Composed of scientific and technical experts and lay members, thousands of research ethics committees—Institutional Review Boards in the United States—must identify and assess the potential risks to human research subjects, and balance those risks against the potential benefits of the research. IRBs handle risk and its uncertainty by adopting a version of the precautionary principle. To assess scientific merit, IRBs use a tacit ``sanguinity principle,'' which treats uncertainty as inevitable, even desirable, in scientific progress. In balancing human subjects risks and (...) scientific benefits, IRBs use uncertainty as a boundary-ordering device that allows the mediation of the science and ethics aspects of their decisions. One effect is the entangling of methodological and ethical review. Some have suggested these should be more clearly separated, but decisions by research ethics committees depend in part on the negotiating space created by incommensurable approaches to uncertainty. (shrink)