 Conditioning on B (for a specific value of B) often reveals a non-causal negative correlation between A and C. This negative correlation has been called collider bias and the "explain-away" effect as in, B explains away the correlation between A and C. The correlation can be positive in the case where contributions from both A and C are necessary to affect B.A mediator node modifies the effect of other causes on an outcome (as opposed to simply affecting the outcome).For example, in the chain example above, B is a mediator, because it modifies the effect of A (an indirect cause of C) on C (the outcome).