For a valid instrumental variable:
(1) the instrument must be correlated with the endogenous explanatory variables, and,
(2) the instrument cannot be correlated with the error term in the explanatory equation.
My question is about point 1. Does the direction of the correlation matter? Suppose I choose Z as an instrument for the endogenous variable X. Z is highly correlated with X (and seemingly unrelated to the outcome of interest), but in the opposite direction that I expected. My hunch is that it undermines my assumptions about point 2, but I wasn't quite sure.