Past exam of the mathematics course of the University of Cambridge / 2021 / iii / Paper 207 / 2 / d / Solution

Among the education-associated variants in the left panel, larger genetic effects on time in education are associated with more negative refractive error, hence greater myopia. Under the stated assumptions, the fitted negative slope supports a causal effect of more education on increased myopia. In the reverse direction, the myopia-associated variants in the right panel show a much weaker slope whose confidence band includes zero, so the figure gives little evidence that genetically increased myopia causally increases time in education. The asymmetry supports education-to-myopia causation over reverse causation, subject to the Mendelian-randomization assumptions.