University of Utah anthropologist Shane Macfarlan, shown here, is first author of a new study with provocative anthropologist Napoleon Chagnon about the Yanomamö, or so called “fierce people” of southern Venezuela and northern Brazil. Critics for years have claimed Chagnon places undue emphasis on biological and genetic roots of human violence, but he has insisted he holds a more nuanced view. The new study with Macfarlan found that Yanomamö cultural rules dictate how men in that society conducted lethal raids. Instead of fighting in so-called “fraternal interest groups” or “bands of brothers” and other close relatives from one village, the study found the Yanomamö often formed alliances with men in other villages and married their allies’ sisters and daughters, in effect becoming “bands of brothers-in-law.”Photo Credit: Lee J. Siegel, University of Utah