Relative willingness to pay and surplus comparison mechanism in experimental auctions

We study the relative willingness-to-pay (WTP) of consumers according to the diversity of supply in a market and we show how the presence of substitutes for a given product leads to question the incentive mechanisms commonly used in experimental auctions. We propose a Surplus Comparison Mechanism (SCM) in order to yield WTP estimates which better take into account the choice set available to consumers. After showing the efficiency of this mechanism we test the SCM in a laboratory experiment, reconsidering WTP for food environmental certifications (Integrated Pest Management and Organic certification). It appears that WTPs are decreasing when more alternative certifications are offered to consumers