1. Institute of Cognitive Sciences and Technologies, Italian National Research Council, Rome 00185, Italy
2. Department of Ecology & Evolutionary Biology, Department of Mathematics, University of Tennessee, Knoxville, TN 37996-1610, USA
3. Graduate School of Business and Department of Psychology, Stanford University, Stanford, CA 94305, USA
4. Department of Anthropology, University College London, London WC1H 0BW, UK