this movie years ago, and was thrilled to see it available for instant streaming on Netflix.
Lobotomy was never used for mental retardation, and examinations of school papers and the ages at which Rosemary did them shows her I. Q, was at least 90, well above retardation level. This was one of the biggest mental health coverups in history. Her mother's book says it was a family decision, which is a lie--her father had it done when her mother was out of town and she never forgave him.
Interestingly, she lived longer than many of the others. She was the fifth of the nine children to die, but the first of natural causes.
Yes, Joe Kennedy apparently did many sinister things, and poor Rosemary was silenced with a lobotomy for reasons we may never know. I remember reading that she was one of the first lobotomy cases at Danvers State, where it was "perfected". Actually, it was very common to put a woman away for making trouble. All you needed was two male members of the family to sign commitment papers. It happened to my grandmother in 1958.