20 Things You Somehow Missed In Friday The 13th (1980)
17. Mrs Voorhees's Unspoken Backstory
Although it is well-known that Betsy Palmer only took the role of Mrs Voorhees to buy herself a new car, she was still determined to give an authentic performance.
Without much information to go on, she used what she knew about the character to develop Mrs Voorhees's past and motivations, reasoning that Mrs Voorhees wore a class ring because she had fallen in love with her high school boyfriend and had accidentally become pregnant. Her boyfriend subsequently deserted her and her parents disowned her when they learned the truth.
Mrs Voorhees then went from job to job, an outcast in society, before becoming the cook at Camp Crystal Lake to support herself and her son, Jason. She snapped when Jason drowned there in 1957, pinning all of her troubles on sex and therefore seeking to stamp it out wherever she found it, whilst also ensuring that the camp never reopened.
Whilst her character was named Mrs Voorhees (the name "Pamela Voorhees" first appeared in 1984's Friday the 13th: The Final Chapter), it is possible that she simply assumed herself to be her absent boyfriend's wife.