Evolutionary psychologists, among others, have used a method called “reverse engineering” to uncover (a) whether a trait was selected for, and (b) if so, why that trait was selected for. In this paper I argue that reverse engineering cannot deliver on either (a) or (b), and tends to pervert, rather than enhance, our knowledge of natural history. In particular, I expose as false a fundamental assumption of reverse engineering—namely, that all traits selected for a particular function will share some nontrivial properties.