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Description: [Current version submitted to BMC Public Health.] Background: Literature on persuasion suggests compliance increases when requests are accompanied with a reason (i.e. the “because-heuristic”). The reliability of outcomes in physical activity research is dependent on sufficient accelerometer wear-time. This study tested whether SMS reminders—especially those that provided a rationale—are associated with increased accelerometer wear-time. Methods: We conducted a within-trial partially randomised controlled trial during baseline data collection in a school-based physical activity intervention trial. Of 375 participants (mean age=18.1), 280 (75%) opted to receive daily SMS reminders to wear their accelerometers. These 280 participants were then randomised to receive either succinct reminders or reminders including a rationale. Data was analyzed across groups using both frequentist and Bayesian methods. Results: No differences in total accelerometer wear minutes were detected between the succinct reminder group (Mdn=4909, IQR=3429–5857) and the rationale group (Mdn=4808, IQR=3571–5743); W=8860, p=0.65, CI95=-280.90–447.20. Similarly, we found no differences in wear time between participants receiving SMS reminders (Mdn=4859, IQR=3527–5808) and those not receiving them (Mdn=5067, IQR=3201–5885); W=10642.5, p=0.77, CI95=-424.20–305.30. Bayesian ANOVA favored a model of equal weartime means, over one of unequal means, by a Bayes Factor of 12.05. Accumulated days of valid accelerometer wear data did not differ either. Equivalence testing indicated rejection of effects more extreme than a Cohen’s d (standardised mean difference) of ± ~0.3. Conclusions: This study casts doubt on the effectiveness of using the because-heuristic via SMS messaging, to promote accelerometer wear time among youth. The because-heuristic might be limited to face-to-face communication and situations where no intention for or commitment to the behavior has yet been made. Other explanations for null effects include non-reading of messages, and reminder messages undermining the self-reminding strategies which would occur naturally in the absence of reminders.