He is a social and economic demographer whose current research focuses on health, demography and social change in developing and developed countries. A key characteristic of his research is the attempt to integrate demographic, economic, sociological and biological approaches in empirical and theoretical models of health and demographic behaviors. In his prior work, he has investigated the role of social and sexual networks for HIV risk perceptions and HIV infection risks, the causal effects of education on health, the consequences of learning one's HIV status on risky behaviors, the interrelations between marriage and sexual relations in developing countries, the role of social interaction processes for fertility and AIDS-related behaviors, processes of aging in low-income populations, the determinants and consequences of low fertility in developed countries, and global family change. His research combines extensive knowledge about the life-course determinants of family change, health, fertility/mortality, social networks, and related economic behaviors in developing and developed countries with considerable experience in sophisticated econometric and demographic analyses, including analyses with controls for endowment and unobserved determinants of individuals' behaviors, models of population and disease dynamics, randomized designs and integration of social science and biomedical research methods. His research has been funded by the National Institutes of Health (NIH), the National Science Foundation (NSF), the Bill and Melinda Gates Foundation and others foundations. He has been awarded the Clifford C.\ Clogg Award for Early Career Achievement by the Population Association of America for his interdisciplinary work on fertility and health, and has been honored with Otis Dudley Duncan Award for Outstanding Scholarship in Social Demography by the American Sociological Association. He has been a Fellow at the Center for Advanced Studies at the Norwegian Academy of Science, served as the president of the Society of Biodemography and Social Biology, and was engaged as lead-paper author in the Copenhagen Consensus Project to evaluate policies to prevent the sexual transmission of HIV (2011, with Behrman), reduce population growth (2012), and the post-2015 UN Development Goals in the area of Population and Demography. He served as the Chair of Penn's Ph.D.\ Program in Demography and the NICHD T32 Training Director for this program (successfully renewed in 2012), and co-direct the Population Aging Research Center (PARC) at Penn. He has extensive experience in the design and implementation of large-scale data collection in sub-Saharan contexts, and he directs Malawi Longitudinal Study of Families and Health (MLSFH) that documents more than two decades of demographic, socioeconomic and health conditions in one of the world's poorest countries.