Perhaps she was naturally left-handed? There was a time when schools required left-handed children to learn to write with their right hand, as it was not considered acceptable to do otherwise. I speak of American schools since I can not say if this was normal in other countries. My aunt was forced to use her right hand, and when I started school my mother was informed they would do the same to me. Fortunately my mother refused and said she would remove me from school if it was attempted…and I was born after we lost Marilyn. There have been studies done that show forcing a child to change handedness can cause psychological damage. Marilyn did show many signs of a right-brained individual, or a lefty. I only hope this was not a cause of additional turmoil in her already difficult life.

FYI, this information is incorrect.
Marilyn was indeed left-handed.
Back then, the studio had total control over all images of their stars, and were very protective of images of Marilyn.
They did not want her to appear and anything than all-American, girl-next-door wholesomeness. As a result, when photos like those above were submitted, the studio would flip the negative so that Marilym appeared to be right handed. (They would even transfer her mole in the picture to the correct side of her face.)
Fact.
Just an FYI. ;)

Ritchie Jackson
Fox Studios