Participants and Procedures
WEST
Participants were a diverse sample of 162 West Coast community-dwelling adults recruited via flyers and web-based platforms (ie, Craigslist and Nextdoor) between September 2017 and September 2019. The posting advertised an opportunity to participate in a paid research study to develop more effective fitness trackers. To be eligible to participate, they had to meet the following criteria assessed via a web-based prescreening survey: walking as the primary source of physical activity in the previous 6 months (to ensure relevance of the step count manipulation), health status allowing for engagement in physical activity according to the Physical Activity Readiness Questionnaire [Each participant attended a personal onboarding and offboarding session in a laboratory of the Computer Science department at Stanford University at the start and end of their 5-week study participation (Study timeline.Throughout the following 5 weeks, participants’ step counts were tracked using the Apple Watch. In addition, participants completed weekly web-based surveys assessing affective and behavioral processes and daily web-based check-ins to ensure step count awareness. A researcher monitored participants’ survey response rates and watch activity to ensure study adherence. When step counts had not been uploaded to the cloud database for an extended time, researchers communicated with participants via SMS text message or email to remind them to wear the watch or assist with any technical issues. At the end of the 5 weeks, participants returned for the offboarding session, completing the same measures as in the onboarding session. They were then fully debriefed, thanked, and paid US $175 for satisfactory participation. Suspicions regarding the experimental manipulations and study purpose were probed with up to 10 progressively specific questions during poststudy interviews. The degree of suspicion was then rated by the researcher (1=not at all, 2=slightly, 3=somewhat, 4=very, and 5=extremely).A total of 45 participants were excluded from the analysis because of incomplete study participation (n=30, 67%), substantial suspicion about experimental manipulations (ie, correctly guessing inflation or deflation of step count, with suspicion rating >3; n=4, 9%), noncompliance (ie, using other activity-tracking apps or comparing step counts with friends; n=9, 20%, including 7, 16%, who also expressed substantial suspicion), or pregnancy discovered during the study period (n=2, 4%), yielding the final sample size of 162.Further details on the methods are included in 