This study grounds a financial incentives intervention in well-documented age-related changes in motivation. The investigators consider these motivational shifts through the lens of socioemotional selectivity theory (SST, Carstensen, 2006). According to SST, perceived time horizons influence motivation across the life span. When time is perceived as expansive (typical of younger adults), exploration and achievement are prioritized, but when time horizons narrow (typical of older adults), emotional meaning and satisfaction are prioritized. Previous research has shown that socioemotional goals may influence decisions about healthcare plans and the effectiveness of motivational messages to increase physical activity (Löckenhoff \& Carstensen, 2007; Mikels et al., 2010; Notthoff \& Carstensen, 2013). Drawing on the tenets of SST, individuals may be more motivated to engage in physical activity when doing so aligns with socioemotional goals. For example, older adults may find physical activity most appealing when it promotes emotional meaning and satisfaction; in contrast, younger adults may find physical activity most appealing when it promotes exploration and achievement.

Recent work has shown that spending on others (termed "prosocial spending") can have greater emotional benefits than spending on oneself (Anik, Aknin, Norton, Dunn, \& Quoidbach, 2013; Dunn, Aknin, \& Norton, 2008). Extending this concept, the researchers posit that, in the context of financial incentives, earning for others may offer older adults a more emotionally meaningful alternative to individual financial incentives, which may, in turn, better support health behavior change for older adults. Because of the profound link between physical activity and risk for cardiovascular disease and stroke, and because previous research suggests that financial incentives are most effective for simple, well-defined, objectively measurable activities, the behavioral outcome of interest in this study focuses on walking measured by pedometer step counts (Scott \& Schurer, 2008). In addition, because SST postulates specifically about motivational priorities in younger relative to older adults, the investigators recruited adult participants of all ages. The researchers designed this study to test the effectiveness of prosocial relative to individual financial incentives for increasing walking in older and younger adults. Reasoning from SST, the investigators hypothesize that younger adults will be relatively more motivated by incentives that produce personal benefits (i.e. individual financial incentives), but older adults will be relatively more affected when incentives were more emotionally meaningful (i.e. benefit another person or social cause). The investigators also predict that, when given the choice of incentive, older adults will be more likely to choose to earn for others rather than to earn for themselves, relative to younger adults. In addition, the investigators will also test the effects of prosocial relative to individual financial incentives on intrinsic motivation for physical activity and behavioral maintenance (maintaining increased steps) once incentives are removed.

Community-residing adults aged 18 and older are recruited via advertisements on Internet bulletin boards and local newspapers to participate in a study that requires four visits to our laboratory and three weeks of pedometer wear in exchange for $50. Importantly, participants are unaware of the incentive component of this study. Once participants contact research staff to indicate their interest, they are phone screened for eligibility (see the eligibility criteria section of this entry).

During the first in-lab session, after providing informed consent, participants respond to a series of questionnaires using Qualtrics survey software. Participants are provided with a pedometer, instructed on its use, and asked to wear it each day. One week later, participants return to complete the second study session. Participants complete questionnaires while pedometer steps are downloaded. Participants are randomly assigned within age group and gender to one of five possible incentive conditions (which can be found in the arms and interventions section of this entry). One week later, participants return to the lab for the third study session where they complete questionnaires while pedometer steps are downloaded. All participants are told their average daily steps from the previous week and how this average relates to the first week's average. Participants in incentivized conditions who increase steps are told how much they earned. Participants in the personal condition and choice participants who elect to keep the earnings receive a check for the amount of their earnings. Participants in the loved one and charity conditions and choice participants who elect to give their earnings to a loved one or charity witness the research staff write a check for the amount of their earnings to their selected loved one or charity and enclose the check along with a cover letter into an envelope to be mailed. Participants in the incentivized conditions who do not increase steps were told that they did not earn any money. All participants in incentivized conditions are reminded that the incentive portion of the study is complete and that they will not earn money by changing steps the final week of the study. One week later, participants complete the final study session. Participants complete questionnaires while pedometer steps are downloaded. All participants are told their average daily steps from the previous week and how this average relates to the second week's average. At the end of the session, participants are debriefed, paid, and thanked for their participation.