This essay focuses on Lionel Shriver’s 2013 novel Big Brother. This novel describes the apparent efforts of its narrator, Pandora, to help her morbidly obese brother, Edison, lose weight. A final section of the book, however, reveals that Pandora has in fact fabricated large parts of her narrative, and in reality, she declined to help Edison, who subse-quently died. The essay situates Shriver’s novel as part of the resurgence of naturalism in contemporary American literature. Big Brother uses its unusual form to consider contested ideas associated with the degree to which humans have free will or—as in naturalism—their actions are prey to deterministic forces. Ultimately, this essay contends that Shriv-er’s use of elements of naturalism is broadly tendentious, especially in terms of its designating characters’ behavior as determined, which is used partly as a means for them to evade moral responsibility. Finally, the essay notes how this manoeuvre reflects Shriver’s unconvincing dis-course—articulated both in Big Brother and in interviews—regarding obesity, its causes, and potential solutions.