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This study assesses the factors influencing the movement of people across health plans. We distinguish three types of cost-related transitions: adverse selection, the movement of the less healthy to more generous plans; adverse retention, the tendency for people to stay where they are when they get sick; and aging in place, enrollees’ inertia in plan choice, leading plans with older enrollees to increase in relative cost over time. Using data from the Group Insurance Commission in Massachusetts, we show that adverse selection and aging in place are both quantitatively important. Either can materially impact equilibrium enrollments, especially when premiums to enrollees reflect these costs.