Furthermore, these studies provide an explanation of why GWASs could not find an effect of SLC6A4 (Border et al., 2019; Culverhouse et al., 2018); as the short form appears to be associated with both a reduced risk of depression (in the setting of positive environments) and an increased risk of depression (in the setting of significant adversity), neither effect would be visible in a GWAS analysis.(Cantor et al., 2010). This evidence concerns the gene SLC6A4 and major depressive disorder.