What Kind of Thing Is Your Construct: Reflective, Formative, or Fiction?

What kind of thing is your construct? We rarely ask, even while certifying its measures reliable and predictive. Measurement theory gives three structurally distinct answers: reflective, a real entity generating interchangeable indicators; formative, an emergent composite of distinct components; and measurement fiction, a label for correlated phenomena with no unified cause. Socioeconomic status, collective efficacy, and social capital, among others, could be any of the three, because all three can produce comparable aggregate signatures (adequate loadings, good fit, significant prediction); the standard validation toolkit, built to assume reflective structure, cannot reliably tell which. We interrogate that assumption at escalating stringency on self-control across 30 countries (ISRD-2), culminating in a predictive concordance test of whether supposedly interchangeable items identify the same individuals. Evidence rules out a unidimensional reflective trait yet cannot separate formative from fiction. We offer a three-question diagnostic and four research designs to avoid reifying indices as unified traits.