The theory that Jenny was schizophrenic has been bantered around the board. Schizophrenia is a neurological condition that may be genetic; certain parts of the brain don't develop properly and by the time the person is a young adult, it starts to show in their behavior. Also, Jenny was traumatized by Quentin jilting her, the Collins family's ongoing hostility toward her, being in an unfamiliar environment (big, proper house on the hill instead of the presumably less-rigid Gypsy camp), being cut off from her own family, and learning she was pregnant. Naturally all of those things would cause her a great deal of stress and if Jenny was also schizophrenic, they would have contributed to her condition. After the babies were born, she might have suffered from post-partum psychosis as well (like Andrea Yates). Being shut away in an isolated drafty tower probably didn't help.
For one of my lower-division psychology classes, I once had to do a project where we chose a charcter from TV or film who exhibited a mental disorder, presented a clip to the class, and described their symptoms. I chose to show a scene from DS with Jenny and Beth. Jenny displays a number of common symptoms of schizophrenia: she believes people are out to get her, (e.g. Judith is jealous of her and has stolen her dresses) she hears and sees things that aren't real, (e.g. babies crying) her speech is disorganized, (e.g. "What color is a lie?") she doesn't groom herself, and she's restless--particularly at night. Whether this portrayal was intentional on the part of the writers and/or Marie Wallace or if they were just shooting for the standard image of the crazy lady, I don't know.
Anyway, that's one answer to your question.
ProfStokes
