This essay offers a unified philosophical account of observer effects in social science. When agents are aware their behavior is subject to scientific inquiry they often act in ways that render measurements of them unreliable. This is the problem of reflexive measurement. In order to develop this novel account, I provide a general characterization of reflexivity which encompasses the full scope of scientific practice: theorizing, prediction, measurement, etc. The characterization captures the insights of contemporary philosophers of science working on reflexive (...) prediction alongside observations by scientists grappling with observer-type effects in the course of their research. (shrink)