He has conducted fieldwork in Paraguay and Bolivia with Ache and Tsimane forager-horticulturalists since 1998. His research has focused on problems of intra-group cooperation and collective action, and the application of life history theory to better understand the evolution of human longevity, development, health and sociality. Since 2002, he has co-directed the Tsimane Health and Life History Project (w/ Hillard Kaplan).