I think that Plath develops Esther Greenwood's descent into insanity through showing how the social configuration of the time period does not embrace a thinking woman.  To a great extent, this descent is inevitable because Plath depicts a social world that is intolerant of a woman who seeks to break through the socially conditioned reality that envelops her.  The result of this collision between an irrepressible subjectivity and an adamantine social order is individual insanity.

Plath develops a dynamic of tension between Esther's subjectivity and the world around her in different settings. Existentially, there is significant textual support to show that Esther clearly realizes that she is different from the world around her and that this difference is the source of her pain and eventual descent into insanity.  Esther is not able to find herself fully integrated into the world around her.  She is unable to accept the constraints of the world around her, the conditions within which she must live.  She is not overly excited to be in the city and does not immediately take to what women are supposed to do in terms of playing a particular role, donning a mask, marrying, and having children. Rather, she sees herself as "different" in specific contexts throughout the narrative:  “I felt very still and empty, the way the eye of a tornado must feel, moving dully along in the middle of the surrounding hullabaloo."  The voice inside her that never relented was one that emphasized her difference in a world of conformity, her dissent in a world of ascent:  

That’s one of the reasons I never wanted to get married. The last thing I wanted was infinite security and to be the place an arrow shoots off from. I wanted change and excitement and to shoot off in all directions myself, like the colored arrows from a Fourth of July rocket.

Plath is able to showcase how a fundamental sense of distance from both herself and her world is the result of this collision between what Esther sees as mutually exclusive realities: "If neurotic is wanting two mutually exclusive things at one and the same time, then I'm neurotic as hell. I'll be flying back and forth between one mutually exclusive thing and another for the rest of my days."  Plath's depiction of Esther's reality as one where she cannot find peace within the world around her is a way she develops Esther's eventual descent into madness.

Plath shows that the existing social options that surround Esther are recipes for discontent.  Esther is unable to silence the voice of individual identity within her:  "I took a deep breath and listened to the old brag of my heart. I am, I am, I am.”  This notion of self is a powerful one and cannot be silenced by the social conformist idea of marriage and subjugation of self towards the role of domesticity.  There are several instances in which Plath shows that Esther cannot find any solace in simply being an ornament for the opposite sex: "There I went again, building up a glamorous picture of the man who would love me passionately the minute he met me, and all out of a few prosy nothings."  Plath shows that the traditional "prosy" or romantic constructions of the good will not appease Esther and help her find happiness.   When Esther describes sex, it is far from liberating, but actually reflective of the social condition that causes frustration, disinterest, and a feeling of being "different" in a world that does not tolerate it: "Then he just stood there in front of me and I kept on staring at him. The only thing I could think of was turkey neck and turkey gizzards and I felt very depressed."  At one point in the narrative, Jay Cee says that Esther wants "to be everything."  The fact that such a statement would be viewed as so very far from the realistic conditions that envelop Esther is another way in which Plath is able to show how insanity is inevitable.  It is through her being insane that Esther is able to fully understand her own definition of self as one apart from a social world that does not embrace difference or divergent notions of identity.  In drawing out the differences between Esther's subjective reality and the external conditions that envelop her, Plath is able to depict how insanity is inevitable for Esther.