[Richard Kimball]

She was. There are three versions as to why. One is that she was mentally retarded. Another is that she had violent outbursts and was unstable. A third is that she was sexually promiscuous and that Joe feared a scandal from her getting pregnant. She was apparently slow, but prior to her lobotomy, she could multiply and divide, and kept a diary, and attended parties. After the procedure, she was incontinent and almost an infant.

It's a tragic and scary story. Apparently, Joe was told that the procedure would calm her down and make her more easy going.

[IndyTiger]

I remember reading once speculation that Rosemary was brain damaged at childbirth due to a forceps delivery. In any event, you are right that she was having violent temper outbursts. In that competitive family, she no doubt could see that she was not as bright or accomplished as her older siblings Joe, Jack, and Kathleen, and this only fueled her resentment.

Joe Sr. thought the lobotomy would calm her down, but the procedure went too far. It is a sad and tragic story.

[Theophilus]

A third is that she was sexually promiscuous and that Joe feared a scandal from her getting pregnant.

She should have been chaste and virtuous like her brothers.