Frankenstein is a birth-fantasy. Since Mary Shelly was already a mother at age 19, would she have really written a fantasy about giving birth, about creating life?
Percy Shelly was envious of Mary Shelly’s parentage: her mother Mary Wollstonecraft wrote Maria: the Wrongs of Woman, an early novel of the feminist movement (about 1797), while Mary’s father was the philosopher William Godwin. Percy thought that with parents like those, Mary should of course be an author. (If you want some wild tales, read about this family. Her mother, Mary Wollstonecraft, as an early feminist, was clearly sleeping around and loathe to marry Godwin, doing so only a few months before Mary was born; Percy had two women pregnant at once; Percy’s wife killed herself, enabling him to marry Mary, etc.) Anyway, Percy clearly wanted his wife to be an author, and may have helped her extensively.
Another argument that Frankenstein was written by a man has to do with the characterizations of men and women. It’s a story of one man, the doctor, telling the tale to another man, a ship’s captain, about creating and chasing the (male) monster. The female characters are poorly drawn stick figures: the virginal wife of the doctor, for example. There are few other examples of 19th century tales – written by women – that so emphasize male characters over female: Silas Marner by George Elliott comes to mind. Even a story like Adam Bede is far more about the women involved than the male of the title.
I would also add that, not only did Percy Shelley admit to writing the preface, but also that it’s written in the first person, “My story… my chief concern…”
In all fairness to Mary Shelley, however, the Frankenstein we are familiar with is not the 1818 version, but the 1834 revision which occurred after Percy Shelley died. And she did manage to make a living with her writing, although much of it was about her travels. I’d like to see someone do a computer analysis of the early version of Frankenstein against her writings after Percy Shelley died, to see how the word use compares.
Interesting question, though, isn’t it?