Stalling COVID-19 vaccination rates threaten public health. To increase vaccination rates, governments across the globe are considering using monetary incentives. We present evidence on the effect of guaranteed payments on COVID-19 vaccination uptake. We ran a large pre-registered randomized controlled trial (N = 8,286) in Sweden and linked the data to population-wide administrative vaccination records. We found that modest monetary payments of $24 (SEK 200) increased vaccination rates by 4.2 percentage points (p = 0.005), from a baseline rate of 71.6%. In contrast, behavioral nudges increased stated intentions to vaccinate but had only small and not statistically significant impacts on vaccination rates. The results highlight the potential of modest monetary incentives to increase vaccination rates.

In an anonymous 4-person economic game, participants contributed more money to a common project (i.e., cooperated) when required to decide quickly than when forced to delay their decision (Rand, Greene & Nowak, 2012), a pattern consistent with the social heuristics hypothesis proposed by Rand and colleagues. The results of studies using time pressure have been mixed, with some replication attempts observing similar patterns (e.g., and others observing null effects (e.g., Tinghög et al., 2013;Verkoeijen & Bouwmeester, 2014). This Registered Replication Report (RRR) assessed the size and variability of the effect of time pressure on cooperative decisions by combining 21 separate, preregistered replications of the critical conditions from Study 7 of the original article (Rand et al., 2012). The primary planned analysis used data from all participants who were randomly assigned to conditions and who met the protocol inclusion criteria (an intent-to-treat approach that included the 65.9% of participants in the time-pressure condition and 7.5% in the forced-delay condition who did not adhere to the time constraints), and we observed a difference in contributions of −0.37 percentage points compared with an 8.6 percentage point difference calculated from the original data. Analyzing the data as the original article did, including data only for participants who complied with the time constraints, the RRR observed a 10.37 percentage point difference in contributions compared with a 15.31 percentage point difference in the original study. In combination, the results of the intent-to-treat analysis and the compliant-only analysis are consistent with the presence of selection biases and the absence of a causal effect of time pressure on cooperation.

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