 When there are multiple paths between cause and effect, the question becomes more complex. Hesslow (1976) provides an example where taking birth control pills promotes a blood-clotting condition called thrombosis via a chemical in the blood, but inhibits it via preventing pregnancy, which itself is a cause of thrombosis. As a result, taking birth control intuitively influences thrombosis both positively and negatively. If one is interested in the net effect of \(C\) on \(E\)—as opposed to the effects via particular paths (Hitchcock 2001)—then one should not condition on mediators. However, conditioning is necessary for calculating path-specific effects (e.g., Pearl 2001; Weinberger 2019).