1. Department of Population Health; London School of Hygiene and Tropical Medicine; Keppel Street London WC1E 7HT U.K
2. Department of Sociology; University of Groningen; Grote Rozenstraat 31 9712 TS Groningen The Netherlands
3. Department of Psychology; University of Lethbridge; Lethbridge Alberta T1K 3M4 Canada
4. Applied Behavioural Ecology and Ecosystems Research Unit; UNISA; Johannesburg South Africa