Little research has deliberately investigated the effects of participant preferences for treatment condition in unblinded randomized controlled trials. We designed a study with a non-patient sample comparing a randomized arm to a preference arm of the same trial to investigate: (1) whether having a choice to select one's preference affects feelings about participation, belief in treatment effectiveness, treatment contamination, intervention adherence and engagement, and trial attrition; and (2) the interaction of preferences and treatment assignment on these variables. Contamination and attrition were rare and excluded from analyses. There was no effect of choice. Participants mismatched to preference felt less positive about their experience, but this did not affect belief in treatment, adherence, or engagement. Stronger effects may occur for patient populations.