Internal and external scope in willingness-to-pay estimates for threatenedand endangered wildlife

Economic theory suggests willingness-to-pay (WTP) should be significantly h igher for a comprehensive good than for a subset of that good. We tested th is using both a split sample design (external scope test) and paired respon ses (internal scope test) for WTP for several endangered fish and wildlife species in the US. In the paired response case we corrected for correlation of willingness-to-pay responses using a bivariate probit model. Surprising ly, the independent split samples passed the scope test but the paired samp les did not As the results contradict each other questions of validity for policy implications are raised. However using either approach, the benefit of maintaining critical habitat for these species exceeds the costs. (C) 19 99 Academic Press.