Experimental Design Details
In March 2021, we conduct an online survey experiment with the following details. We recruit totally 1,600 survey respondents of young people (25-34 years old) and elderly people (65-74 years old) residing throughout Japan from the response monitors of a survey company. We randomly divide the survey respondents into four groups. In the survey, we set a hypothetical question to capture the respondents’ willingness-to pay (WTP) for the COVID-19 vaccine, while displaying messages by group. The control group provides an objective explanation of the vaccine. In addition to the explanation, the three treatment groups display the following nudge-based messages, respectively: “6 to 7 out of 10 people in your age group answered that they would receive this vaccine” (for the young respondents in the treatment group A), “7 to 8 out of 10 people in your age group answered that they would receive this vaccine” (for the elderly respondents in the treatment group A), “The more people who receive this vaccine, the more people who have an intention to do so. Your vaccination behavior can encourage the vaccination behavior of the people around you” (for the treatment group B), and “The more people who receive this vaccine, the more people who have an intention to do so. If you do not receive the vaccine, the people around you also may not do so.” Before and after the hypothetical question, we set questions to capture the respondents’ socio-economic attributes, psychological and behavioral economic characteristics, health status, infection prevention attitudes and behaviors, indicators of autonomous decision making, and attitudes toward the vaccine. At the end of the survey, we explain to the respondents that we have conducted a randomized controlled trial in the survey and its content and purpose. We then ask them if they would like to change their choice regarding whether or not to receive the vaccine.