Figure 1.

Illustrative diagram showing the standard instrumental variable assumptions for genetic variant $G_j$ (solid lines) with potential violations of the assumptions shown by dotted lines (which are marked with a ‘cross’). The genetic effect on the exposure $X$ is ${\gamma }_j$, the direct genetic effect on the outcome $Y$ is ${\alpha }_j$ and the causal effect of the exposure $X$ on the outcome $Y$ is $\beta$.