Salt preferences, which vary widely across individuals, are a function of past exposure to both levels of dietary salt and dehydration. From an evolutionary perspective, such plasticity is puzzling, as the health costs of high salt intake, combined with the increased time and energy needed to obtain large quantities of salt under ancestral conditions, suggest that natural selection should have eliminated the plasticity in preferences that can produce such behavior. This puzzle is resolved once it is recognized that high salt intake may provide protection against sudden dehydration, a benefit that outweighs the costs associated with this pattern. It is proposed that an adaptive mechanism calibrates salt preferences as a function of the risk of dehydration as indexed by past dehydration events and maternal salt intake.