They did know how painful it was beforehand, and 100% of participants said they would pay to not be shocked again. Notably, it was a very small sample size for the shock study, 18 men and 24 women, but similar "sit there and do nothing" studies were done across all age groups (18-77)

Source: https://news.virginia.edu/content/doing-something-better-doing-nothing-most-people-study-shows