Functional variants of the mono-amine oxidase A (MAO-A) and catechol-O-methyl-transferase (COMT) genes have been associated with violence and aggression [69–72], while Lesch et al. [73] found evidence indicating that the short allele of a functional insertion–deletion variant in the SERT gene was associated with anxiety-related personality traits. This evidence concerns the gene SLC6A4 and Anxiety.