I don’t know what study you are referencing, but in ALL the behavior analytic literature on “self-control” that I know of, the experimenters provide the reinforcement as part of the study. Not only that, but there are usually repeated trials with the same individual to establish a baseline and some sort of experimental treatment to see if self-control-related behaviors can be shaped. If the research is done with individuals who are children or have Developmental Disabilities, they usually ask that whatever the reinforcers are be restricted for some amount of time prior to experimental sessions to minimize outside influence on the study. So, families are usually asked not to give that specific candy, or whatever, to the individual during the trial, but there will be a handful of opportunities to receive some.