Effect of P/T depletion on ICR and within-session response inconsistency is COMT genotype dependent. a) Comparison of average ICR in the control and P/T depletion conditions for the sample as a whole. b) Comparison of ICR across beverage conditions when participants are separated by COMT genotype (beverage × group interaction, F(1,13) = 4.96; p = 0.044). c) Comparison of inconsistency of impulsive choices in the control and P/T depletion conditions for the sample as a whole. d) Comparison of choice consistency when participants are separated by COMT genotype (beverage × group interaction, F(1,13) = 6.1; p = 0.029). P/T depletion reduced inconsistent responding among the val/val participants (t(3) = 3.45, p = 0.04). ICR, impulsive choice ratio; P/T[−], phenylalanine/tyrosine-depletion session. met, methionine; val, valine.