The zero risk allele group (who had no DAT1 intron 8 alleles associated with ADHD) had slopes that were significantly steeper (averaging 0.11 ms more of a decrease in RT per trial) than the one risk allele group, which in turn averaged 0.08 ms more of a decrease in RT per trial than the two risk allele group. Here, SLC6A3 is linked to attention deficit-hyperactivity disorder.