The book, "Sins of the Father" by Ronald Kessler explains Rosemary's problem the best.
She was not mentally-ill, she had a learning disability and she was sort of a klutz. When the Kennedy's would all gather on the lawn to play sports, Rosemary did not excel. And old Joe thought that was not normal and he could not fathom having someone in the family who was not athletically perfect. (Eugenics much?)
Some have written that she was mentally retarded, however she could do higher-level math with ease.
What old Joe worried about was that she was so beautiful, he thought that she would become a loose woman and "embarrass" him by getting pregnant and flaunting herself. She did tend to be a bit erratic at times, but then if you were the family klutz in a family where physicality was a top priority, you'd be a bit upset too.
Because she was deemed a "problem" he decided to act without his wife's knowledge and found a doctor who would lobotomize her in order to straighten her out. The procedure caused her to become mentally incapacitated.
Yes, her father paid for someone to cripple his daughter mentally.
So, the Special Olympics, supposedly created in the honor of a mentally retarded Rosemary Kennedy, has a very crippled foundation.
I read “Sins of the Father,” by Ron Kessler a while back. He had to be the biggest scoundrel of his era. He was an unapologetic adulterer even regularly bringing his paramours to the Kennedy Compound in Hyannis Port. Rose even became close friends with some of them. Although he made a fortune bootlegging and importing during Prohibition, he wasn’t an alcoholic. He usually limited himself to one after-dinner Irish whiskey on the rocks. He also made a fortune in the manipulation of securities.
I finished it in three evenings. Fascinating, but sad and anger-provoking. I needn’t dwell on Rosemary as Slyfox covered that ground completely and accurately.