Deposit? Yes, please! The effect of different modes of assigning reward-and deposit-based financial incentives on effort 305 9 External validity The real-effort task used in this experiment differs in many structural ways from health behaviour in the field,e.g: 1. the pay-off structure is completely transparent and predictable as opposed to field behaviour where benefits may be uncertain, 2. we use a one-time tedious task, whereas most field behaviour requires persistent effort, 3. field behaviour may involve additional benefits beyond just monetary incentives (e.g. better health) This (non-exhaustive) list of differences implies that we cannot rule out limited external validity for informing related work in modes of assigning incentives for health behaviour.29, 34, 35 Nevertheless, in designing our study, we deliberately designed effort-contingent rewards such that they bear similarities with health behaviour. For example, respondents completed an effortful task (sliders) at T1 to receive payment a week later at T2, which may be taken to resemble tradeoffs between effort provision now and (health) benefits later that are present in e.g. smoking cessation and when going to the gym. The findings of this study suggest that, at least for simple, tedious tasks completed in one session, choice between incentives (including advice) may be beneficial. Hence, our findings may apply more closely to field behaviour that is also simple and tedious (e.g. hand hygiene, see: Talbot et al.58) or for provision of incentives for one-shot decisions (e.g. vaccination, see: Campos-Mercade et al.59). Regardless, future work should extend our design to field behaviour to further explore the external validity or our findings. Limitations Besides taking into account potentially low external validity, the findings of this study should be interpreted in light of a set of limitations. First, the informed advice we provided respondents with was, in line with Boderie et al.36 based on a set of pragmatic cut-offs determined a priori, which raises several issues. For example, under the current specification a large majority of respondents was given the advice to take-up deposit-based incentives. This may be considered problematic, as in fact, in our study reward-based deposits dominate depositbased incentives, as respondents can only be equally good off and only if they complete all tasks (in all other cases they earn less for the same effort). In our
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