In contrast to some epidemiological reports (Boden and Fergusson, 2011), our results do not support evidence of reverse causation, that is, AD causing MD. One could posit that this is due to the relative power of the AD instrumental variable compared to those for MD and AC given the greater number of GWS variants detected for those traits. However, we would like to bring forward three arguments that support the notion that the null AD→MD result is due to the absence of a causal effect of AD on MD rather than a lack of power. First, our findings are in line with the results of an earlier MR study that explored the causal effect of ADH1B rs1229984 on depression and reported no significant association (Almeida et al., 2014). Since this variant is significantly associated with AD risk (Walters et al., 2018), this supports our premise that AD does not have a causal influence on MD. Second, the AD genetic instrument showed different associations between the traits tested: significant causal effect with respect to AC scales, while no effect on MD.
