How, in general, do you determine whether X causes Y or Y causes X?

How, in general, do you determine whether X causes Y or Y causes X?



Answer: You run a cross-lagged study. You measure both variables at two (or more) points of time, using the same subjects at both times so that you now have (at least) two pairs of X & Y for every subject. If X is the cause of Y, then the correlation between X-at- time-1 and Y-at-time-2 (hereafter: X1-Y2) will be stronger than the correlation between Y-at-time-1 and X-at-time-2 (Y1-X2). On the other hand, if Y is the cause of X, then the Y1-X2 correlation will be stronger than the X1-Y2 correlation. In general, because we think that causation always happens forward in time, the cause should be measured before the effect to get the best correlation.

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