I suggest that we assess the value of public participation at least partly by its effect on the public’s individual and collective self-determination, measured by how well the science aids the public to rationally and effectively pursue its own ends. I explore two areas of expertise relevant to science’s ability to foster rational, effective self-determination: expertise in evidential reasoning and expertise in value identification. I describe ways in which public participation may introduce trade-offs between accurate reflection of public values and evidential quality and precision, where loss of either may lead to a failure of public self-determination.