As a result of being measured, individuals sometimes alter their behavior and attitudes to such extent that subsequent measurement results are affected. This ‘reactivity’ to measurement problematizes prediction and explanation, but some reactivity is nevertheless legitimate. Using the example of the measurement of race in the US Census, this article demonstrates that some forms of reactivity do not affect the accuracy of research. The article argues that legitimacy of reactivity depends on the metaphysical status of the phenomenon being measured. It (...) is argued that reactivity’s legitimacy is affected by the arbitrariness of a measure and the voluntariness of measurement results. (shrink)