Relative to defined benefit (DB) plans, defined contribution (DC) plans have been linked to greater employee mobility. Because employees with different underlying mobility tendencies may sort across plans or firms, the relationship between plan type and mobility may be due to selection. We identify the role of selection by exploiting a natural experiment at an employer, in which the transition froma DB to aDCpension planwas affected by default rules.Using the default assignment as a source of exogenous variation in plan enrollment, we find that employees with higher mobility tendencies self-select into the DC plan. [ABSTRACT FROM AUTHOR]