The rare variant in HFE rs1800562 causes iron overload and is associated with an increase in haemoglobin levels,19, 22 whereas the rare allele in TF rs3811647 is associated with higher transferrin levels20 and lower transferrin saturation.20 But more importantly for this study, as maternal educational level is one of the strongest predictors of childhood cognition, this violates the assumption of no confounding by genotype. This evidence concerns the gene TF and Tangier disease.