The IRS requires study payments totaling $600 or more to be reported on tax returns. This includes all incentive payment methods (i.e. cash, check, Amazon gift code, etc).
If your cumulative annual payments equal or exceed $600 across studies, you will be sent a 1099 form at the beginning of the year. One copy will be sent to the participant and another to the IRS.
For further information regarding taxes, we recommend advising financial counsel for any further information regarding taxes.