Abstract Contingent valuation has been given a psychological interpretation, by Kahneman and colleagues, that claims willingness to pay bids represent psychological attitudes rather than personal economic valuations. Evidence reported here shows the need to qualify the role of this attitudinal explanation. In contradiction to the attitudinal hypothesis, the decision to bid zero or positive appears to represent a complex psychological appraisal. Furthermore, evidence of bid clustering on currency denominations implies fundamental differences concerning how people respond to a monetary scale. Whether interpreted as charitable contributions or imprecise welfare estimates there are serious implications for how economists interpret and use stated preference responses.