After participants had completed the initial survey, which took most of them at least 10 minutes, we measured their volunteering behaviour by asking them whether they would be willing to fill in a ‘a few extra questions’ for charity (59.3% responded yes) rather than skipping directly to the final questions. The participants were informed that their choice would be completely anonymous and that 5 SEK (around 0.50 euro or 50 US cents) would be donated to a charity organization of their choosing if they would fill out the additional questions.
This seems like a potentially meaningful measure. We need to read closely to consider the extent to which that 'desire for consistency with questionnaire response' could be driving/biasing this.