The thesis—or, more precisely, the theme—of "The Lottery" presents tradition as a destructive force in a community. In this story, the residents of a New England village cling to a tradition of annual human sacrifice. At one time, presumably before the advent of modern science and presumably when life was harder for the new settlers, the villagers fell back on the superstitious belief that they would have a good harvest if they selected and stoned to death one member of their community each June.
While the origins of the custom are lost in history, we know the tradition dates back to well before the birth of the oldest villager. Though the villagers must now know by the mid-twentieth century that there is no logical reason to maintain this custom, they do so anyway.
The story is chilling because this tradition has been so normalized. Children are let out of school and gather piles of stones on a beautiful day, as if this community killing is a happy and festive occasion. Tessie Hutchinson goes along with the lottery until she is chosen as its victim, at which point she decides it is unfair. This points to a second theme of the story: it is important to stand up to cruel practices, because you could easily become the next victim.