Altruistic preferences have been found to be important for explaining the substantial nonuse values identifed in numerous stated preference surveys. However, studies analysing the efect of altruism on willingness to pay (WTP) have underestimated the challenges of measuring altruism by stated measures. We exploit a naturally occurring decision domain to investigate the role of altruism in stated preference studies. We employ a novel dataset, collected from an Internet survey panel, that contains respondents’ past donations of earned survey coins to charities and use these data to analyse the efect of donation behaviour on the same respondents’ WTP. We analyse donation behaviour across two contingent valuation surveys on environmental topics....