Systematic reviews of gamification research often focus on effects on motivation and engagement. Fewer studies systematically investigate the effect of gamification on 'adherence', the extent to which individuals use a gamified service and experience its content, as envisioned by the creators, to derive a certain benefit. To this end, this paper presents a systematic review of the effect of gamification on adherence across disciplines, including only studies with experimental designs and empirical measurements of adherence. The results lend support to the hypothesis that gamification has a positive effect on adherence: 19 out of 27 studies report a significantly positive effect or trend. However, we also demonstrate that further debate is necessary on how to conceptualize and measure adherence across disciplines, and suggest that studies on gamification on adherence not only measure the usage behavior of individuals, but equally provide an a priori operationalization of intended use along with a justification.