Causal effect of education on IQ scores using the discordant twin design?

Recently, the twin-control design has been used to test causal models (e.g. exercise→happiness, exercise→¬depression/anxiety symptoms, casual sex→depression/suicidal thoughts). The theory is simple. Suppose we do a standard cross-sectional design study and find that X and Y are correlated. Suppose we suspect that X causes Y. Then, if X causes Y, then one would expect to…

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