There is a beautiful study where they gave half the people a lecture on the importance of getting a vaccination, and only 3 percent of the people went. They gave the other half the same lecture, but they also asked them to indicate on their calendar when they were going to go. In that group, 26 percent of the people went to get vaccinations. Not everybody wants to get vaccinated. But the idea is that we have intentions, and unless they get specified in a concrete way on the calendar, they’re unlikely to be carried out.
Dan Ariely speaking to Wired Magazine