The first time she flies home for the holidays, Iris makes two friends. She is seated between a married couple, because the woman prefers the window and the man prefers the aisle, and they are the kind of people, she discovers quickly, with strong preferences. In general, Iris is the kind of person with mild preferences—preferences that can be painlessly ceded to someone else’s. When the woman clambers over her to use the bathroom, Iris presses her chin against her neck, closes her eyes, doesn’t mind at all.

She is eighteen, living half a country away from her parents.…