At what point does the lady go insane?

I would say it is when the narrator completely identifies herself with the woman imprisoned in the wallpaper.

And what do you think caused her to think this?

The Yellow Wallpaper

In the story, wallpaper, a usually feminine, floral decoration on the interior of walls, is a symbol of female imprisonment within the domestic sphere. Over the course of the story, the wallpaper becomes a text of sorts through which the narrator exercises her literary imagination and identifies with a feminist double figure.

When John curbs her creativity and writing, the narrator takes it upon herself to make some sense of the wallpaper. She reverses her initial feeling of being watched by the wallpaper and starts actively studying and decoding its meaning. She untangles its chaotic pattern and locates the figure of a woman struggling to break free from the bars in the pattern. Over time, as her insanity deepens, she identifies completely with this woman and believes that she, too, is trapped within the wallpaper. When she tears down the wallpaper over her last couple of nights, she believes that she has finally broken out of the wallpaper within which John has imprisoned her. The wallpaper's yellow color has many possible associations - with jaundiced sickness, with discriminated-against minorities of the time (especially the Chinese), and with the rigid oppression of masculine sunlight. By tearing it down, the narrator emerges from the wallpaper and asserts her own identity, albeit a somewhat confused, insane one. Though she must crawl around the room, as the woman in the wallpaper crawls around, this "creeping" is the first stage in a feminist uprising.

Night comes, and the narrator is alone. She locks the door to the nursery and throws the key down into the front path. She wants to astonish John by capturing the woman in the wallpaper and proving that her delusions are real. She has a rope to tie up the woman in case she tries to get away. The narrator continues to strip off the wallpaper, but she cannot reach high up along the wall, and she cannot move the bed to help her. She pulls off what she can reach, and hears within the pattern the "strangled heads and bulbous eyes and fungus growths...shriek with derision."

Frustrated and angry, the narrator wants to jump out the window, but the bars are solid, and she realizes that an action like that might be "misconstrued." Besides, she is afraid to go outside or even look out the window because of all of the women who are creeping about. She wonders if they came out of the wallpaper as she did. She ties herself up with the rope. Though she enjoys creeping about the room, she thinks she will have to get back inside the wallpaper when it "comes night."

John comes home and tries to open the locked door. The narrator tells him where the key is, and he finds it and enters. He asks her what she is doing as she creeps around. She tells him that she has finally gotten out of the wallpaper despite him and Jennie, and that she has pulled off most of the wallpaper so they cannot put her back. John faints, and the narrator keeps creeping over him as she goes around the room.

Analysis

The narrator's insanity climaxes as she identifies completely with the woman in the wallpaper. She believes that not only has the woman come out of the wallpaper, but so has she. Again, the symbolic meaning is that both she and the woman have liberated themselves from masculine oppression; by tearing out of the domesticated prison of the wallpaper, they are free. This moment of liberation again occurs by moonlight when, according to the motif Gilman has drawn, women enjoy a break from the oppression of masculine sunshine.

With her statement that she has gotten out of the wallpaper despite John and Jennie, she suggests that not only her husband, but also the representation of ideal domesticity (in the form of Jennie) has contributed to her imprisonment. She has allowed John and social expectations to dominate her and curb her freedom, but this new self - one made up of the woman in the wallpaper and all the other women she sees "creeping" about - has broken free.

With this in mind, however, the verb "creeping" is an odd choice for this act of breaking free. Creeping - either crawling or walking while hunched over - implies a gesture of subservience. The narrator (and the women creeping outside) is always afraid of being caught, so she must creep about. This may indicate that early feminism needed to "creep" about secretly before it could be respected and acknowledged by the rest of society. The multitudes of women that the narrator sees are perhaps these early practitioners of feminism, who draw strength in their numbers and who, having crept out of the wallpaper, now creep outside.

The narrator’s use of the term “work” is also significant in this context. She approaches the destruction of the wallpaper as “work,” a job that must be done, yet this in itself is contrary to the expectations of society. The narrator brings a strictly masculine activity into the realm of the domestic sphere and uses it to destroy the oppressive wallpaper. With that in mind, Gilman suggests that the way for women to overcome the oppression of a paternalistic society is to assume the roles of men in the public sphere.

Gilman also drops clues in this section to suggest that the nursery may have been previously used to house the insane. The narrator’s total insanity, of course, makes this more evident. The bars on the window are to prevent someone from jumping out, as the narrator contemplates doing; the immovable bed is "fairly gnawed" (and the narrator bites it, too); and the strange mark around the periphery of the room may be from someone else's act of crawling about.

There is one final irony that avenges the narrator's insanity: John's fainting is a stereotypically feminine show of weakness. The narrator finally achieves an authoritative position in her marriage, with John unconscious and her creative imagination finally free of all restraints. Her continual “creeping” over his prone body serves as a repeated emphasis of this liberation, almost as if the narrator chooses to climb over him to highlight his inferiority over and over again.