Background: Educational attainment is associated with many life outcomes, including income, occupation and many health and lifestyle variables. Many researchers use it as a control variable in epidemiological and other social scientific studies, often without specifying exactly what environmental effects or set of personal characteristics is being controlled. Other researchers assume that genetically influenced intelligence drives educational attainment, and think that intelligence is the appropriate control variable. Researchers' different and often unstated causal assumptions can lead to very different analytical approaches and thus to very different results and interpretations.
Methods, results and conclusions: We document several examples of this important variation in the treatment of education and intelligence and their association. We recommend greater clarity in stating underlying assumptions and developing analytical approaches and greater objectivity in interpreting results. We discuss implications for study designs.