As her undergraduate thesis topic, Kagan chose to write about the demise of the American socialist movement, a story which she called “a sad but also a chastening one for those who, more than half a century after socialism’s decline, still wish to change America.... In unity lies their only hope.”
She explained in the acknowledgements that her brother’s “involvement in radical causes led me to explore the history of American radicalism in the hope of clarifying my own political ideas.”
And Michael Goldfarb has more, from May 8, 2010, "Elena Kagan, Radical?":
Yesterday THE WEEKLY STANDARD obtained a copy of Elena Kagan's senior thesis, written almost thirty years ago while an undergraduate at Princeton. The title of the thesis: "To the Final Conflict: Socialism in New York City, 1900-1933."
Plus, from May 8, 2009, "Young Elena Kagan: Hoping For A 'More Leftist Left'."