One of these has been the history and evolution of early human kinship systems. A second interest has been to apply the methods of historical reconstruction from linguistic evidence to issues in anthropological theory and in world history. He has also collaborated with geneticists in seeking to correlate linguistic with genetic findings (e.g., Sarah A. Tishkoff, Floyd A. Reed, F. R. Friedlaender, Christopher Ehret, Alessia Ranciaro, et al., "The Genetic Structure and History of Africans and African Americans", Science 324, 22 May 2009) and in developing mathematical tools for dating linguistic history (e.g., Andrew Kitchen, Christopher Ehret, Shiferew Assefa, and Connie Mulligan, "Bayesian phylogenetic analysis of Semitic languages identifies an Early Bronze Age origin of Semitic in the Near East," Proceedings of the Royal Society B: Biological Sciences, July 2009).