1. Department of Spatial Sciences, University of Groningen, PO Box 800, NL-9700AV Groningen, The Netherlands, and Department of Social Sciences, Wageningen University, PO Box 8130, NL-6700 EW Wageningen, The Netherlands
2. Behavioural Science Institute, Radboud University Nijmegen, PO Box 9104, NL-6500 HE Nijmegen, The Netherlands