Gillian Bentley is a biological anthropologist and reproductive ecologist and a Royal Society Research Fellow at University College London. Her prior work focused on explaining why different human populations occupying a range of environments have varying levels of reproductive hormones. She now directs projects that interface with reproduction and reproductive health, working with the migrant Bangladeshi community in London. Recent publications include Infertility in the Modern World: Present and Future Prospects, edited with C.G.N. Mascie-Taylor (Cambridge University Press, 2000). Ruth Mace is Professor of Evolutionary Anthropology at University College London. She works on the evolutionary ecology of social and subsistence systems. Particular interests include parental investment, mainly in African populations but also in the UK, and also macro-evolutionary studies on the evolution of cultural diversity. Recent publications include The Evolution of Cultural Diversity: A Phylogenetic Approach, edited with C. Holden and S. Shennan (UCL Press, 2005).