[1] CNA Corp, Resource Anal Div, Alexandria, VA 22311 USA
[2] Univ Canterbury, Dept Econ, Christchurch 1, New Zealand
[3] Cornell Univ, Dept Appl Econ & Management, Ithaca, NY USA
[4] Univ E Anglia, Jackson Environm Inst, Norwich NR4 7TJ, Norfolk, England
[5] Univ Victoria, Dept Econ, Victoria, BC V8W 2Y2, Canada
[6] Cornell Univ, Dept Appl Econ & Management, Ithaca, NY USA