A state can attach high value to something that would otherwise be worthless if it is judged to be of heritage or strategic value. Environmental valuation is also likely to vary between countries at different stages of development, and in a given nation through history. It is also likely that in seeking agreement on transnational issues there will be different views and priorities. So environmental valuation is not absolute. Carson (2004) provides a bibliography and case studies of contingent valuation use in over 100 countries.