The sunk costs effect is defined as the tendency to persist in an endeavor once an investment has been made. This project evaluated whether this effect occurs after making a behavioral investment and if forced and free responses have similar effects upon subsequent behavioral persistence. Subjects, 90 students, divided in three groups, were exposed to three conditions of 0, 60 and 110 forced responses in FR 350 schedules. Persistence increased as free responses increased, suggesting that the sunk cost effect is produced by a freely chosen behavioral investment. It was concluded that the sunk cost effect is produced by a behavioral investment and that happens only if each instance of investment is freely chosen