A Good Instrument is a Bad Control: Part II

At a recent seminar dinner the conversation drifted to causal inference, and I mentioned my dream of one day producing a Lady Gaga parody music video called “Bad Control”. 1 A lively discussion of bad controls ensued, during which I offered one of my favorite examples: a good instrument is a bad control . To summarize that earlier post: including a valid instrumental variable as a control variable can only amplify the bias on the coefficient for our endogenous regressor of interest. When used as