Figure 1.

Causal diagram showing two exposures and one outcome

$G$ , genetic instrument; $L$, liability; $X_0$, exposure measured at time 0; $X_1$, exposure measured at time 1; $Y$, outcome; $U$, confounder. Other sources of variability in the liability, exposures and outcome are not shown in this diagram. There is a problem of under-identification here in that the direct effects of $X_0$ or $X_1$ on $Y$ cannot be estimated with a single liability ($L$).