  In medicine, patients can obtain real benefits from a sham treatment. These
benefits are known as the placebo effect. We report two experiments (Experiment
I: N=369; Experiment II: N=100) demonstrating a placebo effect in adaptive
interfaces. Participants were asked to solve word puzzles while being supported
by no system or an adaptive AI interface. All participants experienced the same
word puzzle difficulty and had no support from an AI throughout the
experiments. Our results showed that the belief of receiving adaptive AI
support increases expectations regarding the participant's own task
performance, sustained after interaction. These expectations were positively
correlated to performance, as indicated by the number of solved word puzzles.
We integrate our findings into technological acceptance theories and discuss
implications for the future assessment of AI-based user interfaces and novel
technologies. We argue that system descriptions can elicit placebo effects
through user expectations biasing the results of user-centered studies.
