When inferring causal effects from correlational data, a common practice by professional researchers but also lay people is to control for potential confounders. Inappropriate controls produce erroneous causal inferences. I model decision-makers who use endogenous observational data to learn actions’ causal effect on payoff-relevant outcomes. Different decision-maker types use different controls. Their resulting choices affect the very correlations they learn from, thus calling for equilibrium analysis of the steady-state welfare cost of bad controls. I obtain tight upper bounds on this cost. Equilibrium forces drastically reduce it when types’ sets of controls contain one another.