Mary Shelley's husband was Percy Bysshe Shelley, a well known poet. Many have wondered how much influence Mary Shelley's husband had on her writing. In 2008, Professor Charles Robinson of the University of Delaware published the book The Original Frankenstein, where he argues that Percy Bysshe Shelley made significant changes and additions to the novel.
According to Professor Robinson, Percy Bysshe Shelley changed descriptions that clarified Mary Shelley's writing. He offered vocabulary and added or took away phrases to make the writing clearer. Based on his close study of the original texts, he believes it is obvious they were writing together, even with the same pen.
Other scholars believe he only assisted in editing and shaping the story, while the crux of the story was written by Mary Shelley.
In Mary Shelley's own words, she tells the story of an incredibly wet summer stranded in the home of Lord Byron. In their boredom, Mary Shelley, her husband Percy, Lord Byron, and a fourth person, John William Polidori, challenged each other to write ghost stories. Mary only meant to write a short story, but her husband encouraged her to keep writing.
"At first I thought but of a few pages of a short tale; but Shelley urged me to develope the idea at greater length. I certainly did not owe the suggestion of one incident, nor scarcely of one train of feeling, to my husband, and yet but for his incitement, it would never have taken the form in which it was presented. From this declaration I must except the preface. As far as I can recollect, it was entirely written by him."
From the above we can gather that Percy Bysshe Shelley, as well as Lord Byron and John Polidori, gave each other the idea to write. Percy's confidence in her story and encouragement caused her to keep writing until we have the novel we know today as Frankenstein.