Fig. 2. Graphical representation of several violations of instrumental variable assumptions, for a variant j.

In (A, B) are two similar violations of the exclusion restriction, with causal effects directly on the outcome either from variant $j$ itself or from another variant $k$ in LD with it. C Shows a reverse causation scenario, another violation of the exclusion restriction, with a causal effect of variant $j$ directly on the outcome, which is then mediated onto the exposure by the causal effect of outcome on exposure. D, E show two mediated confounding scenarios which violate the independence assumption, with the confounder $C$ mediating the genetic effect of variant $j$ onto both the exposure and outcome, with in (F) a further variation on (E) with additional direct causal effects of the variant on the outcome.